138 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 (4) i. 60 milli-equivalents of Na 2 SO 4 .* 
 
 (5) 5- milli-equivalents of NaCl. 
 
 (6) 50.00 milli-equivalents of NaCl.* 
 
 (7) 5.00 milli-equivalents of MgCl 2 . 
 
 (8) 50.00 milli-equivalents of MgCl 2 .* 
 
 is added 50 cc. of the ferric hydroxide sol, whereupon 
 precipitation is observed in the cases, marked by an 
 asterisk. 
 
 169. "Protective Colloids." The use of emulsoids in 
 preventing the precipitation of dispersoids is demon- 
 strated as follows: 1 Adding first 200 cc. of a N/5O 
 sodium chloride solution to 200 cc. of a N/5O silver ni- 
 trate solution, containing 5 cc. of strong nitric acid 
 (specific gravity 1.42), a white flocculous precipitate im- 
 mediately forms. 
 
 The experiment is then repeated with equally strong 
 solutions of both salts, containing I per cent, of gelatin 
 dissolved. The mixture becomes opalescent, and the 
 turbidity increases after a while, without forming a pre- 
 cipitate. 
 
 170. The deflocculation of suspensions by the addition 
 of a small amount of acid and the stabilizing effect of 
 hydroxyl-ions are readily demonstrated as follows: 
 
 Ordinary China clay is stirred up in water, so as 
 to form a suspension, which settles out rather quickly, 
 leaving a clear liquid above and a sharply defined sedi- 
 ment below. If, however, a little alkali, or a salt with 
 alkaline reaction is added, it will be observed that the 
 
 1 Noyes, 1. c. p. 91. 
 
COI^OIDS AND ADSORPTION 139 
 
 settling takes place much more slowly, the smallest par- 
 ticles not settling out at all, or if so only very gradually. 
 
 171. The mobility of a clay suspension containing a 
 little acid is very much less than that of the same sus- 
 pension with a trace of alkali as may be shown by allow- 
 ing the suspensions (which must be rather concentrated) 
 to flow down an inclined glass plate. 
 
 172. With a suspension of colophony (rosin) the de- 
 flocculation by one drop of acid is a very striking 
 phenomenon. An opaque suspension of a milky ap- 
 pearance is obtained by dissolving 0.5 gram rosin in 10 
 cc. of alcohol and pouring the solution in 90 cc. water. 
 On adding one drop of 5N hydrochloric acid an immedi- 
 ate deflocculation takes place. A small amount of alkali 
 dissolves the flocks with the formation of a soap. 
 
 F. Adsorption. 
 
 Adsorption includes a number of closely related 
 phenomena, sometimes distinguished as (i) adsorption, 
 (2) absorption, occlusion or solution and (3) formation 
 of absorption compounds. A sharp demarcation between 
 these groups is impossible. In some cases, e. g., that of 
 palladium, taking up hydrogen, it is likely that all three 
 phenomena occur. In order to avoid these cumbrous 
 distinctions some authors speak of "sorption." The fol- 
 lowing mostly well-known experiments on sorption or, 
 using the more familiar term adsorption as a general 
 designation on adsorption refer to the condensation of 
 (a-) gases, (b) liquids, and (c) dissolved substances on 
 different solids. 
 
UNIVERSITY FARM 
 
UNIVERSITY OF CALIFORNIA LIBRARY 
 
 THIS BOOK IS DUE ON THE LAST DATE 
 STAMPED BELOW 
 
 \ 
 
 5rn-10,'22 
 
LECTURE DEMONSTRATIONS 
 
 IN 
 
 PHYSICAL CHEMISTRY 
 
Published by 
 
 The Chemical Publishing Company 
 
 EASTON, PA. 
 
 Publishers of Scientific Books 
 
 Engineering Chemistry Portland Cement 
 
 Agricultural Chemistry Qualitative Analysis 
 
 Household Chemistry Chemists' Pocket Manual 
 
 Metallurgy, Etc. 
 
Lecture Demonstrations 
 
 IN 
 
 PHYSICAL CHEMISTRY 
 
 BY 
 
 HENRY S>VAN KLOOSTER, Ph.D. 
 
 of the Department of Chemistry, Rensselaer 
 Polytechnic Institute, Troy, N. Y. 
 
 EASTON, PA. 
 
 THE CHEMICAL PUBLISHING CO. 
 1919 
 
 LONDON, ENGLAND TOKYO, JAPAN 
 
 WILLIAMS & NORQATE MARUZEN COMPANY, LTD., 
 
 14 HENRIETTA STREET, COVENT QARDEN, W. C. 11-10 NIHONBA8HI TORI-8ANCHOME 
 
COPYRIGHT, 1919, BY EDWARD HART. 
 
CONTENTS. 
 
 PAGE 
 
 Preface v 
 
 CHAPTER I. 
 General Properties of Matter in the Liquid and Solid State ... I 
 
 CHAPTER II. 
 Diffusion 15 
 
 CHAPTER III. 
 Osmosis 24 
 
 CHAPTER IV. 
 Vapor Pressure and Determination of Molecular Weights 31 
 
 CHAPTER V. 
 Chemical Equilibrium and the Law of Mass Action 38 
 
 CHAPTER VI. 
 Catalysis 55 
 
 CHAPTER VII. 
 Electrochemistry and Ionic Theory 67 
 
 CHAPTER VIII. 
 Solubility and Its Changes 108 
 
 CHAPTER IX. 
 Colloids and Adsorption 114 
 
 CHAPTER X. 
 Actino-chemistry 149 
 
 CHAPTER XI. 
 Flame, Combustion and Explosion 159 
 
 CHAPTER XII. 
 Liquid Air Experiments 179 
 
 Bibliography 188 
 
 Index of Authors 190 
 
 Index of Subjects 193 
 
"Quoniam menti humanae nulla corporum 
 "vel qualitatum corporearum est innata 
 "cognitio: omnia, quae ad corpora perti- 
 "nent, observationibus, vel experimentis 
 "addiscenda sunt." 
 
 PETRUS VAN MUSSCHENBROEK, 
 
 Introductio ad Philosophiam 
 Naturalem, p. 4 (1742). 
 
PREFACE. 
 
 This volume of lecture demonstrations has been prepared with 
 the idea that it would be of service to have a set of experiments 
 at hand, suitable to be shown in the lecture for the illustration 
 of our present conceptions on physical chemistry. 
 
 Arrhenius, in the introduction to his "Theory of Solutions" 
 states "that there are very few doctrines in exact science, where 
 so few lecture experiments are shown as in physical chemistry." 
 This is, of course, partly due to the fact that quantitative meas- 
 urements are needed on which the general laws must be based, 
 while lecture experiments, as a rule, can only illustrate the prin- 
 ciples involved in a qualitative way. It may be said, however, 
 that quite a number of experiments well adapted to illustrate the 
 different chapters of physical chemistry can be performed. Some 
 of these are found in any of the well-known standard works of 
 Heumann, Arendt, Newth and Benedikt, but little or no attention 
 is paid in these text-books to physical chemistry as a separate 
 branch of teaching, as the connecting link between chemistry and 
 physics. In fact, the interesting topics of physical chemistry 
 such as osmosis, diffusion, catalysis are treated in connection with 
 some element or compound, the properties of w r hich are under 
 discussion, thereby unconsciously and perhaps unwillingly intro- 
 ducing the idea, that these phenomena are typical or especially 
 characteristic of certain elements or compounds. To take a few 
 instances out of many: absorption is a standing property of 
 charcoal, colloids are discussed in connection with silicon, allo- 
 tropy is taken up with oxygen and ozon a. s. o. The scope of 
 this volume is diametrically opposed to this system in so far that 
 relationships, rather than distinctions are emphasized, the general 
 character of the different topics is stressed and the all-embracing 
 grip of physical or as it is frequently called general chemistry 
 underlined. 
 
 It is interesting to note as can be seen from the references, 
 which have been given wherever available, that many experi- 
 ments along this line originate from the great masters, which 
 
VI PREFACE 
 
 have given to the science of physical chemistry a place in the 
 front ranks of exact sciences. The very fact, that chemists like 
 Faraday, Graham, Ostwald, Fischer and others have spent part 
 of their time in devising suitable demonstrative experiments is 
 sufficient proof for the usefulness of lecture experiments, wher- 
 ever practicable, even in the case of such a "theoretical" subject 
 as physical chemistry. However important the theoretical part 
 may be, the experimental side will remain our first and our final 
 resort; to quote the words of an early Dutch physicist, cited on 
 a preceding page in the original version : "Since the human 
 mind has no innate knowledge of matter or its properties, every- 
 thing pertaining to matter must be learned by observation and 
 experiment." 
 
 It is hoped that this volume will be useful in the preparation 
 of lecture experiments and stimulate the interest of the students 
 in "practical" physical chemistry. 
 
 Any remarks or suggestions as to changes or additions will be 
 gladly welcomed. 
 
 The author takes pleasure in stating his indebtedness to Prof. 
 Bingham, of Lafayette College, for the help received in correct- 
 ing the manuscript and giving valuable additions (Nos. I, 14, 170, 
 171, 172 on pp. i, 12-14, J 38 and 139). Acknowledgment is also 
 expressed to Prof. Hart and Dr. Hunt Wilson, both of Lafayette 
 College, and to Dr. van Rossen, of Bryn Mawr College, for a 
 number of suggestions. In the reading of the proof sheets the 
 writer was assisted by Miss M. S. Cline, of the Moravian College 
 for Women, and by Mr. Ch. F. Fryling and in the preparation of 
 the cuts by Mr. R. ResnikofT, to whom full credit for their pains- 
 taking labor is hereby given. v. K. 
 
 WASHINGTON, D. C., 
 August, 1918. 
 
CHAPTER I. 
 
 GENERAL PROPERTIES OF MATTER IN THE 
 LIQUID AND SOLD) STATE. 
 
 Fundamental to the study of chemistry and physics is 
 the differentiation of matter into the solid, liquid and 
 gaseous states. A distinction between a liquid and a gas 
 is easily made, since they can only merge into each other 
 at the critical point, the constants of which (critical tem- 
 perature and pressure) are readily denned. Solids are 
 usually denned as having a definite form and a definite 
 shape, while liquids have their own definite volume, but 
 take on the shape of the vessel in which they are con- 
 tained. These simple definitions do not hold, however, 
 in the case of very viscous or plastic substances like 
 glass, pitch, sealing wax, clay and similar materials. A 
 sharp demarcation between a solid and a liquid is pos- 
 sible by defining a solid as a substance which requires a 
 definite shearing force to produce a permanent deforma- 
 tion. A liquid on the other hand is permanently de- 
 formed by any shearing force, no matter how small. 1 
 This may be effectively demonstrated as follows : 
 
 1. A bar of pitch is made up I centimeter square and 
 10 centimeters long. A similar bar is made of modeling 
 clay and both laid horizontally on two supports, 9 centi- 
 meters apart. After a time, which depends on the tem- 
 perature, the clay bar remains perfectly straight, while 
 the pitch bar has flowed, showing its essentially liquid 
 condition. 
 
 1 Bingham, An Investigation of the I^aws of Plastic Flow, Bulletin Bureau 
 of Standards, No. 278, p, 309, (1916). 
 
2 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 Starting again with two other bars of exactly the same 
 dimensions a load of 100 grams is placed upon the pitch 
 bar for a moment only. No perceptible sag is noted. On 
 placing the same weight upon the bar of plastic clay, it 
 gives way completely. The clay is, therefore, a soft (or 
 plastic) solid, and the pitch a very viscous liquid. 
 
 Among the properties of chemical compounds in the 
 liquid and solid state, which are most suitably illustrated 
 by lecture demonstrations may be mentioned the phase 
 transitions which are brought about by a change of tem- 
 perature or pressure. Since 1884, when the importance 
 of the phase rule as a guiding principle for the rational 
 classification of heterogeneous equilibria was gradually 
 recognized, a very considerable amount of work on phase 
 transitions in general has been done by Van't Hoff, 
 Bakhuis Roozeboom, Tammann, Bancroft and their co- 
 workers. It is safe to say that their results could hardly 
 ever have been successfully mastered without the aid of 
 the law which was put forward by Willard Gibbs in 1874. 
 
 The following experiments on phase transitions deal 
 with: 
 
 A. Polymorphic transformations of compounds. 
 
 B. Dissociation of solids. 
 
 C. Undercooled liquids. 
 
 D. Liquid crystals. 
 
 E. Allotropy. 
 
 F. Passivity. 
 
 The chapter is concluded with a demonstration of the 
 relation which apparently subsists between the specific 
 
GENERAL PROPERTIES OF MATTER 3 
 
 heat and the atomic weight of elementary solids (Dulong 
 and Petit's law). 
 
 A. Polymorphic Transformations of Compounds. 
 
 Although it has been known for a long time, that cer- 
 tain compounds exist in two or more polymorphic modi- 
 fications, the recognition of the general character of 
 polymorphism dates from the recent investigations by 
 Tammann and others on the polymorphism of a great 
 many inorganic compounds (water and various salts). 
 The greatly improved methods for the measurement of 
 temperatures, due to the introduction of thermo-elements 
 in physico-chemical work, bring us daily in contact with 
 an ever-increasing number of polymorphic compounds. 
 The transition of one solid phase into another is usually 
 made evident by the heat effect at the transformation 
 temperature ; sometimes also by a marked change in color 
 or a noticeable increase or decrease in volume. 
 
 2. The change in color is easily observed by inserting 
 a test-tube with 5-10 grams of cuprous mercuric iodide in 
 a beaker, containing water of about 80. The color of 
 the compound changes from red to black. The color is 
 reversed by dipping the tube in water of 50, or by allow- 
 ing the tube to cool in the air. In preparing this double 
 salt, 1 mercuric iodide is precipitated from a solution of 
 6.8 grams of mercuric chloride in 100 cc. of water by 
 the addition of 50 cc. of a solution containing 8.3 grams 
 of potassium iodide. The precipitate is washed out and 
 dissolved in a solution of 8.3 grams of potassium iodide 
 
 1 cf. H. und W. Biltz, Uebungsbeispiele aus der unorg. Experimental- 
 Chemie, Leipzig, p. 27, (1907). 
 
4 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 in 50 cc. of water. The filtered solution is mixed with a 
 concentrated solution of 12 grams of copper sulphate in 
 water and the mixture reduced with sulphur dioxide. 
 The precipitate is thoroughly washed, dried at 90-100 
 and kept in a closed tube. 
 
 In lecture courses mercuric iodide is usually taken; 
 this substance, however, has the disadvantage, that the 
 reverse change (on cooling) from yellow to red proceeds 
 rather slowly, the transition temperature (126) is fre- 
 quently overshot by more than 100, and it requires sev- 
 eral hours, sometimes a day or more to complete the 
 transformation. The reversible, enantiotropic character of 
 most phase transitions is therefore more clearly demon- 
 strated in the case of cuprous mercuric iodide than with 
 the latter substance. 
 
 3. A considerable change in volume at the transforma- 
 tion from one modification into another occurs in the 
 case of potassium tungstate. This salt is easily prepared 
 by fusing dry potassium carbonate with (previously 
 ignited) tungsten trioxide. It is exceedingly hygroscopic 
 and must be kept in closed tubes. It melts at 921 , x and 
 has one transition point at 388, which temperature is 
 far overshot on cooling, before the transformation starts 
 with increase of volume. Four to five grams of this salt 
 are fused on a square piece of platinum or nickel foil 
 over a Bunsen flame. On solidifying it will be seen 
 keeping the foil inclined towards the audience that the 
 solid crust crumbles after a while and drops as a fine 
 
 1 Van Klooster, Zeitschr. f. anorg. Chem., 85, p. 49, (1914)- 
 
GENERAL PROPERTIES OF MATTER 5 
 
 dust from the foil, owing to the expansion during the 
 transformation. 1 
 
 4. Another instance is potassium bichromate. 2 On 
 fusing about 10 grams in a thin-walled test-tube, and al- 
 lowing the molten salt to cool, it solidifies at 397, form- 
 ing triclinic crystals, which change at 236, 3 with hardly 
 any perceptible heat evolution into a powder, causing 
 the tube to crack by the expansion. 
 
 B. Dissociation of Solids. 
 
 5. The dissociation, which a number of solid com- 
 pounds undergo on heating is easily exemplified in the 
 case of ammonium chloride or ammonium carbamate. 
 With the former substance the demonstration is conven- 
 iently carried out by placing a little ammonium chloride 
 near the middle of a hard glass tube (about 40 centi- 
 meters long, inner bore I centimeter), held in a slightly in- 
 clined position by a clamp, fastened to a ring stand. A 
 loose plug of asbestos wool is placed a little above the 
 salt, and two strips of moist litmus paper inserted, a blue 
 paper at the lower end and a red paper at the upper end. 
 The salt is gently heated, and dissociates into a mixture 
 of hydrogen chloride and ammonia gas. The latter, be- 
 ing the lighter gas of the two, diffuses more quickly than 
 the hydrogen chloride, with the result that the blue paper 
 is reddened by the excess of hydrogen chloride in the 
 lower part of the tube and the red paper is turned blue 
 by the ammonia, which diffuses faster than the hydrogen 
 
 1 Hiittner and Tammann, ibidem, 43, p. 215, (1905). 
 
 2 Mitscherlich, Pogg. Annalen, 28, p. 120, (1833). 
 
 3 Zemczuzny, Zeitschr f. anorg. Chem., 57, p. 267, (1908). 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 chloride. This experiment also serves as a demonstra- 
 tion of atmolysis. The very simple arrangement de- 
 scribed above for demonstrating the heat-dissociation of 
 ammonium chloride, is due to Fenton. 1 Other types of 
 apparatus for the same purpose have been devised by 
 Pebal 2 and Than. 3 
 
 C. Undercooled Liquids. 
 
 6. The familiar phenomenon of an undercooled (also 
 called: supercooled) liquid may be conveniently demon- 
 strated with sodium thiosulphate (Na 2 S 2 O 3 . 5H 2 O). 
 About 100 grams of the salt are 
 heated in a flat-bottomed bulb flask 
 of 250 cc. The compound melts at 
 48 and the molten salt is allowed 
 to cool to about 30. By closing 
 the flask with a loose plug of cotton 
 wool thus preventing the access 
 of minute crystals or dust particles, 
 which occasionally act as "germs" 
 in breaking up the metastable con- 
 dition, the supercooled liquid may 
 be kept for an indefinite time. 
 Crystallization can only be started 
 by a crystal of the salt (which may 
 be almost invisible). By introduc- 
 ing a glass rod, covered at its lower end with a thin crust 
 of the solid salt, without any adhering loose powder, 
 
 1 cf. Mellor, Modern Inorganic Chemistry p. 542, (1916). 
 
 2 lyiebigs Annalen 123, p. 199, (1862). 
 * Ibidem, 131, p. 129, (1864). 
 
 Fig. i. 
 
GENERAL PROPERTIES OF MATTER 7 
 
 into the undercooled liquid, crystallization starts from 
 the end of the glass rod (with simultaneous evolu- 
 tion of heat) and after a few seconds the rod is lifted out 
 of the liquid, covered with a conglomerate of crystals; 
 (Fig. i) at the same time, however, no further solidifica- 
 tion is observed in the liquid, due to the fact, that the 
 solid phase has been completely removed. 1 
 
 7. A case, analogous to the crystallization of an un- 
 dercooled liquid is that of the devitrification of a (silicate) 
 glass, as can be shown with sodium metasilicate 
 (Na,SiO 3 ). This salt melts at io88, 2 and solidifies, 
 when slowly cooled, at temperatures, varying from 1080- 
 1000. The salt is easily prepared by mixing sodium 
 carbonate and silica (quartz) in equivalent quantities, 
 heating the mixture for 1-2 hours at a temperature of 
 6oo-8oo, thereby effecting a partial combination. The 
 sintered mass is pulverized and the above process re- 
 peated two or three times, in order to insure perfect 
 homogeneity. Finally the powder is fused and on slowly 
 cooling changes into a conglomerate of opaque crystals. 
 Ten grams of the salt are heated in a small platinum 
 crucible and rapidly cooled by means of a stream of cold 
 air, 3 whereupon a perfectly clear and transparent glass is 
 formed. This glass is then slowly heated, either in the 
 crucible or on a piece of platinum or nickel foil over a 
 Bunsen flame. At a temperature where the glass just 
 begins to soften (about 550) the devitrification (crystal- 
 
 1 Ostwald, Grundlinien der anorg. Chemie, 3e Aufl, p. 537, (1912). 
 
 2 Jaeger, Journ. of the Wash. Ac. of Sc., 1, p. 53, (1911). 
 '" Guertler, Zeitschr. f. anorg. Chem., 40, p. 268, (1904). 
 
 2 
 
8 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lization) suddenly starts, often accompanied by a strong 
 glowing, indicating an enormous increase of temperature. 
 
 D. Liquid Crystals. 
 
 8. As an example of a group of organic compounds, 
 which are characterized by two melting points, the case 
 of para-azoxyanisol may be quoted. This substance, dis- 
 covered by Gattermann, melts at n 6 to a turbid bright 
 yellow liquid, which on further heating, suddenly clears at 
 135. The phenomenon is suitably projected on the screen 
 by heating the substance in a small glass trough with 
 parallel walls of rectangular cross section. The first 
 melting point (116) represents the conversion of a crys- 
 talline solid into (anisotropic) liquid crystals, which 
 change at 135 into an (isotropic) liquid. 
 
 E. Allotropy. 
 
 The recent work of Cohen and his co-workers on this 
 topic have clearly brought out the frequent occurrence of 
 polymorphism among elements, especially heavy metals. 
 Since in most cases the change from one solid phase into 
 another at the transition point is accompanied by an ap- 
 preciable change in volume the method chiefly employed 
 is that, which makes use of a dilatometer. 
 
 9. The following lecture experiment 1 gives a good idea 
 of the enormous decrease in volume, resulting from the 
 transformation of grey tin into white. At the tempera- 
 ture of transformation (18) the specific gravities, as de- 
 termined by Cohen, 2 are 5.79 and 7.28 respectively. The 
 
 1 Cohen, Transactions of the Faraday Soc., 7, p. 6, (1911). 
 
 2 Zeitschr. f. phys. Chem., 30, p. 601, (1899). 
 
GENERAL PROPERTIES OF MATTER 9 
 
 dilatometric apparatus (Fig. 2), consists of a glass cylin- 
 der (A), filled with 60-70 grams of grey tin, and a con- 
 necting U-shaped tube, containing mercury. The space 
 
 Fig. 2. 
 
 between mercury and tin is filled, as far as the stopcock 
 K, with distilled water. On the mercury in the open 
 limb of the U-tube floats a small cylindrical weight, con- 
 nected by means of a thin thread with the disk S, which 
 turns around an axis, kept in its place by the beam H. A 
 pointer fastened to the disk and moving along a graduated 
 scale, follows the displacements of the mercury in the 
 U-tube. The zero-position is reached by opening the 
 stopcock and pouring water in the apparatus, until the 
 
IO DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 pointer is adjusted. The stopcock is then closed and the 
 cylinder A warmed up with water of about 8o. The 
 mercury sinks in the open limb and a sudden upward 
 move of the pointer over three or more scale divisions is 
 observed. 
 
 It has been found, that the reverse change, from the 
 white modification, in which tin is usually known, into 
 the grey form goes fastest at a temperature of 45, 
 and also, that the transformation is accelerated in 
 the presence of pink salt solution. In the absence of 
 the grey modification white tin can be kept below 18 
 several months, or even years, without the slightest indi- 
 cation of any transformation. If, however, the white tin 
 is "infected" with a trace of grey tin, the transformation 
 goes on, until the "tin pest" has entirely affected the 
 white modification. 
 
 10. The phenomenon of dynamical allotropy in the 
 liquid state is shown by sulphur and was thoroughly in- 
 vestigated, first by A. Smith and his pupils, and after- 
 wards by Kruyt and his co-workers. The peculiar be- 
 havior of molten sulphur in the neighborhood of 160 
 and the formation of plastic and amorphic sulphur are 
 usually demonstrated in first courses on inorganic chem- 
 istry and need no special description at this place. It 
 may be remarked, that from a colloid-chemical stand- 
 point this behavior is interesting, when sulphur is con- 
 sidered, as W Ostwald proposes 1 as an "allo-colloid." 
 
 F. Passivity. 
 
 11. The change in condition, which some heavy metals, 
 
 1 Ostwald-Fiseher, Handbook of Colloid Chemistry, p. 104, (1915). 
 
PROPERTIES OF MATTER II 
 
 especially iron and chromium undergo, when inserted in 
 strong nitric acid (specific gravity 1.50), usually 
 called "passivity," 1 may be demonstrated in the following 
 manner. A square piece of thin sheet iron, well cleaned, 
 is attached to a platinum wire, and lowered in a beaker 
 containing dilute nitric acid, in which the iron is imme- 
 diately attacked. It is then transferred to another 
 beaker with concentrated nitric acid (specific gravity 
 1.50) ; nothing happens. Having removed the adhering 
 acid by inserting the iron in a beaker with distilled water, 
 the now passive iron is brought in a fourth beaker con- 
 taining a dilute solution of copper sulphate. No film of 
 copper is formed on the iron, which remains grey as be- 
 fore. Care has to be taken, that the iron is not touched 
 in some way or other, because hammering, bending or 
 scratching immediately restores the active state as will 
 be seen by the formation of a thin copper coating. 
 
 12. The iron can also be brought in the passive state by 
 dissolving the metal electrolytically, using the iron as an 
 anode in electrolyzing an aqueous solution of sulphuric 
 (or nitric acid), or a solution of a nitrate or sulphate. 
 
 Passivity was discovered by Keir 2 and studied more in 
 detail by Faraday (1836) and simultaneously by Schon- 
 bein. A good explanation for this peculiar phenomenon 
 is still lacking. Some authors 3 ascribe the activity to the 
 presence of hydrogen ions at the surface of the iron ; by 
 dipping the metal in concentrated nitric acid the hydrogen 
 is oxidized and the metal becomes passive. Activity is 
 
 1 Schonbein, Pogg. Ann. 37, pp. 390, 590, (1836). 
 
 2 Phil. Transact., 80, p. 374, (1790). 
 
 3 cf. Rathert, Zeitschr. f. phys. Chera., 86, p. 567, (1914). 
 
12 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 restored by heating in hydrogen gas or inserting the metal 
 as a cathode in a ferrous sulphate solution. Another 
 explanation, first advanced by Faraday, traces the cause 
 of passivity to the formation of a protecting skin of oxide. 
 Quite recently Smits 1 has given an entirely new explana- 
 tion, based on the assumption of different kinds of mole- 
 cules or ions in the metal, which are in mobile equilibrium. 
 Passivity, according to Smits, would be nothing but a dis- 
 turbance of this internal equilibrium. 
 
 13. The following experiment, taken from Smits' 
 
 ^rx^ paper, shows that passivity can be 
 
 " overcome by bringing the iron in con- 
 tact with solutions of chlorides, bro- 
 mides, or iodides, a fact, which cannot 
 well be reconciled with the oxide 
 theory. A piece of sheet iron, pro- 
 vided with an elbow (Fig. 3), is first 
 inserted in strong nitric acid, and 
 
 Fig. a. then in a concentrated solution of cop- 
 
 per sulphate. No copper is deposited, but on bringing the 
 elbow-appendix in contact with a solution of potassium 
 chloride, bromide, or iodide, activity is restored at once. 
 A solution of mercuric chloride has no effect, hence the 
 activating action is exerted by the Cl/ Br' and I'-ions 
 respectively. 
 
 14. The law, discovered by Dulong and Petit in 1819, 
 stating that the heat capacity of atoms is approximately 
 the same for all solid elements, is very striking as is 
 readily seen from the following table which contains a 
 
 i Chem. Weekblad 12, p. 676. (1915). 
 
PROPERTIES OF MATTER 
 
 number of elements (metals) selected at random from a 
 list of more than fifty elements arranged in the order of 
 increasing atomic weight: 
 
 Element 
 
 Atomic 
 weight 
 
 Specific 
 heat 
 
 Specific 
 gravity 
 
 Atomic 
 volume 
 
 Atomic 
 heat 
 
 Aluminium .... 
 
 27.1 
 65.4 
 
 0.2T7 
 O.OQ4 
 
 2.7 
 
 7.1 
 
 10.4 
 0.2 
 
 5-9 
 6.1 
 
 Tj n 
 
 118 7 
 
 o 0^*1 
 
 7 ^ 
 
 16 ; 
 
 6 s 
 
 Lead 
 
 2O7 2 
 
 O O^I 
 
 1 1 7 
 
 18 2 
 
 64 
 
 
 
 
 
 
 
 The importance of this law is frequently not realized 
 to its full extent, especially in elementary courses of 
 inorganic chemistry because of the lack of a suitable lec- 
 ture demonstration. This is, however, a very simple mat- 
 ter, since it is merely necessary to take amounts of two 
 elements in ratio of their atomic weights, heat them to 
 1 00 and then plunge them in equal volumes of water at 
 room temperature. The rise in temperature is approxi- 
 mately the same as can be readily seen at a distance by 
 using two large air thermometers of equal size. The 
 apparatus used by Prof. Bingham in his lectures 1 consists 
 of a lead weight of 4144 grams (20 gram atoms) and a 
 zinc weight of 1308 grams (20 gram atoms) of the same 
 cross section (10 centimeters square) as shown in Fig. 4. 
 From the table given above, it is seen that the atomic 
 volume of lead (18.2) is twice that of zinc so that the 
 volume of the lead weight as judged from its height must 
 be double that of the zinc weight. Both weights, provided 
 with brass handles for ease of manipulation are heated 
 in a pail (or dish) containing boiling water, simultane- 
 
 1 Obtainable from Eimer and Amend, New York. 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 ously removed and plunged into two glass jars of equal 
 size (diameter 16 centimeters, height 9 centimeters) 
 filled with 500 cc. of water at room temperature. In 
 the center of the jars are placed two air thermometers 
 
 Fig. 4. 
 
 filled with a colored liquid and carefully adjusted, so that 
 both show the same rise of liquid in the stem for equal 
 increments of temperature. The initial position of the 
 liquid in the stem is marked by means of a clip. The 
 rise of liquid will be found to be several centimeters 
 (dependent upon the bore of the thermometer stem) but 
 the same for both thermometers. 
 
 If desired, the experiment may be repeated using equal 
 -weights of lead and zinc in which case the rise of tem- 
 perature will be more than three times greater for the 
 zinc than for the lead. 
 
CHAPTER II. 
 
 DIFFUSION. 
 I. Diffusion in Gases. 
 
 15. The process of diffusion of gases has been the sub- 
 ject of exhaustive researches by Graham (1832), to 
 whom we owe the laws governing gaseous diffusion and 
 the related phenomena of effusion and transfusion. As 
 Graham has shown, the relative speeds of diffusion of 
 gases are inversely proportional to the square roots of 
 their relative densities. That hydrogen, being the light- 
 est of all known gases, diffuses faster than air through 
 the walls of a thin porous membrane, while air itself, 
 
 Fig. 5. 
 
 being lighter than carbon dioxide travels faster through 
 the membrane than does carbon dioxide, is readily 
 
l6 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 shown by the use of unglazed, porous, porcelain cylin- 
 ders, connected with a long narrow glass stem, as were 
 first recommended for this purpose by Wohler. 1 The 
 whole arrangement may be seen from the figure (Fig. 5). 
 Both cylinders contain air, under atmospheric pressure 
 as indicated by the open manometers, with which the 
 stems are connected. An inverted beaker filled with 
 hydrogen is brought over one pot and another beaker 
 filled with carbon dioxide over the second cylinder. The 
 different speed of diffusion instantly causes, in one case, 
 a (temporary) excess of pressure, and in the other a 
 reduction of pressure, until, after a few minutes, 
 equilibrium is re-established. On removing the beakers 
 the reverse takes place. 
 
 16. The different speed of diffusion can also be demon- 
 strated in an elegant manner by the use of small glass 
 bulbs (of about 1.5-2 centimeters in diameter) filled with 
 liquid bromine, as used by Biltz. 2 These bulbs are 
 made by drawing out a glass tube into capillary ends and 
 blowing the intermediate piece of tubing into the re- 
 quired shape. Two of these bromine bulbs are placed 
 in two glass cylinders (height 27 centimeters, width 6.5 
 centimeters) closed at both ends by well-fitting glass 
 plates coated with a little grease, so as to insure gas- 
 tight connections. The upper glass plates are perforated 
 and closed by rubber or cork stoppers. One of the stop- 
 pers has one hole, which allows the passage of a long 
 glass rod, bent at right angles at its end in the form of a 
 circle, in order to crush the bromine bulb at the proper 
 
 1 Ber. d. chem. Ges., 4, p. 10, (1871). 
 
 2 Zeitschr. f. phys. Chem., 9, p. 152, (1892). 
 
DIFFUSION 
 
 moment. The other stopper has two holes, through 
 which passes a glass tube (inner bore 0.4 centimeter) 
 similar in shape to the glass rod in the first mentioned 
 cylinder and serving for a like purpose, and another L,- 
 shaped tube, provided with a piece of rubber tubing and 
 a pinchcock for the introduction of hydrogen gas (Fig. 
 6). The dry gas is passed through in a rapid stream, 
 
 
 Fig. 6. 
 
 expelling at the same time the air through the "crushing" 
 tube. After 2 minutes the cylinder is filled and both 
 tubes closed by the pinchcock and a cork stopper re- 
 
l8 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 spectively. The two bromine bulbs are crushed simul- 
 taneously and the difference in behavior of the air- 
 bromine and hydrogen-bromine mixture becomes visible 
 in the course of 3-5 minutes. Using white screens to 
 make the colors visible at a distance, it will be seen that 
 in the hydrogen cylinder the bromine fills the space half 
 way up, while in the air cylinder the bromine has moved 
 only one-fourth upward. 
 
 17. On the different speed of diffusion through a 
 porous septum is based a method first applied by Gra- 
 ham, called atmolysis to separate one gas from another. 
 Ostwald 1 has given the following arrangement to show 
 the separation of detonating gas into hydrogen and oxy- 
 gen by this method. 
 
 The gas is generated in a wide mouth bottle (Fig. 7), 
 
 Fig. 7. 
 
 filled with a rather strong solution of caustic soda, which 
 is electrolyzed by the current from two storage cells. 
 Two cylindrical iron or nickel sheets are used as elec- 
 trodes. The gas is dried by a U-tube filled with granu- 
 lated calcium chloride and enters first the left (glass) 
 
 1 Ostwald-McGowan, The Scientific Foundations of Anal. Chem., srded., 
 p. 232, (1908). 
 
DIFFUSION 19 
 
 arm of a branched tube, the one stopcock being turned 
 on and the other turned off; the gas collected in a test 
 tube over water explodes with a lighted match. When 
 however the gas is made to pass through the right tube 
 of unglazed porcelain, it will be seen that, under proper 
 conditions the hydrogen diffuses out almost completely 
 with the result that the collected gas rekindles a glowing 
 splint, thereby showing that it is oxygen that is left. 
 
 Fig. 8 
 
 18. The same effect is obtained by passing the elec- 
 trolytic gas through two crossed "churchwarden" clay 
 pipes, connected by a piece of thick walled rubber tubing 
 (Fig. 8). Both in this and in the preceding experiment 
 
20 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the proper rate at which the gas mixture travels has to 
 be found out beforehand. If the gas stream is too rapid, 
 the hydrogen has no time to diffuse out ; on the other 
 hand, if the rate is too slow, air will diffuse into the tube 
 so that a glowing splint will not burst into flame. 
 
 19. That the law of diffusion also holds good for ef- 
 fusion, i. e., the passage of a gas 
 through a fine orifice, was also 
 found by Graham (1832) and 
 may be shown for hydrogen and 
 oxygen with an apparatus de- 
 vised by Freer 1 (Fig. 9), con- 
 sisting of a U-tube, connected on 
 one side with a two-hole stop- 
 cock (A) and on the other side 
 with a barometer tube. The left 
 limb of A (a) contains a piece of 
 glass rod and is drawn out into 
 an extremely narrow tip, while 
 the right outlet tube (b) is left 
 unchanged. After the bend of 
 the U-tube has been covered 
 with mercury, so as to separate 
 the air space on both sides, dry 
 hydrogen gas is passed through 
 the tube, escaping through b (a 
 being closed). After a few minutes A is turned off, and 
 mercury poured in the long limb of the U-tube up to a 
 certain height, the hydrogen in the short limb occupying 
 
 1 Zeitschr. f. phys. Chem., 9, p. 669, (1892). 
 
 Fig. 9. 
 
DIFFUSION 21 
 
 a volume of about 80 cc., marked off by a strip of paper. 
 The gas is then allowed to escape through the tip a, a 
 metronome being used to note the time necessary to drive 
 the gas out to a mark just below the stopcock. This 
 ought to require about 7 seconds. The experiment is then 
 repeated replacing the hydrogen by oxygen. If proper 
 care is taken in filling the tube with an equal volume of 
 pure oxygen, the time of effusion will be four times as 
 long as before. 
 
 II. Diffusion in Liquids. 
 
 20. Diffusion in liquid state, (and taking as a typical 
 instance that of salt solutions in water), first carefully 
 studied by Graham (1850-51) requires such a consider- 
 able time to show a visible result that the effect of diffu- 
 sion can only be seen after half a day 
 or longer. The experiment may be 
 carried out, following Graham's di- 
 rections by filling a bottle with a 
 concentrated solution of the salt, 
 (copper nitrate or chloride) and plac- 
 ing this bottle in a cylindrical vessel 
 which is then filled to the top with dis- 
 tilled water (Fig. 10). 
 
 A sharp boundary surface between Fi s- 10 - 
 
 the water and the solution may also be obtained by con- 
 necting the cylinder containing the water through a bent 
 capillary glass tubing with a separatory funnel (Fig. n), 
 and allowing the heavy salt solution to slowly push the 
 aqueous layer upward without any perceptible mixing. 
 
22 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 Still another scheme is to cover the salt solution, filling 
 
 a cylindrical jar halfway, 
 with a thin cork disk and to 
 allow the water to drop 
 slowly on the disk. The 
 original sharp demarcation 
 line between the two layers 
 disappears as the diffusion 
 progresses. 
 
 III. Diffusion in Solids. 
 
 Diffusion of solids into 
 each other, requires months 
 and years to show a notice- 
 able result, as has been 
 demonstrated by the work 
 of Roberts-Austen 1 on the 
 diffusion of gold in lead at 
 20, 100 and 250. An 
 
 experiment, that takes a few weeks and illustrates to a 
 certain extent the diffusion in solids, is the following: 
 
 21. A 5 per cent, solution of gelatine in water (150 cc.) 
 is made and divided in three equal volumes. One part is 
 left uncolored, the other two portions are dyed with 
 congo-red and methyl violet, or any other organic dyes. 
 The solutions are poured in three crystallizing dishes 
 of 10 centimeter diameter, and when coagulated, taken 
 out in the form of thick plates, which are placed in a 
 
 1 Transactions of the Royal Soc., 187, p. 383, (1896). 
 
 Fig. 11. 
 
DIFFUSION 23 
 
 large glass jar, one on top of the other, the uncolored 
 plate being put in the middle. The jar is covered by a 
 cork stopper and set aside in a suitable place, where the 
 result of the diffusion of the colors can be observed at 
 any time. 
 
CHAPTER III. 
 
 OSMOSIS. 
 
 I. Osmotic Experiments with Gases. 
 The property of palladium, especially when heated, of 
 dissolving hydrogen readily, but not nitrogen has been 
 used by Ramsay 1 to carry out osmotic experiments with 
 a nitrogen-hydrogen mixture. Since it is necessary to 
 work at high temperatures, in order to obtain satisfactory 
 results, it is more convenient to carry out a similar ex- 
 periment at the ordinary tempera- 
 ture with air and ammonia, replac- 
 ing the palladium by animal mem- 
 brane moistened with water. 
 Ammonia is extremely soluble in 
 water, while hydrogen, oxygen and 
 nitrogen are difficultly soluble in 
 this solvent. The thin film of 
 water on the membrane acts in this 
 way as a semipermeable membrane. 
 22. A thistle tube covered with 
 the moist membrane, is bent in the 
 form of a U, and contains air, 
 under atmospheric pressure as in- 
 dicated by the height of some col- 
 ored oil in both limbs of the U-bend 
 (Fig. 12.). 2 If now a beaker is 
 inverted over the head of the thistle 
 
 Fig. 12. 
 
 i Phil. Mag., 38, p. 206, (1894). 
 
 Stieglitz, Elements of Qualitative Chemical Analysis, Vol. I, New York 
 (1916), p. 22, also: Alex. Smith, Introduction to Inorg. Chemistry, 3d ed., New 
 York, p. 329, (1917). 
 
OSMOSIS 25 
 
 tube and hydrogen admitted, no increase of pressure 
 inside the thistle tube is observed. On substituting an 
 atmosphere of ammonia for the hydrogen, the gas dis- 
 solves quickly in the water on the membrane until satu- 
 ration, and then enters the inside of the tube producing 
 an increase in pressure. A piece of red litmus paper 
 changes color at the same time. 
 
 II Osmotic Experiments with Liquids. 
 
 23. A very simple osmotic experiment, which forms a 
 modification of the original experiment, performed by the 
 discoverer of osmosis, the abbe Nollet 1 was described by 
 Lupke 2 as follows : 
 
 A 100 cc. glass jar is filled with a nearly saturated 
 solution of cane sugar and closed with bladder. On sub- 
 merging the jar in water the volume increases consider- 
 ably in the course of 2 or 3 hours and the membrane 
 swells up to such an extent, that on piercing the latter 
 with a thin needle a stream of liquid, about 20 centimeters 
 high, is thrown up. 
 
 24. The realization of practically semipermeable mem- 
 branes rests on the discovery by M. Traube of the copper 
 ferrocyanide precipitation membrane, the formation of 
 which may be shown in a way suitable for projection, in 
 a small trough with parallel walls (Fig. 13), filled with 
 a half-saturated solution of copper sulphate. 3 From a 
 pipette, containing a nearly saturated solution of potas- 
 
 1 M6moires de 1'Ac. Royale des Sc., p. 57, (1748). 
 
 2 Grundziige der Blectrochemie 50 Aufl., p. 91, (1907). 
 
 'c.f. Nernst, Theoretische Chemie, 6e Aufl., p. 133 (1909). Thiel, Zeitschr. 
 f. Electrochemie, 12, p. 229, (1906). 
 
26 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 sium ferrocyanide, one drop is allowed to fall on the 
 copper sulphate solution. An exceed- 
 ingly tenuous membrane of the brown 
 copper ferrocyanide is formed, through 
 which water passes into the solution, en- 
 closed by the precipitate. The result is, 
 that the solution, surrounding the drop 
 becomes more concentrated and sinks in 
 
 Fig. is. thread-like streaks to the bottom. These 
 streaks are easily seen, owing to the different refractive 
 indices of solutions of different densities. Tammann 1 
 has used this method of "streaks" in an ingenious way 
 to detect isosmotic (isotonic) solutions. 
 
 25. Precipitation membranes, like the above men- 
 tioned, are obtained in a similar manner, by pouring a 
 moderately concentrated sodium silicate solution (specific 
 gravity I, i) into a number of lecture jars containing a 
 few crystals of copper-, iron-, manganese-, nickel- and 
 cobalt salts respectively. After standing over night in a 
 quiet place, peculiar, coralline shoots (so-called "chemi- 
 cal gardens") are formed, of different shape and color, 
 characteristic of the salts used. The mode of formation 
 is the same as in the case of the copper ferrocyanide. 
 
 26. The precipitation of efficient copper ferrocyanide 
 membranes in the pores of unglazed porcelain cells (after 
 Pfeffer) is connected with considerable experimental 
 difficulties, as was clearly brought out by Morse and his 
 collaborators. It is, therefore, preferable to use for lec- 
 ture experiments, demonstrating osmotic pressure, parch- 
 ment thimbles (as may be obtained from Schleicher and 
 
 1 Wiedemaiis Annalen, 34, p. 299, (i-sS). 
 
OSMOSIS 
 
 Schiill, in the dimensions of 100 by 16 millimeters, No. 
 579), tightly fastened to a long narrow tube 1 (Fig. 14). 
 The cell is filled by pouring through 
 the funnel a colored, concentrated 
 solution of cane sugar. The stopcock 
 is then closed and the cell placed in a 
 beaker of distilled water. Although 
 the parchment is not quite imperme- 
 able to sugar, it will be seen that the 
 water passes very easily through the 
 parchment membrane causing a rapid 
 rise of the solution in the narrow tube. 
 The initial height of the liquid is 
 marked by a strip of paper. The rise 
 amounts to several centimeters in the 
 course of an hour. 
 
 27. Nernst 2 has constructed an 
 osmotic cell based on selective solu- 
 bility, similar to the osmotic gas cell, 
 described above, consisting of an in- 
 verted thistle tube (7 centimeters 
 wide), to which a piece of pig bladder, 
 thoroughly soaked in water of 40 is tightly fastened by 
 means of a string. The cell, filled with ether in which 
 benzene has been dissolved, is pressed against a wire 
 gauze, suspended in a 400 cc. beaker, and is held in its 
 place by a clamp from a ringstand (Fig. 15). The 
 beaker contains ethyl ether, saturated with water, and is 
 covered by a one-hole stopper (allowing the passage of 
 
 1 Alex. Smith, 1. c. p. 328. 
 
 2 Zeitschr. f. phys. Chem., 6, p. 37, (1890). 
 
 Fig. 14. 
 
28 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the stem) in order to limit the loss of ether by evapora- 
 tion. The wire gauze serves to prevent the bladder 
 from distending as a result of the passage of ether 
 through the bladder into the thistle tube. The water con- 
 tained in the bladder, dissolves the 
 ether, but not the benzene and acts in 
 this manner as a semipermeable mem- 
 brane. A rise of 10-20 centimeters in 
 the course of an hour will be observed, 
 provided the stem of the thistle tube 
 is not too wide. 
 
 28. In a very striking manner Crum 
 Brown 1 has illustrated the role of a 
 perfectly semipermeable membrane. 
 A strong solution of calcium nitrate 
 is shaken with a little phenol, until 
 saturation is reached and the mixture 
 poured into a high and narrow cylin- 
 drical jar. The phenol, left undissolved, 
 floats as a thin layer, (which should not 
 be more than a few millimeters thick) 
 on top of the calcium nitrate solution, 
 saturated with phenol. The phenol- 
 layer is then cautiously covered with a 
 saturated solution of phenol in distilled water. The cal- 
 cium nitrate being insoluble in phenol, the latter acts as 
 a semipermeable membrane dissolving the water and 
 allowing its passage from the upper layer into the lower, 
 and the result is, as a daily observation and demarca- 
 
 1 Proc. of the Royal Soc. of Edinburgh, 22, p. 439, (1898). 
 
 
 Fig. 15. 
 
OSMOSIS 
 
 2 9 
 
 tion of the height of the thin phenol layer by means of 
 strips of paper shows, that the phenol is gradually dis- 
 placed upward until finally only 2 layers are left: a 
 dilute, calcium nitrate solution, surmounted by a thin 
 layer of phenol. 
 
 The experiments with three liquid layers, of which 
 the middle acts, to a certain extent 
 as an osmotic membrane, were first car- 
 ried out by a French scientist, Lhermite. 1 
 As liquids he used an aqueous solution 
 of alcohol (35 per cent.), castor oil (or 
 turpentine) and water; also ethyl ether, 
 water and oil of bitter almonds (or car- 
 bon disulphide). 
 
 29. A modification of one of Lher- 
 mite's three liquid combinations, was 
 introduced by Kahlenberg, 2 who fixes 
 the middle layer (in this case water) in 
 its position, in order to be able to demon- 
 strate the osmotic pressure, resulting 
 from the difference in solubility of ether 
 in water and carbon disulphide. A glass 
 apparatus (Fig. 16), contains at the 
 bottom and in the communicating narrow 
 side tube, mercury. The latter is cov- ** 16 - 
 ered by a layer of carbon disulphide (C), then follows 
 a tightly pressed cork-disk (B), soaked with water, and 
 finally a layer of (aqueous) ethyl ether (A). The 
 apparatus is closed by a loosely fitting cork to avoid 
 
 1 Ann. de Chim. et Phys. (3) 43, p. 420, (1855). 
 
 ' 2 Outlines of Chemistry, Revised ed., New York, p. 443, (1916). 
 
3O DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 evaporation of the ether. The initial position of the 
 mercury in the narrow gauge tube is marked by a strip 
 of paper. The gradual rising of the mercury may be 
 still better observed by pouring some colored water on 
 to the mercury in the side tube. 
 
CHAPTER IV. 
 
 VAPOR PRESSURE AND DETERMINATION OF 
 MOLECULAR WEIGHTS. 
 
 A. Vapor Pressure. 
 
 Of the two methods for measuring vapor pressures, 
 the static and the dynamic, the former is easily carried 
 out as follows : x 
 
 30. In three out of four barometer tubes, all inverted 
 over mercury, are inserted, by means of pipettes, with 
 their ends curved upward small quantities of water, 
 ethyl alcohol and ethyl ether respectively. The fourth is 
 kept as a standard showing the atmospheric pressure. The 
 fall of the mercury in the three tubes as compared with 
 the height of the mercury pile in the fourth tube is a 
 direct measure of the vapor pressure of the liquids at 
 room temperature. Expressed in centimeters of mercury 
 at 20, these pressures are: 
 
 for water 1.74 
 
 for alcohol 4.40 
 
 for ether 44.24 
 
 By mounting the tubes within a jacket, connected with 
 a distilling flask containing a suitable distilling liquid, 
 the vapor pressure can be demonstrated for any desired 
 temperature. 
 
 The dynamic method, which consists in a slow but con- 
 tinuous diminution of pressure while keeping the liquid 
 constantly at ebullition, allows the pressure and tempera- 
 
 1 Bigelow, Theor. and General Chemistry, New York, p. 274, (1914). 
 
DEMONSTRATIONS IN PHYSICAL, CHEMISTRY 
 
 / Z 3 4 
 
 ture to be read at the same time; but, owing to the fact 
 that only the decrease in pressure can be made visible to a 
 large audience, this method is less fit for a lecture dem- 
 onstration. 
 
 31. The vapor pressure of solutions is shown in the 
 
 same way as for pure liquids, viz., 
 by the use of four barometer tubes 
 over mercury, of which one is kept 
 as standard of comparison. Into the 
 vacuum of the first tube is intro- 
 duced a little ethyl ether, in the 
 second is placed a few drops of a 
 solution of 12.2 grams benzoic acid 
 in 100 cc. ether (molecular weight 
 of benzoic acid = 122) and in the 
 third tube an ethereous solution of 
 benzoic acid of double this strength 
 (22.4 grams in 100 cc. ether). 1 
 After a while the difference in 
 mercury level between tube one 
 and two is about half as much as 
 that between tube one and three showing that the lower- 
 ing of the vapor pressure is proportional to the concen- 
 tration of the solute. (Fig. 17.) 
 
 32. The depression of the vapor pressure at the boil- 
 ing point (under atmospheric pressure) of the solvent, 
 is directly connected with the ebullioscopic methods for 
 the determination of molecular weights and is conven- 
 iently carried out with the aid of an apparatus as 
 
 1 lyiipke-Bose, Grundziige der Electrochemie, 5e Aufl. p. 106. 
 
 17 - 
 
VAPOR PRESSURE; AND MOLECULAR WEIGHTS 
 
 33 
 
 sketched in Fig. iS, 1 consisting of an outer jacket and 
 an inner "test"-tube ( to which a narrow 
 gauge tube has been sealed), held by a two- 
 holed cork stopper, provided with a second 
 tube for the escape of the vapor of the solv- 
 ent. Pure solvent is poured in the outer 
 jacket and also in the test-tube, to a 
 height of 7 centimeters above the bend. In 
 order to make any difference in level better 
 visible, a trace of some aniline dye is added. 
 On gently heating the solvent in the outer 
 jacket, the vapor condenses along the walls 
 and finally escapes in the open through the 
 outlet tube. The latter is then closed by a 
 cork stopper and the vapor is forced 
 through the narrow gauge tube, expelling 
 the remaining air, and condensing above the 
 constricted part of the test-tube. After the 
 vapor has bubbled through for a few min- 
 utes, the cork stopper above the constric- 
 tion is pushed down, and at the same time 
 
 Fig. 18. 
 
 the stopper removed from the outlet tube, thus allowing 
 the vapor to escape directly in the open as before. It 
 will be found that the level is practically the same in the 
 test-tube and the gauge tube. A weighed quantity of a 
 solid, easily soluble in the chosen solvent (about 0.3 
 gram) is introduced in the inner tube, and the operation 
 is repeated When equilibrium is reached, the liquid 
 stands lower in the gauge tube (2 centimeters or more) 
 
 1 Journ. Am. Chem. Soc., 40, p. 193, (1918).' 
 
34 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 than in the test-tube, thus clearly showing that the pure 
 solvent has a higher vapor pressure than the solution at 
 the same temperature. On adding the same quantity of 
 solute once more, the fall in level in the gauge tube will 
 be, after equilibrium is re-established, approximately 
 twice as much as before. A suitable solvent for a lecture 
 demonstration is carbon tetrachloride, on account of its 
 low boiling point (76). its non-inflam- 
 mability, and its low surface tension (the 
 capillary ascension being negligible) . As 
 solute naphthaline or any other organic 
 compound, which dissolves readily in 
 this solvent may be used. 
 
 3, Determination of Molecular Weights. 
 33. The above described method of 
 heating the solution by means of the 
 vapor of the solvent has been used by 
 several investigators for the determin- 
 ation of molecular weights. In cases 
 where it is only necessary to decide what 
 multiple of the empirical formula re- 
 presents the molecular weight, the use of 
 a modified Landsberger apparatus as the 
 one devised by Eykman 1 (Fig. 19), or 
 a similar one, by McCoy, 2 allows a mol- 
 ecular weight determination to be carried 
 out with an accuracy of 5-10 per cent, in the course of a 
 few minutes. For a lecture demonstration a weighed 
 
 1 Journ. de Chimie Physique, 2, p. 47, (1903). 
 
 2 Am. Chem. Journ., 23, p. 353, (1900); obtainable from Eimer and Amend, 
 New York. 
 
 19. 
 
VAPOR PRESSURE AND MOLECULAR WEIGHTS 
 
 35 
 
 quantity of naphthalene preferably in the form of a tablet 
 (0.3 gram) is introduced into the inner tube of the 
 Landsberger apparatus containing 12-16 cc. of benzene. 
 The outer jacket is filled with about 50 cc. of the solvent. 
 The thermometer, on which the boiling point is read, need 
 not be a "Beckmann;" one graduated in tenths of a de- 
 gree is quite sufficient for this purpose. As a matter 
 of course, the reading of the boiling point and the volume 
 of the solution can only be made by the lecturer or his 
 assistant. 
 
 34. The vapor density method, due to Victor Meyer, 
 is applied for substances which can be readily evapor- 
 ated. A description can be found in any textbook on 
 organic chemistry. Care has to be taken, that all con- 
 nections are gas-tight (thickwalled India rubber tubing 
 should be employed) and that the water in the graduated 
 glass tube is saturated with air. Using 
 aniline (boiling point 184) in the outer 
 jacket, and xylene (ortho, meta or para, 
 boiling point 140) in the glass-stop- 
 pered weighing bottle, about 20 cc. of air 
 will be collected from a weight of about 
 o.i gram of liquid. 
 
 Instead of glass-stoppered weighing 
 bottles it is more convenient to use small 
 glass bulbs with a sealed capillary stem, 
 bent at the end, so that by a short pull 
 of the copper wire from which the bulb 
 is suspended the stem breaks off and the bulb falls in 
 the cylinder below. The arrangement is readily under- 
 
 Fig. 20. 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 stood from Fig. 20. A side tube for the passage of a 
 glass rod as the Meyer-apparatus usually contains, is 
 then unnecessary. 
 
 35. The cryoscopic method is conveniently carried out 
 with the Eykman depressimeter (Fig. 21). This simple ap- 
 paratus 1 consists of a short thermom- 
 eter, divided into twentieths of a de- 
 gree and fitted at the end in the neck 
 of a small flask (contents about 20 
 cc.), fastened inside a glass cylinder 
 by means of a cork stopper at the top 
 and a plug of cotton at the bottom. 
 The thermometer, a modified "Beck- 
 mann" has to be "set" before use. A 
 weighed quantity of the solvent, say 
 water (10 grams) is poured in the 
 flask and the freezing point is read. 
 A quantity of solute of known weight 
 is then added and the freezing point 
 again determined. Taking for ex- 
 ample 0.2 gram of sodium chloride, 
 the depression will be found to be 
 about 1.30, instead of a calculated 
 value (for undissociated molecules) 
 of 0.65. 
 
 36. Whenever it is desired to make the readings vis- 
 ible to the auditory, the use of a large air thermom- 
 eter is unavoidable. Ciamician has proposed the follow- 
 ing arrangement. 2 A cylindrical glass reservoir (Fig. 22) 
 
 * Zeitschr. f. phys. Chem., 2, p. 964, (1888). 
 2 Ber. d. chem. Ges., 22, p. 31, (1889). 
 
 Fig. 21. 
 
VAPOR PRESSURE AND MOLECULAR WEIGHTS 
 
 37 
 
 is sealed to a glass tube (inner cross section 1.5 milli- 
 meters), twice bent at right angles and provided with two 
 bulbs at a distance of 50-60 
 centimeters. The lower part 
 of the glass tube is inserted in 
 an alcoholic solution of fuch- 
 sine, serving as a confining 
 liquid. Around the glass cyl- 
 inder a copper stirrer moves 
 in a large test-tube (20 by 3 
 centimeters). This test-tube 
 is filled successively with 
 equimolecular solutions o f 
 cane sugar (34.2 grams in 100 
 cc. of water), mannite (10.2 
 grams in 100 cc.) and acetic 
 acid (16 grams in 100 cc.) 
 and after inserting the tube 
 each time in a freezing mix- 
 ture of salt and ice the freez- 
 ing point is determined. In 
 all three experiments the con- 
 fining liquid rises to about the same height. 
 
CHAPTER V. 
 
 CHEMICAL EQUILIBRIUM AND THE LAW OF 
 MASS ACTION. 
 
 Since the publication of Van 't Hoff's epochmaking 
 "Etudes de dynamique chimique" (1884) chemical equi- 
 librium and the law of mass action have become the 
 nucleus of modern physical chemistry. The frequent 
 and manifold applications of these fundamental prin- 
 ciples, in analytical chemistry and elsewhere make it de- 
 sirable, to take up the discussion of this subject, directly 
 after the properties of gases, liquids and solids have been 
 expounded. It goes without saying, that lecture experi- 
 ments in this field especially, can only illustrate the gen- 
 eral laws in a qualitative way; nevertheless they may 
 be considered of great use, in clearly demonstrating the 
 effect of concentration, temperature and pressure on the 
 course of chemical reactions. Taking up first of all, the 
 question of reversibility, a few typical reversible reac- 
 tions are mentioned, then the concentration of the react- 
 ing substances is considered more in detail, followed by 
 a demonstration of the change of equilibrium by varying 
 temperature or pressure. In connection herewith, the 
 effect of the factors, which influence the velocity of 
 chemical reactions is illustrated. Finally some space is 
 devoted to the rule of successive reactions. 
 
 Thus the chapter may be subdivided under the fol- 
 lowing headings : 
 
 I. Reversible reactions. 
 II. The law of mass action. 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 39 
 
 III. Displacement of equilibrium. 
 
 IV. Time reactions. 
 
 V. Velocity of chemical reactions. 
 VI. The rule of successive reactions. 
 
 I. Reversible Reactions. 
 
 In spite of the prevailing tendency in analytical chem- 
 istry, of carrying out reactions along "irreversible" lines, 
 it has been recognized in the past decades, that in prin- 
 ciple every reaction is reversible, and that it is only a 
 question of choosing the proper conditions in order to 
 revert the course of a reaction. Out of the great many 
 cases at our disposal, the following examples, easily per- 
 formed, may be quoted : 
 
 37. BiCl s -f H 2 O ;F BiOCl + 2HC1. 
 
 To a small quantity of bismuth trichloride in a 
 conical lecture jar a few cubic centimeters of 5N hydro- 
 chloric acid are added, until a clear solution is obtained. 
 When water is added, hydrolysis occurs and a white pre- 
 cipitate is formed, which redissolves on the addition of 
 concentrated hydrochloric acid. 
 
 38. Sb 2 S s -f 6HC1 ^ 2SbCl, + 3H 2 S. 
 
 Concentrated hydrochloric acid in a separatory fun- 
 nel is allowed to drop slowly on red antimony sul- 
 phide in a fractionating flask, and the escaping gas passed 
 into a solution of antimony chloride, forming a precipi- 
 tate of red antimony trisulphide. 
 
 4 
 
40 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 39. H 2 S displaces CO 2 from its salts. 
 
 That hydrogen sulphide can displace carbon diox- 
 ide from its salts, just as well as the latter can liberate 
 hydrogen sulphide from its salts, was shown by Emil 
 Fischer 1 as follows : A strong current of hydrogen 
 sulphide is passed into a solution of sodium bicarbonate 
 thereby liberating carbon dioxide. The gas mixture, bub- 
 bling through a barium hydroxide solution precipitates 
 barium carbonate. The reverse takes place by passing 
 carbon dioxide into a solution of sodium hydrosulphide ; 
 the hydrogen sulphide evolved, precipitates lead sulphide 
 from a solution of lead acetate. 
 
 40. CH 8 COOH displaces CO, from its salts. 
 
 In the same way acetic acid (dilute) displaces carbon 
 dioxide from a solution of potassium carbonate, while on 
 the other hand carbon dioxide passed into a solution of 
 potassium acetate in absolute alcohol, produces a precip- 
 itate of potassium carbonate, the acetic acid, set free, re- 
 maining in solution. 2 
 
 41. 2 H 2 O ^ 2H 2 + O 2 . 
 
 The decomposition of water and its re-formation 
 from the resulting 2:1 mixture of hydrogen and oxygen 
 can easily be shown, 3 by fastening a coil of platinum 
 wire (0.6 millimeter in diameter) to stout copper wires, 
 
 1 Heumann-Kuhling, Anleitung zum Experimentieren, 3e Aufl. p. 95, 
 (1904). 
 
 2 I^e Cha teller, logons sur le carbone, p. 210, (1908). 
 
 * Hofmann, Ber. d. chem. Ges., 23, p. 3303, (1890), also I^ash Miller and 
 Kenrick, Journ. Am. Chem. Soc., 22, p. 296, (1900). 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 4! 
 
 passing air-tight through small thick-walled glass-tubes 
 (Fig. 23) held by a cork, which closes the neck of a frac- 
 tionating flask. Water is boiled in the flask, until the 
 vapor escapes free from air; the platinum wire is cau- 
 tiously heated electrically to white heat, by connecting 
 the copper wires with a strong current cable provided 
 
 Fig. 23. 
 
 with ampere meter and a rheostat (for currents up to 
 20 amperes). The arrangement resembles Deville's 
 cold-hot tube in bringing about a dissociation of the 
 water vapor. The gas mixture, formed in the reaction 
 is collected over water in a eudiometer and then, after 
 passing the spark recombined to water. 
 
42 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 II. The Law of Mass Action. 
 
 As illustrations of the law of mass action several in- 
 structive lecture experiments have been devised. Those 
 given below have been found to be most convenient. 
 
 42. FeCl 3 + 3 NH 4 CNS ^ Fe(CNS) 3 + 3NH 4 C1. 
 
 This reaction, which was first systematically in- 
 vestigated by Gladstone, 1 may be carried out, following 
 the directions of L,ash Miller and Kenrick 2 as follows : 
 
 Approximately equivalent solutions of ferric chloride 
 and ammonium thiocyanate are prepared, the first, con- 
 taining 6 grams of commercial ferric chloride, 25 cc. of 
 concentrated hydrochloric acid (specific gravity, 1.175) 
 and water to make up 200 cc. ; the second solution con- 
 tains 7.5 grams of ammonium thiocyanate dissolved in 
 200 cc. of water. Five cc. of each solution are mixed and 
 2 liters of (tap) water added. The orange-colored mix- 
 ture is equally divided between four beakers of 600 cc. 
 each. From the color of the four solutions it is evident 
 that the equilibrium is considerably displaced to the left, 
 ferric thiocyanate [Fe(CNS) 3 ] being dark red in so- 
 lution; ferric chloride is more or less yellow, while am- 
 monium salts are colorless. Therefore, the amount of 
 ferric thiocyanate, present in solution, can be fairly well 
 judged from the depth of color of the solution. 
 
 On adding to the first beaker 5 cc. ammonium thiocy- 
 anate solution and to the second 5 cc. ferric chloride so- 
 lution the color becomes in both cases dark red, showing 
 an equilibrium displacement from left to right (-*). 
 
 1 Phil, Trans. Royal Soc., p. 179, (1855). 
 
 2 Journ. Am. Chem. Soc., 22, p. 292, (1900). 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 43 
 
 An addition of 50 cc. of a saturated solution of ammon- 
 ium chloride to the third beaker makes this solution al- 
 most colorless, thereby showing, in accordance with the 
 law of mass action, that the equilibrium is now displaced 
 from right to left ( *). The fourth beaker is kept for 
 comparison. 
 
 43. Ag- + Fe" Ag + Fe"'. 
 
 This reaction was recently studied in detail by A. 
 A. Noyes and Brann 1 and its equilibrium conditions (at 
 25) determined carefully. As a lecture experiment 
 this reaction may be performed in the following 
 manner :- 
 
 Pure powdered ferrous sulphate (FeSO 4 /H 2 O) is dis- 
 solved in cold water (previously boiled, to drive out the 
 air), to which a few drops of sulphuric acid have been 
 added. The liquid is then quickly filtered and kept in an 
 Erlenmeyer flask with a coil of thin, rust-free iron wire. 
 
 A second solution is made by adding a solution of 
 sulphuric acid (specific gravity 1.25) to a nearly satu- 
 rated solution of ferric ammonium alum [Fe(NH 4 ) 
 (SO 4 ) 2 i2H 2 O] until the solution is almost colorless, or 
 slightly yellow. The addition of sulphuric acid serves to 
 hinder hydrolysis. 3 Five to ten cc. of a dilute silver ni- 
 trate solution are then poured into a conical lecture jar 
 and mixed with enough ferrous sulphate solution to form 
 a precipitate of silver. The latter is redissolved after the 
 subsequent addition of enough ferric alum solution 
 
 1 Ibidem, 34, p. 1016, (1912). 
 
 2 I^uther, Die chemischen Vorgange in der Photographie, Halle p. 35 (1899). 
 8 Ostwald, Grundliuien der anorg. Chem., 2 Aufl., p. 594, (1904). 
 
44 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 (about four to six times the quantity of the ferrous sul- 
 phate solution, required for precipitating the silver). 
 
 In connection with this experiment it is interesting to 
 point out the mechanical conceptions by which Luther 1 
 and later on Van 't HofP and Baur 2 have tried to illus- 
 trate the chemical equilibrium in this reversible reaction. 
 The former imagines a balance beam (Fig. 24) kept in 
 its position by wire coils Fe" and Ag* combining their 
 efforts at opposite ends in one sense, while Ag and Fe'" 
 
 re" ton 
 
 coils act in the reverse. More appealing to the mind is 
 the idea of Van 't Hoff and Baur, as represented in Fig. 
 25. By plotting the percentage composition as abscissa 
 against the energy of the system as ordinate, we see that 
 like a rolling ball, reaching from whatever side of a 
 curved line it comes down, the lowest level, the energy 
 of the system reaches a minimum value. Similar views 
 have been expressed by L,eveing 4 in 1885. 
 
 1 I^uther, 1. c. p. 35. 
 
 2 Baur, Themen der physik. Chemie, lyeipzig, p. 6, (1910). 
 
 Van 't Hoff, Physical Chemistry in the Service of the Sciences, p. 88, 
 Chicago, (1903). 
 
 * I^eveing, Chemical Equilibrium, Cambridge, p. 35, (1885). 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 45 
 
 /oo re 
 ore" 
 
 Percentage composition 
 
 Fig. 25. 
 
 44. HgCl + OH' ^ HgOH + Cl'. 
 
 Von Dieterich and Wohler 1 propose the reversible 
 reaction, expressed in the above equation as another suit- 
 able illustration of mass action. Phenolphthalein being 
 used as indicator, a N/ioo solution of potassium hy- 
 droxide shaken with calomel remains red; this means an 
 incomplete consumption of hydroxyl ions ; if instead of a 
 N/ioo solution a N/iooo solution is used, the color of 
 the solution changes from red to grey on shaking with 
 calomel ; an addition of a few drops of potassium chlor- 
 ide restores the original red color. 
 
 1 Zeitschr. f. anorg. Chem., 34, p. 194, (1901). 
 
46 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 in. Displacement of Equilibrium. 
 
 45. The change of equilibrium by lowering or raising 
 the temperature, can be easily shown, in dealing with gas- 
 eous mixtures, e. g., nitrogen tetroxide, partly dissociated 
 in nitrogen dioxide : 
 
 N 2 O 4 ^ 2N0 2 . 
 
 The thermal equilibrium displacement can be made vis- 
 ible in this case first by the accompanying change in 
 color, and secondly by the abnormal change in pressure 
 at constant volume. 
 
 Taking first two glass tubes (size 10 by 1.5 inches) 
 filled with the carefully dried gas mixture (prepared by 
 heating lead nitrate) and closed at both ends by round- 
 ing off the ends in the blast flame, one is lowered into a 
 cooling mixture of alcohol and carbon dioxide, while 
 the other tube is carefully heated with proper precau- 
 tions, by moving the flame of a Bunsen burner along 
 the tube. The result is then shown by placing both tubes 
 simultaneously against a white background. 
 
 46. For the second experiment, two round-bottom 
 flasks of exactly the same size (contents 600-800 cc.) 
 with air-tight fitting ground glass stoppers, to which U- 
 shaped open manometer tubes (inner bore 2 millimeters) 
 have been sealed, are filled, i or 2 hours in advance, 
 with carbon dioxide and nitrogen tetroxide respectively. 
 Care has to be taken that the stoppers, carrying the man- 
 ometer tubes, are well attached to the necks of the flasks, 
 so as to hold an excess pressure (Fig. 26). At the out- 
 set both manometers show the same pressure, the gases 
 being under atmospheric pressure. On inserting the 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 
 
 47 
 
 flasks held by clamps, to the same depth in a large water 
 bath of 50, the manometer of the nitrogen tetroxide 
 flasks held by clamps, to the same 
 depth in a large water bath of 50, 
 the manometer of the nitrogen tetra- 
 oxide flask indicates a much higher 
 pressure than the second manometer, 
 due to an equilibrium displacement 
 from left to right (). When 
 taken out of the bath, the difference in 
 pressure gradually decreases, until fi- 
 nally the initial state is reached. 
 
 47. The effect of heating on gaseous 
 dissociation can also be demonstrated 
 in the reaction : 
 
 PBr s -f Br 2 ^ PBr 5 . 
 
 Here, too, the progress of dissocia- 
 tion is directly visible by a more in- 
 tense color, since on raising the tem- 
 perature, the equilibrium is displaced 
 from right to left ( - ) . By using 
 equimolecular quantities in one tube Fi - 26 - 
 
 and an excess of phosphorus tribromide in another tube, 
 the same experiment also illustrates the mass action law 
 in a very satisfactory way. Following the directions, 
 given by Stieglitz, 1 two small sealed bulbs, blown at the 
 end of glass capillaries, containing 0.029 gram bromine 
 and 0.058 gram phosphorus tribromide (a little more 
 than i molecule) respectively, are placed in a piece of 
 
 1 Am. Chem. Journal, 23, p. 404, (1901). 
 
48 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 thick-walled glass tubing, closed at one end and drawn 
 out at the other end into a capillary. The length of the 
 tube is about 10 centimeters and its capacity 40 cc. The 
 air is exhausted to 20-30 millimeters mercury pressure, 
 the capillary end sealed off and bent into a loop. By 
 vigorous shaking the bulbs are broken. A second tube 
 of the same size is filled in the same way with a mixture 
 of 0.029 gram bromine (i molecule) and 0.45 gram tri- 
 bromide (9 molecules). 
 
 The tubes are suspended by means of the glass loops 
 at the upper ends in a tall beaker of water and a glazed 
 white porcelain tile or a piece of white cardboard placed 
 behind the beaker, to make comparison of colors easier. 
 On heating the first tube is slightly colored at 50, the 
 second not at all. At about 85 the most favorable stage 
 for comparison, the first tube appears reddish brown and 
 opaque, while the latter is reddish yellow, through which 
 the white of the tile or cardboard can still be seen. A 
 similar experiment with greater differences in color may 
 be carried out, using phosphorus trichlordibromide with 
 and without an excess of the trichloride. 
 
 IV. Time Reactions. 
 
 48. The fact, that certain reactions require a percep- 
 tible time, until separation of one of the reaction prod- 
 ucts starts, is illustrated by an experiment, performed by 
 Landolt, 1 demonstrating the reduction of iodic acid by 
 sulphurous acid, according to the equation : 
 
 5H,SO, + 2HI0 3 = 5H,S0 4 + H 2 O + I,. 
 
 1 Ber. d. chem. Ges., 19, p. 1317, (1886) ; 20, p. 745, (1887). 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 49 
 
 Two solutions are made up, one of 1.8 grams iodic 
 acid in i liter water and the other composed of 0.9 
 gram sodium sulphite (Na 2 SO 3 7H 2 O), 5 grams dilute 
 sulphuric acid (1:10) and 9.5 grams soluble starch 
 (rubbed to a thin paste by adding a little water) in I 
 liter water. 1 These stock solutions serve to make up 
 solutions of one-half and one-quarter of the two original 
 concentrations. On mixing rapidly 100 cc. of the orig- 
 inal and of the more dilute solutions, different times 
 will be found before noticeable separation of iodine sets 
 in. Sodium sulphite, being easily decomposed in contact 
 with air, it will be found that on repeating the experi- 
 ment, for the same concentration, not necessarily the 
 same time as before is registered. 
 
 49. More reproducible values can be obtained in per- 
 forming another time reaction, also studied in detail by 
 L,andolt: 2 the decomposition of thiosulphates by acids: 
 
 H- + S 2 O 8 " = HSO 8 ' + S. 
 The experiment may be carried out as follows: 
 To three beakers (of 200 cc. each) containing respec- 
 tively o.i, 0.2 and 0.3 gram sodium thiosulphate 
 (Na a S 2 O 3 5H 2 O) 100 cc. distilled water is added and 
 after complete solution of the salt, in each beaker is 
 poured, at the same time, a solution of i cc. concentrated 
 hydrochloric acid in 20 cc. distilled water (ready at hand 
 in three test-tubes). After 14, 6^ and 4 minutes respec- 
 tively a milky suspension of finely divided sulphur be- 
 comes visible. In a fourth beaker, containing 0.2 gram 
 
 1 H. u. W. Biltz, I.e. p. in. 
 
 2 Ber. d. chem. Ges., 16, p. 2958, (1883). 
 
5<3 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 thiosulphate dissolved in 100 cc. distilled water and kept 
 at 50 is poured, simultaneously i cc. concentrated hy- 
 drochloric acid ; the result, in this case is a visible sulphur 
 separation after i% minutes. 
 
 It is interesting to notice, that the different investi- 
 gators, 1 who studied this reaction carefully, all agree in 
 admitting an immediate formation of sulphur on mixing 
 the salt and acid solutions. The sulphur is supposed 
 to remain in solution, until a definite concentration is 
 reached or a certain change has set in, which causes the 
 appearance of visible sulphur drops. 
 
 V. Velocity of Chemical Reactions. 
 
 50. That the rate, at which a chemical reaction takes 
 place, is proportional to the concentration of the react- 
 ing substances, is shown by the following experiment of 
 A. A. Noyes and Blanchard, 2 referring to the reaction, 
 expressed by the equation : 
 
 HBr0 3 + 6HI = 3 H 2 O + 3 I 2 + HBr. 
 
 Four 500 cc. glass-stoppered bottles, 8 centimeters in 
 diameter, are filled with 400 cc. dilute hydrochloric acid, 
 made up by mixing 1600 cc. distilled water and 50 cc. 
 N/2 hydrochloric acid, to which is added 40 cc. of a 
 starch solution (obtained by rubbing i gram potato 
 starch with 5 cc. cold water and pouring 150 cc. boiling 
 water over it). 
 
 From four 10 cc. graduates, in front of these bottles, 
 
 1 Holleman, Rec. d. Trav. Chim. des Pays-Bas 14, p. 71, (1895). 
 v. Oettingen, Zeitschr. f. phys. Chem., 33, p. i, (1900). 
 Ostwald, Grundlinien der anorg. Cheinie, 3 Aufl., p. 337, (1912). 
 
 2 Journ. Am. Chem. Soc., 22, p. 739, (1900). 
 
CHEMICAL EQUILIBRIUM AND MASS ACTION 51 
 
 are added respectively 5, 10, 5 and 10, cc. of a N/2 solu- 
 tion of potassium bromate, and then, simultaneously 
 as far as possible, from another set of four 10 cc. grad- 
 uates, 5, 5, 10 and 10 cc. respectively of a N/2 solu- 
 tion of potassium iodide. The glass stoppers are quickly 
 inserted and the bottles vigorously shaken. The first 
 mixture will become of the same shade of blue as a 
 standard starch-iodine solution* in about 120 seconds, 
 the second and the third will both require half that time, 
 (about 60 seconds) while the fourth takes on the color 
 of the standard solution after the lapse of only 30 
 seconds. 
 
 51. After Nernst and Handa 1 the velocity of chemical 
 reaction can clearly be demonstrated by saponifying 
 methyl formate, the progress of the decomposition of the 
 ester being shown by the change in color of different 
 indicators. For this purpose a set of five 100 cc. flasks 
 are filled with 50 cc. (previously boiled) water, brought 
 to the titer of N/iooo with barium hydroxide. The fol- 
 lowing indicators, a few drops in each case, are added : 
 phenolphthalein, litmus, ganin, p-nitrophenol, and 
 methyl orange, respectively. From five small test-tubes, 
 all attached to the same strip of wood, in order to in- 
 sure a simultaneous action, and each containing I cc. of 
 methyl formate, the ester is poured into the five flasks. 
 Besides these, another set of five flasks without addition 
 of ester and a third similar set of five flasks, to which 
 
 * The standard solution kept in a fifth 500 cc. glass-stoppered bottle is 
 made by adding to 400 cc. water 10 cc. of the starch solution and i cc. of a solu- 
 tion of i gram iodine and 2 grams potassium iodide in 500 cc. water. 
 
 1 Ber. d. chem. Ges., 42, p. 3178, (1909). 
 
52 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 some acid has been added, are kept for comparison, both 
 sets containing the indicators in the above-mentioned 
 order. The times required for changing the color of the 
 indicators will be about o, I, 15, 30 and 120 minutes re- 
 spectively, and give an idea of the sensibility of the in- 
 dicators towards OH'-ions. 
 
 52. The influence of temperature on velocities of 
 chemical reactions is preponderant and may be shown 
 by cooling concentrated hydrochloric acid and a piece of 
 marble separately, in test-tubes to 80 in a mixture of 
 carbon dioxide and alcohol, and then bringing both to- 
 gether by dropping the marble on the acid. No percept- 
 ible gas evolution is seen. 
 
 53. Even comparatively small temperature differences 
 bring about considerable changes in velocities of reac- 
 tion. An instance was given above (page 49) in connec- 
 tion with the decomposition of sodium thiosulphate by 
 acids. A more detailed experiment, illustrating the gen- 
 eral rule, that equal increments of temperature cause an 
 equal multiplication of the velocity of any chemical re- 
 action (roughly speaking: every increase of temperature 
 by about 10 doubles the velocity of the reaction), was 
 given by Noyes and Blanchard. 1 
 
 The reaction, carried out, was the same as given on 
 page 50: 
 
 HBrO, 4- 6HI = 3H 2 O + 3!, + HBr. 
 
 A solution of 100 cc. N/2 hydrochloric acid and 30 cc. 
 starch solution in noo cc. water is made and 400 cc. of 
 
 1 Noyes and Blanchard, 1. c. p. 741. 
 
VAPOR PRESSURE; AND MOLECULAR WEIGHTS 53 
 
 this solution poured into each of three 500 cc. glass- 
 stoppered bottles and kept in three water baths at 
 4, 16, and 28* respectively. The temperature is con- 
 trolled by a large-size thermometer, inserted in succes- 
 sion in each of the bottles. Another solution is made up 
 by mixing 10 cc. N/2 potassium bromate, 10 cc. N/2 
 potassium iodide and 25 cc. water. Ten cubic centimeters 
 of this mixture is placed into each of three 10 cc. grad- 
 uates. In a fourth bottle is prepared a blue iodine starch 
 solution, in order to serve as a standard. At a definite 
 moment, when the clock (or stop watch) shows a full 
 minute, the three bromate solutions are poured in the 
 three bottles and then after being stoppered, vigorously 
 shaken. All four bottles are placed against a white back- 
 ground and the times noted, at which the three solutions 
 show in succession the standard blue color. If properly 
 carried out, it will be seen, that these times are approxi- 
 mately 32, 58 and 105 seconds. Thus a rise in tempera- 
 ture of 12 and 24 multiplies the velocity of the reac- 
 tion by 1.8 (58/32) and (i.8) 2 . 
 
 54. The influence of pressure on the velocity of chem- 
 ical reactions is usually of no great importance. One 
 well-known instance, however, where pressure has a 
 marked effect, may be quoted, an exception also to the ad- 
 age of the ancients, that substances do not react, except 
 when dissolved, viz., the formation of mercuric iodide : 
 
 HgCl 2 -f- 2KI = Hgl, + 2KC1. 
 
 * The temperature of the third solution must not be above 30, because the 
 depth of color of the blue solution is lessened on raising the temperature, 
 owever, not perceptibly below 30. 
 
54 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 A mixture of powdered potassium iodide and corrosive 
 sublimate, in equivalent quantities, shaken in a wide- 
 mouth bottle, is only slightly colored yellow, due to the 
 slow formation of mercuric iodide, but on rubbing the 
 mixture with a pestle in a glass mortar, the color changes 
 to red, owing to a more rapid formation of the red iodide 
 of mercury. 
 
 VI. The Rule of Successive Reactions. 
 
 That many reactions take place with the temporary 
 formation of less stable intermediate compounds, was first 
 observed by Gay-Lussac and as a result of many observa- 
 tions the "law" of successive reactions was introduced 
 by Ostwald 1 and simultaneously by Bancroft. The valid- 
 ity of this "law" taken in the categorical sense in which 
 it was pronounced by these two investigators has been 
 questioned by Nernst, Bakhuis Roozeboom, Mellor and 
 others. More acceptable is the formulation of this prin- 
 ciple (avoiding the much abused term "law"), proposed 
 by Alexander Smith, 2 stating, that "transformations, 
 which proceed spontaneously and with evolution of heat, 
 may go forward by steps, when there are intermediate 
 substances or allotropic forms capable of existence." 
 
 55. The following well-known example is easily car- 
 ried out. On adding stannous chloride to a solution of 
 mercuric chloride first a white precipitate of calomel is 
 observed, which changes after a while (and rapidly on 
 heating) into metallic mercury. 
 
 1 Zeitschr. f. phys. Chem., 22, p. 306 (1897). 
 
 2 1. c. p. 545. 
 
CHAPTER VI. 
 
 CATALYSIS. 
 
 The term catalysis was introduced in chemistry early in 
 the 1 9th century by Berzelius. The importance of catalytic 
 processes, not only in the laboratory but also for indus- 
 trial purposes has since then been generally recognized. 
 A recent author speaks of catalysis as a chemical "short- 
 circuit." 1 Ostwald has defined this phenomenon as a 
 change (mostly increase) of velocity of chemical reaction, 
 by the addition of substances, which do not appear in the 
 final products of the reaction. This definition covers a 
 great many different types of reactions, which are all 
 labelled as "catalytic" and which may be distinguished, 
 following the classification of A. A. Noyes and Sam- 
 met 2 in seven types : 
 
 1. Reactions, catalyzed by carriers ("translation" 
 
 agents), 
 
 2. Reactions, catalyzed by absorbent contact agents, 
 
 3. Reactions, catalyzed by electrolytic contact agents, 
 
 4. Reactions, catalyzed by water, 
 
 5. Reactions, catalyzed by dissolved electrolytes, 
 
 6. Reactions, catalyzed by enzymes, 
 
 7. Reactions, catalyzed by inorganic colloids: 
 
 to which might be added three other types : 
 
 8. Autocatalytic reactions, and closely related with 
 
 this type, 
 
 9. Reactions, with intermediate formation of catalytic 
 
 agents, and 
 
 10. Reactions, catalyzed by "germs." 
 
 1 Baur, 1. c. p. 62. 
 
 2 Journ. Am. Chem. Soc., 24, p. 498, (1902). 
 
 5 
 
56 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 The experiments described in this chapter, are ar- 
 ranged according to these ten different types. Particulars 
 concerning most experiments described below, were 
 taken from the interesting article by Noyes and Sammet, 
 which gives complete details for making the perform- 
 ance as easy as possible. 
 
 TYPE i. 
 
 56. Reaction catalyzed : 
 
 C 6 H 6 -f Br 2 = C 6 H 5 Br + HBr. 
 
 Catalyzer: ferric bromide (FeBr 3 ). 
 
 A 250 cc. distilling flask 1 is supported upon a ring 
 stand and its side arm connected with the stem of a fun- 
 nel, the mouth of which dips just below the surface of a 
 potassium hydroxide solution. In the flask is brought 
 4 cc. of bromine and then 30 cc. of benzene are poured 
 through a long-necked funnel, nearly reaching the bot- 
 tom of the flask. No reaction occurs. On adding 0.5 cc. 
 of powdered iron and after blowing some gas into the 
 neck of the flask from a small wash bottle, containing 
 strong ammonia, a tight-fitting cork stopper being finally 
 inserted, great clouds of white fumes are seen, escaping 
 through the side arm, and being absorbed in the caustic 
 potash solution. 
 
 57. Reaction catalyzed : 
 
 3 C 6 H 6 + CHC1 3 = CH(C 6 H 5 ) 3 + 3 HC1. 
 
 Catalyzer: Aluminum Chloride (A1C1 3 ). 
 
 This is the type of reaction, known in organic 
 chemistry as the synthesis of Friedel and Crafts. The 
 experiment is carried out by pouring in a test-tube 5-10 
 
 1 Noyes and Sammet, 1. c. p. 501. 
 
CATALYSIS 57 
 
 cc. of benzene, to which a few drops of chloroform are 
 added. No reaction takes place, not even on gently 
 heating over a small flame. As soon as a small quantity 
 of anhydrous aluminium chloride, contained in a small 
 tightly stoppered tube is added, a copious evolution of 
 dense white fumes becomes visible and at the same time 
 the contents of the test-tube turns dark brown, due to the 
 formation of triphenylmethane. The reaction has to be 
 carried out under a glass hood. Its success depends en- 
 tirely on the quality of the aluminium chloride used for 
 the experiment. Good results are obtained by using the 
 granulated, absolutely dry product, manufactured by 
 Kahlbaum, of which small samples are to be kept in 
 well-corked small tubes, ready for use. Noyes and 
 Sammet (1. c.) recommend a similar reaction, viz., the 
 formation of acetophenone from benzene and acetyl 
 chloride, requiring a more complicated apparatus. The 
 arrangement chosen above is just as efficient and far 
 more simple, since only a test-tube is needed. Other re- 
 actions of this type may be looked up in the original 
 paper. 1 
 
 TYPE: 2. 
 
 58. Out of numerous examples, belonging to this type, 
 sc. catalysis by absorbent contact agents, may be chosen 
 the reaction : 
 
 2 H 2 + 2 = 2H 2 0. 
 
 Catalyzer : Finely divided Platinum. 
 
 A mixture of hydrogen and oxygen, in the propor- 
 tion to form water (about 10-15 cc.) is collected over 
 mercury in a eudiometer tube. By introducing a few 
 
 1 Noyes and Sammet, 1. c. p. 499.502. 
 
58 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lumps of platinized, granulated pumice stone (prepared 
 by soaking the pumice in a 10 per cent, solution of chloro- 
 platinic acid and prolonged heating in a Bunsen flame, 
 until the platinum is left in a finely-divided state) to the 
 mixture, combination takes place, demonstrated by a de- 
 crease in volume and the formation of a water nebula. 
 
 59. Another instance of this type is the decomposition 
 of hydrogen peroxide by the catalytic action of platinum 
 black and bone black. 1 
 
 2 H 2 O 2 = 2H 3 O + O 2 . 
 
 In each of two lecture test-tubes is placed a solution 
 of commercial, concentrated hydrogen peroxide (about 25 
 cc.), which has been made slightly alkaline by the addi- 
 tion of ammonia. To the first tube is added i cc. of 
 bone black, to the second a small portion of platinum 
 black. In both tubes a violent development of oxygen 
 gas takes place and a glowing wood splinter inserted in 
 each tube rekindles. The required platinum black is pre- 
 pared by soaking two 9 centimeter filter papers in a 10 
 per cent, solution of chloroplatinic acid and igniting them 
 in a large porcelain crucible, until the carbon is burned 
 off. 
 
 TYPE; 3. 
 
 60. Most electrolytic catalyzers accelerate reactions 
 through the formation of a voltaic couple. A well- 
 known case is the increase of reaction velocity by addi- 
 tion of one drop of a solution of chloroplatinic acid to a 
 pure dilute solution of sulphuric acid, in which a sheet of 
 pure zinc is inserted. 
 
 1 Noyes and Sammet, 1. c. p. 504. 
 
CATALYSIS 59 
 
 61. A similar reaction 1 is the following, also catalyzed 
 by platinum: 
 
 Sn + 2HC1 = SnCl 2 + H s . 
 
 In a 300 cc. Erlenmeyer flask, provided with a two- 
 hole rubber stopper, through which passes a thistle tube 
 and a delivery tube (ending in a beaker of water) is 
 placed a layer of pure feathered tin, covering the bottom 
 of the flask to a depth of 2 centimeters. On pouring 
 through the thistle tube enough hydrochloric acid (specific 
 gravity 1.12) to entirely cover the tin, only a slight action 
 occurs. As soon as a little chloroplatinic acid (or copper 
 sulphate solution) from a medicine dropper is added, a 
 rapid gas evolution occurs. 
 
 TYPE 4. 
 
 62. Catalysis by water is shown in the reaction : 
 
 Zn -f I, = Znl,. 
 
 Four cubic centimeters of powdered iodine are placed 
 into a test-tube and 2, cc. of zinc dust in a 25 cc. wide- 
 mouthed glass-stoppered bottle. The iodine is poured 
 into the bottle and the mixture vigorously shaken. 
 Nothing visible happens. The mixture is then brought 
 into a 4 liter glass balloon, supported on a suitable ring 
 and a fine stream of water from a wash bottle directed 
 on the dry powder. A violent action, attended by a 
 whizzing noise and a copious evolution of iodine vapors 
 takes place. 2 
 
 TYPE 5. 
 
 Hydrolysis, accelerated by an acid, in virtue of its 
 hydrogen ions, is the most important example of re- 
 
 1 Noyes and Sammet, 1. c. p. 505. 
 
 2 Noyes and Sammet, 1. c. p. 508. 
 
60 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 actions of this type. Noyes and Sammet suggest that 
 ions are mostly hydrated (already verified by recent in- 
 vestigations) and that water carried as hydrate is more 
 active than ordinary water. If this suggestion is ac- 
 cepted the fifth type is reduced to the first type, mentioned 
 above, viz., that of carriers, the hydrogen ions acting as 
 water-carriers. 
 
 63. The hydrolysis of starch, catalyzed by sulphuric 
 acid: 
 
 (C 6 H 10 5 )- + *H,0 = *C 6 H 12 6 , 
 is carried out as follows 1 : 
 
 In each of two large size test-tubes is brought an equal 
 weight (i gram) of starch. To one of the tubes is 
 added 25 cc. of water, to the other 25 cc. of a 5 per cent, 
 sulphuric acid solution. Both solutions are boiled for 
 about half a minute. The acid solution is neutralized 
 with 9 cc. of a 50 per cent, caustic potash solution. The 
 contents of both tubes are then boiled and after adding 
 5 cc. of Fehling's solution to each tube, both are boiled 
 again. It will be seen that only in the tube, to which 
 acid had been added, a red precipitate of cuprous oxide 
 is formed. 
 
 TYPE 6. 
 
 64. An experiment, showing the catalysis by enzymes, 
 is the hydrolysis of starch solution by the ptyalin, present 
 in saliva : 
 
 (C 6 H 10 6 )* + *H 2 == *C 6 H ]2 6 . 
 
 In a test-tube is placed a small quantity of starch, 
 about the volume of a split pea. Ten cc. of water 
 is added and after heating to boiling 10 cc. more of 
 
 1 Noyes and Sammet, 1. c. p. 510. 
 
CATALYSIS 6l 
 
 cold water is added and then 2 or 3 drops of a i per cent, 
 iodine solution. The liquid turns deep blue, which color 
 disappears immediately on addition of 25 cc. of fresh 
 saliva. Even when a second portion of iodine solution is 
 added, the color is not restored. 1 
 
 TYPE 7. 
 
 65. Inorganic colloids possess strong catalytic power. 
 Colloid platinum for instance greatly accelerates the de- 
 composition of hydrogen peroxide. 
 
 2 H 2 O 2 == 2H 2 O -f O 2 . 
 
 A colloid platinum solution, prepared according to 
 Bredig's directions (Chapter IX, p. 117) is used. Placing 
 25 cc. of hydrogen peroxide (commercial, concentrated) 
 in each of two lecture test-tubes, made slightly alkaline 
 with ammonium hydroxide, 10 cc. of the colloid 
 platinum solution is added to each. To the first, how- 
 ever, has been added previously 5 drops of a saturated 
 potassium cyanide solution. The difference in gas evolu- 
 tion in both tubes is striking. A vigorous effervescence 
 starts in the solution, which contains no cyanide, while 
 in the other solution hardly any gas evolution occurs, 
 thus proving, that potassium cyanide acts as a poison in 
 retarding or entirely hindering the reaction. 
 
 TYPE; 8. 
 
 66. A case of autocatalysis is the action of nitric acid 
 on metals. 2 A sheet of pure copper or silver is 
 inserted in a lecture jar, filled with pure nitric acid 
 (about 20 per cent, solution). The reaction proceeds 
 
 1 Noyes and Sammet, 1. c. p. 512. 
 
 1 Ostwald, Grundrisz der allgem. Chemie, 4e Aufl., Leipzig, p. 336, (1909). 
 
62 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 very slowly until after a while a vigorous gas evolution 
 takes place. If on the other hand, instead of pure nitric 
 acid, fuming nitric acid, containing several oxides of ni- 
 trogen, is taken, an immediate solution of the metal will 
 be seen. The same happens on addition of a small 
 amount of sodium or potassium nitrite to the pure acid. 
 
 67. That the acceleration of the velocity of reaction 
 is wholly due to the formation of lower oxides of nitro- 
 gen during the reaction is clearly shown by an inter- 
 esting experiment of Quartaroli. 1 
 
 The reaction, studied by him is expressed by the 
 equation : 
 
 2 KN0 3 + 6HCOOH == N 2 O 3 + 
 
 4 C0 2 -f sH 2 O + 2HCOOK. 
 
 Taking 5 cc. of absolute formic acid, heated in a test- 
 tube at 40, to which is added 0.3 gram of potassium 
 nitrate, the reaction sets in slowly, but after 2 minutes 
 a violent gas evolution occurs, which is finished after 5 
 minutes. 
 
 The same experiment is performed simultaneously in 
 another test tube, to which I milligram of potassium 
 chlorate has been added. A slight retardation is per- 
 ceptible. 
 
 On adding 3 milligrams of the chlorate to a third tube, 
 containing as before 5 cc. of formic acid and 0.3 gram of 
 the nitrate, a visible gas evolution takes place after about 
 10 minutes. 
 
 To a fourth tube, is added 5 milligrams of chlorate; 
 
 1 Gazz. chim. ital. 41, (II), p. 64, (1911). 
 
CATALYSIS 03 
 
 no reaction at all, not even after half an hour. Thus a 
 small amount of a less active substance is able to hamper 
 or even to completely paralyze the catalytic action of a 
 more virulent agent. The potassium chlorate may be 
 called a negative catalyzer in regard to the nitrogen triox- 
 ide, the catalyzer formed in the course of the reaction. 
 
 Reactions of this "autocatalytic" type are called byBaur 1 
 "fever reactions/' owing to the strong resemblance which 
 
 Time 
 
 Fig. 27. 
 
 they show with the fever process in the human body. A 
 period of scarcely perceptible reaction (incubation stage) 
 is followed by one of ever increasing velocity (induction 
 stage) and finally by a decrease in activity, generally 
 more rapid than the foregoing increase (period of ex- 
 tinction), as is graphically represented in Fig. 27, where 
 
 1 Baur, 1. c. p, 66. 
 
64 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the velocity of the reaction is plotted against the time as 
 abscissa. 
 
 TYPE 9. 
 
 A similar behavior is shown by reactions, in which the 
 catalyzing agent acts temporarily, being formed and de- 
 stroyed again during the reaction. The same periods of 
 incubation, induction and extinction can be distinguished, 
 as, for instance, in the reduction of potassium permanga- 
 nate by oxalic acid. 1 
 
 68. The reaction takes place in three stages: 
 
 I Mn vn + V, CA" -> Mn" + 5CO 2 (incubation). 
 II Mn + 4 Mn 11 5 Mn in (induction). 
 
 Ill Mn 111 + V, CA" -~ Mn 11 + CO 2 . 
 
 The experiment is carried out in four 200 cc. beakers, 
 the first of which contains a solution of 10 cc. N/io potas- 
 sium permanganate in no cc. of water, serving as a stand- 
 ard. In the second is placed a mixture of 10 cc. N/io 
 potassium permanganate solution, 10 cc. saturated oxalic 
 acid solution and 10 cc. of water. The third beaker and 
 the fourth have the same contents as the second, with the 
 addition of one drop of manganous chloride (or sul- 
 phate) solution to the third and of an excess of the same 
 reagent to the fourth beaker. A temporary brown col- 
 oration becomes visible in all three solutions, but with 
 largely different velocity. 
 
 69. A reaction belonging to the same type is the cataly- 
 
 1 Baur, 1. c. p. 68. 
 
CATALYSIS 65 
 
 sis of hydrogen peroxide by chromic acid, studied 
 by Spitalsky. 1 
 
 The reaction is performed by carefully heating in a 
 test-tube at 4O-5OC. 20 cc. of a 20 per cent, solution of 
 hydrogen peroxide (Merck's solution), to which has 
 been added 5 cc. of a N/ioo solution of chromic acid. 
 The reaction starts slowly, the solution becoming blue ; 
 after about 10 minutes a violent reaction takes place, the 
 color of the solution changing into red-violet (induc- 
 tion period). Finally the reaction slackens and comes to 
 a stop, and the original color of the chromic acid is re- 
 stored. 
 
 TYPE 10. 
 
 70. The part played by so-called "germs" in catalysis 
 is illustrated by an experiment, due to Luther, 2 and de- 
 scribed by him as follows : 
 
 Some word is written, with an alum crystal, on a 
 clean glass plate. Invisible minute crystals remains, 
 where the crystal has been passed. On pouring a super- 
 saturated alum solution over the glass, crystallization 
 starts at the "germs" and the word becomes visible. 
 
 71. As a final experiment on catalysis a case may be 
 quoted, studied by Bredig and Wilke, 3 which shows the 
 periodic character of some catalytic reactions. In a 
 test-tube is brought a mixture of 3.3 cc. of hydrogen 
 peroxide (the authors use Merck's "perhydrol"), 6.7 
 
 1 Zeitschr. f. anorg. Chem. 56 p. 72, (1908). 
 
 2 1. c. p. 24, 
 
 Verh. des Naturhist, med. Vereins Heidelb, N. F. 8, p. 165, (1905). 
 
66 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 cc. of water and 33 cc. of concentrated sodium acetate so- 
 lution. A rather small drop of pure mercury is added, 
 and after a while a periodic gas evolution becomes 
 visible, due to the alternate formation and decomposition 
 of a bronze colored peroxide coat on the mercury drop. 
 
CHAPTER VII. 
 
 ELECTROCHEMISTRY AND IONIC THEORY. 
 
 Arrhenius' theory of electrolytic dissociation (1887) 
 has such an important bearing on the science of electro- 
 chemistry that a joint consideration of both is nowadays 
 a matter of course, the one being inseparably connected 
 with the other. Numerous instructive lecture demonstra- 
 tions illustrating the present conceptions on this subject 
 have been devised by various physico-chemists. In the 
 selection, chosen below, a review of the material at hand 
 is made under the following headings : 
 
 I. Electrolysis. 
 II. Migration of ions. 
 
 III. Electromotive chemistry. 
 
 IV. Conductivity and degree of ionization. 
 V. The common ion effect. 
 
 VI. Hydrolysis. 
 
 VII. Ionization and chemical activity. 
 VIII. Ionization and color of solutions. 
 
 I. Electrolysis. 
 
 72. Experiments on electrolysis of salt solutions and 
 fused salts are so well known, that a special description at 
 this place seems superfluous. Familiar demonstrations in 
 elementary chemistry courses are : the electrolysis of cop- 
 per sulphate solutions between platinum and between cop- 
 per electrodes, also of sulphuric acid, usually carried out 
 in a Hofmann apparatus, formerly frequently called "ap- 
 paratus of electrolysis of water," and of potassium sul- 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 phate solutions. The electrolysis of stannous chloride 
 and of lead acetate is interesting on account of the for- 
 mation of tin and lead "trees" and is for projection pur- 
 poses conveniently performed in small glass troughs with 
 parallel walls, using small metal rods, running through 
 a cork, as electrodes. 
 
 73. In order to obtain the easily decomposable alkali 
 
 metals in the form of amal- 
 gams Nernst 1 has devised the 
 following arrangement : 
 
 A large test-tube (12 by 1.5 
 centimeters) connected with a 
 capillary outlet tube (Fig. 28), 
 contains mercury, covered by 
 a layer, about 3 centimeters 
 thick, of chloroform and an- 
 other layer of concentrated 
 potassium chloride solution. 
 The tube is closed by a three- 
 hole cork stopper, allowing 
 the passage of (a) a glass 
 tube for the escape of gases 
 during the electrolysis, (b) a 
 strong platinum wire with 
 ' v spiral windings and (c) a 
 
 *' funnel of about 25 cc. capac- 
 
 Flg - 28> ity, drawn out into a capillary 
 
 tip, and filled with mercury. A platinum wire is inserted 
 in the mercury and both wire electrodes connected 
 
 1 Zeitschr. f. Electrochemie 3, p. 308, (1897). 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 with a battery of three lead accumulators. The mercury, 
 dropping into the solution forms potassium amalgam, pro- 
 tected from the decomposing action of water by the 
 chloroform layer and collects in the beaker placed under 
 the outlet. On pouring water, containing a few drops 
 of phenolphthalein into the beaker, the liquid instantly 
 turns red and evolution of hydrogen becomes visible. 
 
 n. Migration of Ions. 
 
 74. As introductory to lecture experiments, showing 
 migration of ions, the change in concentration near the 
 electrodes may be demonstrated. 
 
 A small glass trough (Fig. 29), with parallel walls, 
 as used for projection purposes, is 
 filled with a dilute copper sulphate 
 solution acidified with some sul- 
 phuric acid. Two L-shaped copper 
 wires are inserted and connected 
 with the poles of a storage cell. 
 After a while it will be seen on the 
 screen that the blue color at the 
 cathode brightens, whereas a more 
 concentrated solution collects near the anode. 1 
 
 29 
 
 75. A similar experiment was devised by Palmaer. 2 
 A U-shaped glass tube, about 70 centimeters high, with 
 an inner bore of 1.5 millimeters, filled with a 4N solu- 
 tion of hydrochloric acid is used. A silver wire is used 
 as anode, while a platinum wire serves as cathode; both 
 
 1 Coehn in Miiller-Pouillet's Handbook IV, p. 493, (1909). 
 
 2 Zeitschr. f. Electrochemie 12, p. 513, (1906). 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 are inserted as far as the middle of the limbs (Fig. 30). 
 The electrodes are connected 
 
 with a loo-volt circuit. The 
 current is about 0.02 ampere at 
 the start, but sinks in the course 
 of the electrolysis to about 0.015 
 ampere, the silver wire being 
 gradually covered with a thin 
 layer of silver chloride. The 
 difference in level amounts to 4 
 millimeters after 5 minutes, and 
 increases on further passing the 
 current through the solution. 
 
 A silver anode is employed in 
 order to avoid an increase of 
 specific gravity by dissolved 
 chlorine. The wire must have 
 been used several times since the 
 
 gas is not easily taken up by a polished, uncorroded 
 
 surface. 
 
 76. Lodge 1 first introduced the use of gelatin jellies 
 for the direct measurement of ionic velocities. These 
 jellies may be safely used instead of pure aqueous solu- 
 tions provided the percentage of gelatin does not exceed 
 4-5 per cent, since careful investigations have brought 
 out the fact that dissolved salts diffuse through gelatin 
 jellies at about the same rate as through pure water. The 
 method of Lodge is as follows : 
 
 A graduated glass tube, 40 centimeters long and 8 
 
 1 Brit. Ass. Report, p. 393, (1886) ; p. 389, (1887). 
 
 Fig. 30. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 millimeters wide, is twice bent at right angles and the end 
 slightly curved upward, as shown in Fig. 31. The tube 
 
 Fig. 31. 
 
 is rilled with a solution of sodium chloride in gelatin, 
 made up by dissolving 10 grams of gelatin in 140 cc. of 
 hot water, and adding 7 grams of salt and a few drops 
 of a slightly alkaline solution of phenolphthalein. This 
 mixture readily gelatinizes upon cooling. Both ends of 
 the tube are then inserted in two beakers, containing di- 
 lute sulphuric acid. On passing a current through the 
 solutions and the jelly, applying a storage battery of ten 
 cells (about twenty volts) as electromotive force, a 
 gradual decoloration of the jelly will be observed. It will 
 be seen that the boundary surface moves at the rate of 
 1.5 centimeters in i hour. 
 
 77. In the particular case that colored ions are con- 
 sidered, the migration is easily shown by means of a 
 simple apparatus, originally devised by Nernst 1 and 
 slightly modified, as described below : 
 
 A U-shaped glass tube, 1.2 centimeters in diameter and 
 
 i Zeitschr. f. Electrochemie 3, p. 308, (1897). 
 
 6 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 io centimeters high, is connected in the lower part of 
 the bend with a piece of capillary glass tubing (length 
 20 centimeters, inner bore 2 millimeters) bent upward, 
 to which is sealed a separatory funnel of 100 cc. con- 
 tents. The solution used for this experiment is made 
 up by dissolving 0.5 gram of potassium permanganate in 
 100 cc. of distilled water, the specific gravity of which 
 has been increased by the addition of 5 grams of urea. 
 In order to fill the capillary tube, 
 some of the solution is poured in 
 the bend and sucked up into the 
 funnel, until the liquid has risen 
 above the stopcock which is then 
 turned off. The liquid remaining 
 in the bend, is rinsed out with dis- 
 ">-N tilled water and the latter removed 
 ^ by turning the U-tube upside down. 
 71 j|7 The funnel is then filled with the 
 rest of the permanganate solution. 
 In the now empty U-tube, is poured 
 by means of a io cc. pipette, a 
 solution of 0.5 gram of potassium 
 nitrate in I liter of water. Both 
 limbs are closed with two-hole 
 rubber stoppers, allowing the pas- 
 sage of two platinum wires, pro- 
 vided with perforated platinum 
 electrodes, and of two small glass 
 
 tubes for the escape of the gases evolved during the 
 electrolysis. On carefully opening the stopcock the per- 
 
ELECTROCHEMISTRY AND IONIC THEORY 73 
 
 manganate solution drives the colorless nitrate solution 
 with sharp boundary surfaces into the limbs of the U- 
 tube to a certain height, marked by a white and a black 
 strip of paper respectively (Fig. 32). 
 
 When a current, not exceeding 0.2-0.3 ampere is 
 passed through the tube, the violet boundary surface is 
 gradually displaced in the direction of the anode, distinct- 
 ly visible after about 5 minutes The current is turned off 
 as soon as the boundary surface on the anodic side be- 
 comes irregular owing to convection currents. In the 
 left (cathodic) limb, the boundary surface remains ex- 
 tremely sharp and, as Nernst has pointed out, 1 the migra- 
 tion velocity of the MnO 4 '-ion can be approximately 
 calculated from the lowering of the boundary surface. 
 
 78. Kiister, 2 with the aid of two U-tubes, as described 
 in the foregoing experiment, shows how copper in a cop- 
 per sulphate solution moves towards the cathode and in 
 Fehling's solution in the opposite direction. 
 
 The bend of the left U-tube is filled with a light blue 
 (dilute) solution of copper sulphate, separated by a sharp 
 boundary surface from a dilute sodium sulphate solution 
 in both limbs. The second U-tube is filled in an exact- 
 ly similar way with a dark blue Fehling's solution cov- 
 ered in both limbs by a dilute alkaline solution of 
 Rochelle (Seignette) salt. After inserting the platinum 
 electrodes, joined in parallel, an electric current, derived 
 from a storage battery of 15-20 accumulators, is passed 
 through both tubes. After 5-10 minutes the copper sul- 
 phate boundary has moved several millimeters towards 
 
 1 Nernst, 1. c. p. 309. 
 
 2 Zeitschr. f. IJlectrochemie 4, p. 112, (1898). 
 
74 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the cathode, while in the other tube a movement in oppo- 
 site direction has taken place, a sure indication that in 
 this case the copper forms part of a complex anion. 
 
 79. Instead of an apparatus as used by Nernst, a simple 
 U-tube (suitable dimensions: height 16 centimeters, in- 
 ner bore 2 centimeters) will serve the requirements, when 
 in place of aqueous solutions, agar-agar jellies are em- 
 ployed, 1 thus returning to Lodge's original device. 
 
 A solution of agar-agar is first made by cutting 25 
 grams of this substance in small pieces and treating with 
 500 cc. of distilled water. The mixture is then heated 
 until a clear solution is formed, which is, while still 
 hot, strained through a piece of cloth. To 50 cc. 
 of this hot solution is added about 10 cc. of a saturated 
 copper sulphate solution and this mixture poured into 
 the U-tube to about 4 centimeters above the bend (Fig. 
 33). The jelly is allowed to harden and a little bone 
 black sprinkled on the surface to mark the boundary. 
 In order to fix the bone black in its place a solution of 
 potassium nitrate, saturated at o, containing agar-agar 
 is poured in each limb of the tube, and after hardening, 
 the tube on both sides filled up with potassium nitrate 
 solution. The U-tube is placed in a large beaker with 
 ice water, to preserve the jellies from melting on elec- 
 trolyzing the copper sulphate. Electrodes of platinum 
 wire are inserted in the potassium nitrate solution and 
 connected with a 16 candle-power lamp in series, with 
 the terminals of a no- volt lighting circuit. On passing 
 the current through the tube for 5-10 minutes, the effect 
 
 1 A. Smith, I.e. p. 346. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 75 
 
 of the displacement of the blue boundary surface towards 
 the cathode becomes apparent. The movement of the 
 colorless SO 4 "-ions can be demonstrated by interposing, 
 
 Fig. 33. 
 
 on the positive side a thin layer of jelly containing some 
 barium salt, in which case a cloudy layer of barium 
 sulphate jelly is formed. 
 
 80. A similar experiment with a jelly containing a 
 solution of copper chromate enables the demonstration 
 of the simultaneous movement of the blue copper ion and 
 the yellow chromate ion in opposite directions (Noyes 
 and Blanchard 1 ). 
 
 81. The relative velocity of migration of different ions 
 can be demonstrated in an instructive experiment given 
 
 1. c. p. 729. 
 
76 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 by Noyes and Blanchard. 1 Careful determinations have 
 established that at room temperature the ionic mobilities 
 per hour, in dilute aqueous solutions for a potential dif- 
 ference at the electrodes of I volt amounts to 2.05, 2.12, 
 10.8, 5.6, and 1.6 centimeters for the ions K', 
 Cl', H*, OH' and Cu" respectively. Broadly speak- 
 ing, K* and Cl'-ions move at the same speed, 
 H'-ions move about five times as fast, double as fast 
 as OH '-ions and eight times as fast as Cu"-ions. 
 Therefore, it is advisable to use a potassium chloride so- 
 lution in which these different ions are all present, Cu"- 
 ions being visible by their color, H'-ions being recognized 
 by decoloration of phenolphthalein and OH'-ions by 
 coloring this indicator. 
 
 The bend of a U-tube, as described above, and the 
 right limb (Fig. 34), up to a point 5 centimeters from 
 the top, is filled with a jelly made by mixing 32 cc. of 
 saturated potassium chloride solution, i cc. of a I per 
 cent, solution of phenolphthalein in alcohol, 100 cc. of 
 a 2 per cent, agar-agar solution and 8 drops of a normal 
 solution of potassium hydroxide. The other limb, up to 
 5 centimeters from the top, is filled with the same mix- 
 ture, to which has been added twice the amount of hydro- 
 chloric acid, necessary for decolorizing the liquid. The 
 boundaries in both limbs are fixed by sprinkling a little 
 bone black on the surfaces and covering the bone black 
 with a thin layer of the underlying jelly in order to 
 keep the black demarcation surface intact. The platinum 
 wire electrodes are placed at the top of the limbs of the 
 
 i 1. c. p. 731. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 77 
 
 U-tube and connected through a 32 candle-power elec- 
 tric lamp with the terminals of a no-volt direct current 
 circuit. Just before starting the experiment the left 
 limb is filled up with a mixture of 2 cc. of a 10 per 
 cent, potassium hydroxide solution and 20 cc. of a satu- 
 rated potassium chloride solution; the other arm of the 
 tube is then filled up with a mixture of 0.5 cc. of hydro- 
 
 Fig. 34. 
 
 chloric acid (specific gravity 1.12), 6 cc. of a saturated 
 copper chloride solution and 20 cc. of water. The U- 
 tube is placed in ice water, to prevent the melting of the 
 jelly by the heat generated during the electrolysis. On 
 closing the switch and allowing the current to pass for 
 about 15 minutes, it will be observed, that a colorless 
 zone (due to the H'-ions) descends into the pink jelly 
 in the right limb to a depth of about 5-6 centimeters, fol- 
 
78 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lowed by a blue zone (accounting for the Cu**-ions) of 
 about i centimeter deep. (See Fig. 346.) In the other 
 arm a pink zone (due to the OH'-ions) descends into the 
 colorless jelly to a depth of about 2.5 centimeters. 
 
 Electromotive Chemistry. 
 A large number of reactions involving ionogens are 
 known, in which chemical changes are accompanied by 
 the liberation of electrical energy. Since all these ar- 
 rangements for obtaining electric currents are in reality 
 nothing but voltaic cells, this special branch of chemistry 
 may very appropriately be designated as electromotive 
 chemistry. 1 The essential feature about the combina- 
 tions for the production of electric currents consists in 
 preventing the active substances from coming in contact 
 with each other. This can be done in different ways, as 
 will be seen from the following interesting lecture ex- 
 periments, mostly suggested by Kiister and by L,iipke. 
 
 82. The first type of cell to be considered is the "dis- 
 placement cell." Iron, displacing copper from its solu- 
 tion according to the equation : 
 
 Cu" + Fe^ Cu + Fe". 
 
 produces a current in the connecting wire, running from 
 the copper to the iron, as indicated by the arrow. As a 
 current indicator for this and the following demonstra- 
 tions a sensitive lecture galvanoscope (of Keiser and 
 Schmidt) or a suitable milli-ampere meter may be used. 
 A Weston station voltmeter, in which the series resist- 
 ance coil has been short-circuited, will also serve the pur- 
 
 i A. Smith, 1. c. p. 786. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 79 
 
 pose. A full scale deflection is obtained with a current 
 of about o.oi ampere. The experi- 
 ment is carried out as follows: 1 
 
 A disk-shaped copper electrode 
 and a polished iron rod electrode 
 are inserted in a large crystalliza- 
 tion dish filled with a solution of 
 sodium sulphate (Fig. 35). On 
 connecting both electrodes with the 
 current indicator, no current, or at 
 least no lasting current, is noticed. 
 As soon as some solid copper sul- 
 phate is placed on the copper disk, 
 thereby surrounding the electrode 
 with Cu"-ions a strong current re- 
 
 SultS ' Fig. 35. 
 
 83. In the same way the reaction : 
 
 2Fe"' -f Fe !^ 3Fe" 
 
 will give an electric current, in the direction of the ar- 
 row. The same apparatus is used as in the preceding ex- 
 periment, replacing the sodium sulphate solution by a so- 
 lution of sodium chloride and the copper electrode by a 
 platinum disk. 2 No perceptible current is observed, but 
 on bringing some solid ferric chloride on the platinum 
 disk, which is thus surrounded by Fe*"-ions, a current 
 in the direction of the arrow results (Fig. 36). The 
 process that takes place consists in discharging the tri- 
 
 1 Kiister, Zeitschr. f. Electrochemie 4, p. 107, (1897). 
 
 2 Kiister, 1. c. p. 107. 
 
So 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 valent iron ion and the simultaneous loading of the un- 
 charged iron. 
 
 84. Another way of producing 
 an electric current is by discharging 
 a cation and at the same time giv- 
 ing another cation a higher charge : 
 
 Sn" + Hg" = Sn-f Hg. 
 
 An apparatus, as devised by 
 Lupke 1 may be used, consisting of 
 two beakers, one of which con- 
 tains an acidulated solution of 
 stannous chloride, (112 grams in I 
 liter), while the other is filled with 
 an acidulated normal sodium chlor- 
 ide solution. The beakers are con- 
 nected by a wide siphon, filled 
 Fig. 36. with the same solution of sodium 
 
 chloride. Platinum electrodes, bent at right angles, are 
 inserted in the beakers, and attached to copper wires, 
 leading to a galvanoscope. No current is observed. On 
 placing a few crystals of corrosive sublimate on the right 
 electrode, a current flows through the wire circuit from 
 right to left, as shown by the galvanoscope. (Fig. 37.) 
 
 85. Instead of cations, anions, may be used to furnish 
 electricity as in the reaction : 
 
 I' + Br ^ Br' + I. 
 
 In a H-shaped vessel (Fig. 38) two circular plati- 
 num foils are sealed in near the bottom, and the connect- 
 
 1 l,upke-Bose, 1. c. p. 164. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 8l 
 
 ing copper wires attached to a galvanoscope. A 10 per 
 cent, potassium chloride solution is poured into the ves- 
 sel and the platinum disk in the left limb covered with 
 a few drops of bromine. No current is observed, but 
 
 Fig. 37. Fig. 38. 
 
 on placing a crystal of potassium iodide on the elec- 
 trode in the right limb, the galvanoscope indicates a cur- 
 rent in the wire circuit from left to right. At the same 
 time the solution on the left side is colored brown by 
 the separation of iodine. 1 
 
 86. The reversible ionic reaction : 
 
 Fe" + 1 ^ Fe-" + I', 
 
 1 Kiister, 1. c. p. 109. 
 
82 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 in which both cations and anions take part, can also pro- 
 duce an electric current. 
 
 Following again Kuster's directions, 1 a large size 
 crystallization dish, (Fig. 39), filled with moderately di- 
 luted hydrochloric acid, is used. 
 Two small dishes are placed in- 
 side, so that the liquid covers 
 both. Platinum foils, bent at 
 right angles, are inserted in each 
 dish to serve as electrodes. The 
 platinum foil, on the left i s 
 covered with some iodine crys- 
 tals, the other by a few pure 
 ferrous sulphate crystals (even- 
 tually 2 or 3 drops of a freshly 
 prepared ferrous chloride solu- 
 tion). The galvanoscope shows 
 a current flowing through the 
 wire from left to right. After a 
 while, enough Fe"'-ions being 
 formed, the current can be re- 
 verted by adding potassium iodide crystals to the iodine 
 electrode. 
 
 87. Another reversible ionic reaction, already men- 
 tioned in a preceding chapter, viz. : 
 
 Fe" + Ag- = Fe ; " + Ag, 
 
 can be adapted to give a current of electricity in the 
 manner described by L,ermontoff. 2 The experiment is 
 
 1 Id., 1. c. p. 108. 
 
 2 Meldola, the Chemistry of Photography, I^ondon.p. 179, (1891). 
 
 Fig. 39. 
 
ELECTROCHEMISTRY AND IONIC THEORY 83 
 
 well fitted for projection on the screen by dividing a glass 
 cell with parallel sides into two partitions by means of 
 a piece of brown paper cemented in a vertical position, 
 water-tight to each side and to the bottom. The cell is 
 then filled on one side with a 2 per cent, solution of 
 silver nitrate and on the other with a cold saturated so- 
 lution of ferrous sulphate. On connecting both solutions 
 through a bent silver wire, dipping in each partition 
 half way to the bottom, a crystalline growth of silver on 
 the wire can be observed on the side which contains the 
 silver nitrate. 
 
 The above mentioned reactions deal with the type of 
 galvanic cells, called "displacement cells." 1 Two other 
 types are the "combination cell" and the "oxidation cell." 
 
 88. A combination cell may be set up for instance by 
 taking a glass vessel, divided in two partitions by a por- 
 ous diaphragm (of unglazed porcelain) and filled on one 
 side with a sodium chloride solution, in which a zinc rod 
 is dipped, and on the other side with the same solution 
 to \vhich some bromine has been added. A platinum wire 
 or a rod of carbon is inserted in this solution and both 
 poles connected with copper wires to a galvanoscope. A 
 current flows through the wire circuit from the platinum 
 (or carbon) to the zinc and the reaction, that takes place 
 in the solutions on both sides of the septum is the fol- 
 lowing : 
 
 Zn + 2Br = Zn" -f 2Br'. 
 
 89. The same arrangement may be used for illus- 
 
 A. Smith, 1. c. p. 788. 
 
8 4 
 
 DEMONSTRATIONS IN PHYSIC AI, CHEMISTRY 
 
 trating the operation of an oxidation cell. 1 such as is ex- 
 pressed by the equation : 
 
 Sn" + 2Cl = Sn-" + 2CF. 
 
 Both partitions are filled with the same sodium chlor- 
 ide solution, and then some stannous chloride dissolved 
 in the left hand solution, while on the other side free 
 chlorine is introduced. Two platinum wires are in- 
 serted and connected with a galvanoscope which in- 
 dicates a current from right to left. 
 
 A fourth type of cell, vis. concentration cells, will be 
 considered later. 
 
 90. The different "types" of gal- 
 vanic cells may be divided in two 
 groups : inconstant and constant cells. 
 Thus the combination Fe-NaCl-Pt 
 (page 79) e. g. is an inconstant cell. 
 The ferric chloride added to the plati- 
 num electrode, acts as a "depolarizer." 
 Liipke 2 has modified the apparatus, as 
 sketched in the figure (Fig. 40), the 
 ferric salt being poured on the plati- 
 
 Ynum disk through the side tube. 
 91. The "polarization" current can 
 ke easily demonstrated by electrolyz- 
 ing dilute sulphuric acid (1:10) in a 
 H-shaped vessel (Fig. 41), communi- 
 cating with a large crystallization dish, filled to two- 
 thirds of its height with the same acid. The electrodes 
 are platinized platinum foils, the cathode being inserted 
 
 1 Ibidem, 1. c. p. 792. 
 
 2 Riidorff-Iyiipke, Grundrisz der Chemie, 12e Aufl., p. 306, (1902). 
 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 twice as deep into the acid as the anode. The acid is 
 electrolyzed with one storage cell and the electrolysis 
 continued until the lower end of the platinum foils just 
 touches the liquid in both limbs. The current is then 
 turned off and connection is 
 made with a galvanoscope, which 
 indicates a current in the con- 
 necting wire, flowing in the op- 
 posite direction. 
 
 The electrodes are platinized 
 by placing the platinum foils, 
 previously cleaned by means of 
 chromic acid, in a solution of 3 
 grams of platinum chloride and 
 0.02-0.03 gram of lead acetate in 
 100 cc. water, and connecting 
 the electrodes with a battery of 
 two lead accumulators. The cur- 
 rent is passed for 10-15 minutes, 
 reverting its direction through a 
 commutator every half minute. 1 
 
 92. During the electrolysis of 
 dilute acids both platinum elec- 
 trodes are polarized. On adding oxidizing agents to 
 the cathode, the evolution of hydrogen is stopped and 
 cathodic polarization prevented. 
 
 This is clearly shown in the following experiment, 
 devised by L,iipke : 2 
 
 Three U-shaped tubes with sealed platinum foil 
 
 1 Findlay, Practical Physical Chemistry, p. 171, (1915). 
 - Rudorff-IyUpke, 1. c. p. 305. 
 
 Fig. 41. 
 
86 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 electrodes (Fig. 42), are connected in series with a 
 storage battery of eight lead accumulators. The first 
 tube is filled with a 19 per cent, nitric acid solution, 
 the second with a 52 per cent, nitric acid solution and 
 the third with a chromic trioxide solution. As soon as 
 the current is turned on, it will be noticed that oxygen 
 
 Fig. 42. 
 
 is evolved at all three anodes; hydrogen is only set free 
 in the first tube, while in the second vapors of nitrogen 
 oxide escape. In the third tube the color of the liquid 
 turns gradually to a darker shade. 
 
 93. A well-known inconstant cell is the combination 
 Zn-H 2 SO 4 -Cu. By eliminating polarization a constant cell 
 results, as is proved by the following lecture experiment. 1 
 
 1 Brauer, lyehrbuch der anorg. Chemie 2e Aufl., p. 188, (1913). 
 
ELECTROCHEMISTRY AND IONIC THEORY 87 
 
 The funnel A (Fig. 43), about two-thirds filled with di- 
 lute sulphuric acid (i no) is connected by means of rub- 
 ber tubing with a second, leveling funnel (provided with 
 stopcock) containing a copper sulphate solution. A con- 
 ical sheet of zinc, which is amalgamated with mercury 
 in order to minimize the direct action of the zinc on the 
 
 Fig. 43. 
 
 acid acts as anode, while a copper disk, farther down to 
 the bottom serves as cathode. On connecting the elec- 
 trodes with a low resistance ampere meter, the latter in- 
 dicates right at the start a current of about 1.5 ampere, 
 rapidly decreasing, however, to 0.2-0.3 ampere. When 
 the copper sulphate is allowed to flow into the cell, cov- 
 7 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 ering the cathode and forming a "Daniell" or "gravity" 
 cell, the current increases in strength and becomes con- 
 stant. 
 
 94. Very simple in construction is the apparatus, de- 
 scribed by lyiipke, 1 which enables the use of several 
 depolarizers in succession. 
 
 The electrolyte, dilute sulphuric acid, (1:25) is con- 
 tained in a narrow mouth bottomless bottle held upside 
 down by a clamp fastened to a rin stand (Fig. 44). 
 The cathode is a large copper disk sol- 
 dered to a copper rod, passing through 
 the cork stopper. The anode, a platinum 
 disk, is separated from the cathode by a 
 crystallization dish. On connecting the 
 electrodes with a galvanoscope, no ap- 
 preciable current is indicated, but on 
 pressing crystals of potassium perman- 
 ganate, corrosive sublimate, silver ni- 
 trate, small cubes of manganese dioxide 
 or red lead on the platinum disk, the 
 Fig. 44. pointer immediately deviates. 
 
 95. Concentration cells are cells in which two different 
 concentrations of the same salt are used. Such a cell is 
 readily set up by half filling with a concentrated solu- 
 tion of copper chloride (20 grams CuCl 2 in 40 cc. water), 
 a wide glass tube (15 by 2.5 centimeters) closed at its 
 lower end by a one hole cork stopper, through which 
 passes a copper rod. On top of this is poured a dilute 
 
 i Riidorff-I,upke, 1. c. p. 307. 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 8 9 
 
 solution of the same salt (20 grams in I liter water), 
 taking care that a sharp boundary surface is maintained. 
 The tube is closed by another perforated stopper carry- 
 ing a copper rod dipping in the dilute solution (Fig. 45). 
 When connection is made with a galvano- 
 scope, the pointer indicates a current, 
 ing outside the tube, from the lower 
 rod to the upper. 1 Similar concentration 
 cells may be constructed with Ag-AgNO 3 - 
 and Zn-ZnSO 4 - solutions. 
 
 96. With a slight modification "short- 
 circuited," concentration cells are formed, 
 first described by Bucholz in i8c>4. 2 
 
 A lecture jar (15-20 centimeters high) is 
 filled to one-half of its contents with a 
 concentrated solution of stannous chloride, 
 obtained by dissolving 15 grams of tin in 
 dilute hydrochloric acid and evaporating to 
 40 cc., and this solution is covered with a 
 very dilute solution of the same salt. A 
 tin rod, inserted in the jar, so that it passes 
 through both layers, is partly corroded by 
 dissolving in the dilute solution, and below f* 
 the boundary surface covered with a "tin 
 tree." (Fig. 46.) 
 
 97. An experiment, showing that shortcircuited gal- 
 vanic cells are possible, which are entirely built up of 
 liquids, has been devised by Kriiger and Dolezalek. 3 
 
 1 lyiipke, Grundziige der Electrochemie, 5, Aufl., p. 144. 
 
 2 Coehn, 1. c. p. 555. 
 
 3 Zeitschr. f. Electrochemie 12, p. 669, (1906). 
 
DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 An O-shaped glass vessel (Fig. 47), with a bore 
 of 6 centimeters, is half way filled with a 35 per cent, 
 solution of sulphuric acid, colored with litmus. On the 
 left side a layer of sodium acetate solution (30 per 
 cent.), 2 centimeters high, is placed. In order to obtain 
 
 Fig. 46. Fig. 47. 
 
 a sharp boundary, the solution is cautiously dropped 
 from a pipette on a thin cork disk, floating on the acid. 
 The ring is then filled up, in the same way, with a 20 
 per cent, lithium chloride solution, containing a few 
 drops of ammonia, colored with litmus. The ring is 
 brought around a small magnet system, so that the lat- 
 ter occupies the center of the ring. The system con- 
 
ELECTROCHEMISTRY AND IONIC THEORY $1 
 
 sists of several small magnets, suspended from a wire, 
 3 centimeters long, and enclosed in a thick-walled cop- 
 per box (5 centimeters high and 2 centimeters wide) pro- 
 vided with a small glass window. A mirror is fixed on 
 the magnet system, which allows to throw the image of 
 an illuminated arrow on a graduated screen at 2-3 meters 
 distance. A curved, astatic bar magnet, the distance of 
 which from the magnet system can be regulated, is placed 
 underneath the ring, in order to increase the sensibility 
 of the measuring instrument, which indicates a slight 
 current flowing through the ring. By turning the ring 
 through 180, the arrow moves in the opposite direc- 
 tion, over the same number of scale divisions on the other 
 side of the zero-point. Upon shaking the ring the liquids 
 become mixed and the arrow returns to its initial posi- 
 tion. The experiment may be taken as proof that Volta's 
 law does not hold for solutions. 
 
 IV. Conductivity and Degree of lonization. 
 
 98. Pure water does not conduct an electric current 
 perceptibly. A current of appreciable strength is only 
 noticed by dissolving salts, acids and bases in water. 
 This is shown by filling a beaker of 200 cc. with distilled 
 water and inserting two platinum foils (3 by 4 centi- 
 meters) parallel to each other, at a distance of 1-2 centi- 
 meters. On connecting the electrodes with a galvano- 
 scope and a battery of three lead accumulators, no cur- 
 rent is indicated; but on allowing concentrated hydro- 
 chloric acid to drop from a pipette into the water, the 
 
92 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 instrument shows a constantly increasing deviation from 
 the zero point. 
 
 99. That, on the other hand, it is not the acid alone 
 which is responsible for the conductivity, can be proved 
 by passing dry hydrochloric acid gas into carefully pre- 
 pared toluene, from which all traces of water have been 
 removed. On inserting two platinum electrodes, con- 
 nected with a galvanoscope and a battery of 70 volts, 
 into the solution, no current is indicated. A few drops 
 of water, however, immediately have the effect of pro- 
 ducing a current of noticeable strength. 1 
 
 100. The following experiment, due to Scriba, 2 illus- 
 trates the same fact for sodium chloride. Solid rock 
 salt, like pure water, does not perceptibly conduct elec- 
 tricity, but when it is dissolved in water, the solution 
 shows itself a good conductor. A glass tube, 20 centi- 
 meters long and with a diameter of 2.5 centimeters, 
 closed at one end and provided with two platinum wires 
 sealed in the glass at the lower end of the tube, is half 
 filled with distilled water. A cubical piece of solid rock 
 salt (if necessary, dried with absolute alcohol) is fixed be- 
 tween two brass clamps (Fig. 48). Connection is made 
 with the terminals of a no- volt direct circuit, a switch 
 and a 32 candle-power lamp being interposed, so that the 
 salt and water are in parallel in the circuit. On 
 closing the circuit, no electrolysis is observed and the 
 lamp does not glow, but on dropping a small piece of the 
 rock salt in the water the lamp gradually shines with a 
 
 1 Kiister, 1. c. p. 109. 
 
 2 Zeitschr. f. phys. u. chem. Unterricht 28, p. 94, (1915). 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 93 
 
 bright yellow light and electrolysis takes place in the so- 
 lution of rock salt. The experiment may be repeated, 
 replacing the rock salt by a 
 large crystal of cane sugar. 
 The result in this case is 
 negative. 
 
 101. That the conductiv- 
 ity of a given weight of 
 electrolyte increases with 
 increasing dilution is readi- 
 ly demonstrated by the fol- 
 lowing experiment of 
 Stieglitz, 1 adapted from a 
 similar one by Noyes and 
 Blanchard. 2 
 
 A rectangular glass 
 trough, of about i liter 
 capacity, 4.6 centimeters 
 wide 11.5 centimeters long 
 and 20 centimeters high is 
 fitted with copper elec- 
 trodes, 4.6 centimeters broad and 21 centimeters high 
 (Fig. 49) connected with a lead accumulator and an am- 
 pere meter. On bringing 20 cc. of a 4N hydrochloric 
 acid solution in the trough, the current registered by the 
 amperemeter will be after a few seconds, 0.17 ampere. 
 On adding successively 20, 40, 80, 160 and 320 cc. of 
 distilled water, the mixture being well stirred after each 
 
 1 Qualitative Analysis. Vol. 1, p. 49, (1916). 
 
 2 1. c. p. 726 ; similar experiments have been described by Iupke and 
 by Ostwald. 
 
 Fig. 48. 
 
94 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 addition, the current is increased to 0.22, 0.26, 0.30, 0.31, 
 and 0.32 ampere respectively, thus showing that the in- 
 crease in strength grows smaller, the greater the dilution. 
 
 Fig. 49. 
 
 102. Different acids of the same molecular concentra- 
 tion exhibit marked differences in conductivity and hence 
 in degree of dissociation. This may be shown in a simple 
 way 1 by means of 3 U-shaped capillary tubes of exactly 
 the same size (18 centimeters long, inner bore 3 milli- 
 meters) filled with normal solutions of hydrochloric, sul- 
 phuric and acetic acid respectively. The limbs in each 
 tube are widened up, so as to allow the passage of disk- 
 like platinum electrodes of the same diameter, placed at 
 the same height in the solutions (Fig. 50). Each tube 
 is connected in its turn with the aid of a switch to a 
 battery of 20 accumulators and a galvanoscope or milli- 
 
 1 Riidorff-I^iipke, 1. c. p. 136 ; Ostwald, Grundlinien, 3 Aufl. p. 282, 
 (1912). 
 
ELECTROCHEMISTRY AND IONIC THEORY 
 
 95 
 
 ampere meter. It will be seen that the deviation from the 
 zero-point is greatest for hydrochloric acid, somewhat 
 less for sulphuric acid and exceedingly small for acetic 
 acid. 
 
 Fig. 50. 
 
 103. The same principle can be demonstrated in a very 
 elegant manner with a more complicated apparatus, de- 
 vised by Whitney and described by Noyes and Blanch- 
 ard. 1 
 
 Four glass tubes, as nearly alike as possible (inter- 
 nal diameter 3 centimeters; length 20 centimeters), are 
 closed at their lower ends with a one-holed rubber stop- 
 per, in which has been inserted a thick-walled capillary 
 glass tube containing a stout copper wire to which a thin 
 platinum disk, covering the small end of the stopper has 
 been soldered, and attached to it by means of sealing 
 wax. The tubes are set up in a vertical position and 
 held in place by a suitable wooden frame. In the upper 
 end of each tube, a one-holed rubber stopper is inserted 
 carrying a moveable thick- walled glass tube (22 centi- 
 meters long) containing a stout copper wire, to the lower 
 end of which is soldered a thin platinum disk (diameter 
 
 1 1. c. p. 736. 
 
9 6 
 
 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 about 2.8 centimeters) reinforced by a conical layer of 
 sealing wax. Each lower electrode is connected with a 
 32 candle-no volt lamp, and all other connections made 
 as shown in Fig. 51. The upper electrodes are connected 
 
 Fig. 51. 
 
 through an open switch with one terminal and the lamps 
 with the other terminal of an alternating no-volt cir- 
 cuit. (In case no alternating current is available, the up- 
 per electrodes are suitably shaped conically in order to 
 allow the gases, evolved during the electrolysis, to escape. 
 For the same reason the upper rubber stoppers must be 
 
ELECTROCHEMISTRY AND IONIC THEORY 97 
 
 provided with a second hole, and the circuit be closed for 
 as short a time as is necessary). After placing 120 cc. of 
 distilled water in the tubes, they are filled with 5 cc. of 
 half-normal solutions of hydrochloric acid, sulphuric 
 acid, monochlor-acetic acid (freshly prepared) and acetic 
 acid respectively and the mixtures thoroughly stirred. 
 The upper electrodes are re-inserted at the same height, 
 (one-third of the distance from the bottom), the lecture 
 room is somewhat darkened and the circuit closed. The 
 lamp beneath the hydrochloric acid solution is found to 
 glow brightest, the resistance in this case being least; 
 the other lamps follow in brightness in the order given 
 above, the fourth lamp not glowing perceptibly. 
 
 The electrodes are next adjusted so that the lamps are 
 equally bright, when it is seen, on re-admitting light to 
 the room, that if the upper electrode in the hydrochloric 
 acid is at the top, in the second solution (H 2 SO 4 ) it is 
 about one-quarter of the distance down, in the third 
 (CH 2 C1COOH), three-quarters of the distance down, 
 while in the acetic acid tube both electrodes are almost in 
 contact. Finally, in order to show, that the alkali salts 
 of these acids all have nearly the same conductivity and 
 degree of dissociation, the solutions are neutralized 
 (about the same amount of potassium hydroxide being 
 required in each case) and then the equal brilliancy re- 
 established. It will be found this time that the upper 
 electrodes stand approximately at the same height. The 
 same apparatus may be used for the demonstration of the 
 so-called Ostwald's dilution law and for the illustration 
 of the conductivity and dissociation of other substances. 1 
 
 1 Noyes and Blanchard, 1. c. p. 739. 
 
98 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 104. That the "strength" of acids does not bear any 
 relation to the "potential" amount of hydrogen ions, as 
 found by titration (see foregoing experiment), but is in- 
 timately connected with the "actual" amount of H'-ions 
 in solution, may be further illustrated by the different 
 speed of reaction of normal solutions of different acids on 
 equal-sized pieces of metal (zinc or magnesium). On 
 bringing the dilute acids 
 
 (N HC1, N H 2 SO 4 , N CH 3 COOH) 
 with the metal in small Erlenmeyer flasks, connected 
 through rubber tubing with gas collecting tubes of the 
 same size and diameter, the volumes of gas, collected 
 over water in the same time (5-10 minutes), are dif- 
 ferent. The solutions are most suitably treated before- 
 hand with equal amounts of a dilute copper sulphate so- 
 lution and the gases allowed to escape for some time, be- 
 fore the experiment is started. 1 
 
 V. The Common Ion Effect. 
 
 The effect of a common ion represents a special case of 
 the mass action principle, of which several instances were 
 given in Chapter V, some other examples will be discussed 
 in the chapter on solubility. Some further applications, 
 in which the dissociation of either H*-ions or OH' -ions 
 is driven back by the addition of salts with common 
 anions or cations, may be considered here. 
 
 105. H'-ions. A typical case is the following, given 
 by Crum Brown. 2 A dilute solution of ferrous sulphate 
 
 1 Ostwald, Grundlinien, p. 281 ; Rudorff-I<upke, Grundrisz, p. 136. 
 
 2 Proc. Royal Soc. of Edinburgh, 21, p. 57, (1896). 
 
ELECTROCHEMISTRY AND IONIC THEORY 99 
 
 or ferrous ammonium sulphate (Mohr's salt) is acidified 
 with such an amount of acetic acid solution, that on ad- 
 dition of hydrogen sulphide gas no ferrous sulphide is 
 precipitated. On throwing a few particles of solid so- 
 dium acetate in the solution, each crystal forms the start- 
 ing point of a long streak of iron sulphide. The experi- 
 ment fits admirably well for projection on the screen. 
 
 106. The same result is obtained by the use of an indi- 
 cator, 1 viz. methyl orange, which colors a dilute acetic 
 acid solution pink. On adding a large volume (about six to 
 eight times the amount of acid) of a 10 per cent, solution 
 of sodium acetate, which, to prevent the suspicion of alka- 
 linity, has been made very slightly acid, the color turns 
 to a bright yellow. If, instead of methyl orange, congo 
 red is used as an indicator, the observed change in color 
 is from blue to violet red. 
 
 107. Ostwald 2 has shown the effect of salt addition on 
 acids with common anion in the following manner : 
 
 In two small conical flasks are placed two equal-sized 
 pieces of zinc, covered by dilute acetic acid. The escaping 
 gases are collected after some time over water or glycer- 
 ine in eudiometer tubes. The distance between the gas 
 bubbles in both tubes is observed. A concentrated so- 
 dium acetate solution is poured into the flask, from which 
 the bubbles are coming forth somewhat faster than in 
 the other. The gas evolution instantly slackens and con- 
 sequently the distance between the rising gas bubbles is 
 considerably increased. 
 
 1 Kiister, 1. c. p. 109. 
 
 2 Ostwald, Wiss. Grundlagen der Anal. Chem., 5 Aufl., p. 225, (1910). 
 
IOO DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 108. Another good illustration is given by Tread- 
 well. 1 On adding dilute acetic acid to a solution con- 
 taining potassium iodide and potassium nitrite, the solu- 
 tion turns yellow or brown, owing to the separation of 
 iodine, in accordance with the equation: 
 
 2HI + 2HNO 2 == 2H 2 O -f 2NO -f I r 
 If, however, before adding the acid, the solution is 
 mixed with a concentrated potassium (or sodium) ace- 
 tate solution, the addition of dilute acetic acid causes no 
 separation of iodine, although enough H'-ions are present 
 in solution to turn blue litmus paper red. This proves 
 that the amount of H'-ions is not sufficient to reduce the 
 nitrous acid. Addition of a few drops of a strong min- 
 eral acid causes the immediate liberation of iodine. 
 
 109. OH'-ions. The forcing back of the concentration 
 of hydroxyl-ions is easily carried out as follows : A con- 
 centrated ammonium chloride solution is made very 
 slightly alkaline with ammonia, so as to make sure that 
 it contains no free acid (hydrolysis is likely to cause a 
 slight acidity). It is then poured into an aqueous solu- 
 tion of ammonia, colored by phenolphthalein. The red 
 color fades to a scarcely perceptible pink. 
 
 VI. Hydrolysis. 
 
 The slight ionization of water (according to Kohl- 
 rausch and Heydweiller 2 the degree of dissociation for 
 H' and OH'-ions, expressed in gram equivalents per liter 
 is 1.05 times io~ 7 at 25) accounts for the hydrolysis of 
 
 1 Analytische Chemie, 7e Aufl., p. 311, (1911). 
 
 2 Zeitschr. f. phys. Chem., 14, p. 317, (1894). 
 
ELECTROCHEMISTRY AND IONIC THEORY IOI 
 
 salts, viz., the decomposing effect of water on dissolved 
 salts. The salts of a strong base and a strong acid are 
 not appreciably hydrolyzed; the solutions show a neutral 
 reaction. However, under exceptional conditions, water 
 is able to decompose for example sodium chloride, ac- 
 cording to the equation : 
 
 NaCl + HOH =:= NaOH + HC1. 
 
 110. The following experiment demonstrates this de- 
 composition. 1 
 
 A platinum crucible is heated in a blast lamp or over 
 a Meker burner to a bright yellow heat (1100) and a 
 small quantity of sodium chloride (melting point 884) 
 fused in the crucible. One cubic centimeter of water is 
 dropped from a pipette drop by drop on the fused salt. 
 When half of the water has been evaporated, which takes 
 about 30 seconds, the remaining aqueous solution is 
 poured off into a beaker containing a blue litmus solution 
 which turns red. After the crucible has cooled down the 
 residual salt is dissolved in water, and the solution poured 
 into a solution of red litmus which changes into blue. 
 
 111. Salts of weak acids with strong bases, show an 
 alkaline reaction, readily detected by litmus, viz: Aque- 
 ous solutions of alkali salts of hydrocyanic acid, carbonic 
 acid, boric acid, hydrosulphuric acid. Addition of a 
 little alkali hydroxide will stop hydrolysis as can be 
 proved in the case of potassium cyanide solution when 
 the characteristic odor of hydrocyanic acid disappears. 
 
 112. Salts of strong acids with weak bases react acidic 
 as for instance solutions of copper sulphate, zinc sul- 
 
 1 Stieglitz, 1. c. p. 179. 
 
IO2 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 phate, ferric and aluminium chloride, bismuth and anti- 
 mony chloride. Addition of acid in this case prevents 
 or at least forces back hydrolysis and is always applied 
 in case oxy-salts should be formed. Thus the brown- 
 ish red color of acqueous ferric sulphate disappears on 
 the addition of sulphuric acid. 
 
 113. Salts of weak acids and weak bases are largely 
 hydrolyzed and react either alkaline or acidic, according 
 to the nature of the base and acid. In case both are 
 equally weak, the solution is neutral : ammonium acetate 
 for instance. If the acid is stronger the solution is acidic, 
 if the base is stronger the solution is basic. Thus am- 
 monium carbonate reacts alkaline, while ferric acetate is 
 acid. 
 
 Hydrolysis is favored both by dilution and by boiling. 
 
 114. The effect of dilution may be shown by adding 
 water to a slightly acid solution of bismuth chloride : 
 
 BiCl 3 + H 2 O ^ BiOCl + 2HC1. 
 
 115. The effect of boiling is demonstrated by heating 
 a ferric acetate solution prepared by treating freshly pre- 
 cipitated ferric hydroxide with dilute acetic acid when a 
 voluminous precipitate of basic ferric acetate will be 
 formed : 
 
 Fe(C,H,0,)3 + 2H J = F< + 2 HC 2 H S O,. 
 
 That boiling favors hydrolysis is due to the fact that 
 the water is dissociated to a greater extent at its boiling 
 temperature, than when cold. From the data obtained by 
 Kohlrausch and Heydweiller (1. c.) it will be seen that 
 
ELECTROCHEMISTRY AND IONIC THEORY IO3 
 
 the dissociation at 50 is three times greater than at or- 
 dinary temperature and at the boiling point about ten 
 times greater. 
 
 116. Schoorl 1 recommends the change in color of aque- 
 ous indicator solutions on boiling as an easy way for 
 demonstrating the increasing dissociation of water with 
 the rise of temperature. 
 
 To 600 cc. of pure, distilled water is added 3-5 cc. 
 of the indicator solution, and then a few drops of a base 
 or an acid, in order to bring the color to its neutral point 
 ("mix-color"). The solution is divided into three equal 
 parts and each portion is poured into a 500 cc. flat- 
 bottom flask of Jena or Pyrex glass, previously steamed 
 out. Two of the solutions are made slightly alkaline and 
 acid respectively and kept for comparison. The third 
 flask, closed by a plug of cotton wool is heated on a wire 
 gauze. A change in color is visible, even before the boil- 
 ing point is reached. By placing the flask in cold water, 
 the original color returns. Indicators sensitive to bases, 
 cause a change in color towards the alkaline color. Thus 
 methyl orange turns yellow, congo red changes from vio- 
 let to red, lacmoid from violet to blue. Semi-sensitive or 
 neutral indicators (litmus), remain unchanged, while in- 
 dicators, sensitive to acids show a change in color 
 towards the acid color, e. g., phenolphthalein turns from 
 pink to colorless. 
 
 The following table gives the approximate concentra- 
 tion of H'- and OH'-ions at 25 expressed in gram-equiva- 
 
 i Chem. Weekblad, 3, p. 771, (1906). 
 8 
 
104 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lents per liter, at which a marked change in color is per- 
 ceptible. 
 
 Indicator 
 
 H- i X to" 3 
 OH' I X io IX 
 
 H' i X IQ 4 
 OH' i X io I0 
 
 H- i X io5 
 OH' i X iQ9 
 
 H- i X lo" 6 
 OH' i X io~ 8 
 
 Methyl orange 
 Congo red . 
 
 LitmUS 
 
 red orange 
 blue 
 
 orange 
 violet 
 
 yellow 
 red 
 
 red 
 
 Phenol- 
 phthalein 
 
 Indicator 
 
 H- i x io 7 
 OH' i X iQ7 
 
 H' i X io 8 
 OH' i X io 6 
 
 H' i X io-9 
 OH' i X io~ 5 
 
 H' i X io I0 
 OH' i X io4 
 
 Methyl orange 
 Congo red - . . 
 
 
 hlup 
 
 
 
 Phenol- 
 phthalein 
 
 
 colorless 
 
 pink 
 
 red 
 
 VTI. lonization and Chemical Activity. 
 
 117. lonization easily explains the immediate interac- 
 tion of chemical compounds in aqueous solution, and the 
 inactivity or slow reaction in non-aqueous (non-ionized) 
 solutions. A comparison of the properties of hydro- 
 chloric acid dissolved in water and in toluene may be 
 taken as an illustration. 
 
 An aqueous solution of hydrochloric acid dissolves 
 marble rapidly while a solution in toluene, a non-con- 
 
ELECTROCHEMISTRY AND IONIC THEORY 10$ 
 
 ductor of electricity, has little or no effect on calcium 
 carbonate. 
 
 118. Another experiment of the same kind is the fol- 
 lowing: 1 One hundred cc. of nearly saturated aqueous 
 solutions of potassium bromide and zinc bromide and 
 equal volumes of 5 per cent, alcoholic solutions of isopro- 
 pyl bromide, ethyl bromide and phenyl bromide are pre- 
 pared and placed in five lecture jars. A nearly saturated 
 alcoholic solution of silver nitrate (2.5 grams in 100 cc.) 
 is then prepared and 20 cc. of this solution added to each 
 of the five jars after which the mixtures are thoroughly 
 stirred. An immediate precipitate is formed in the 
 first two cases in which the reactions are ionic. An in- 
 complete reaction, although immediate turbidity is visible, 
 is shown in the case of isopropyl bromide. A slow reac- 
 tion takes place in the ethyl bromide solution, while ap- 
 parently no reaction is going on in the solution containing 
 phenyl bromide. Care must be taken that the organic 
 bromides are free from hydrobromic acid and bromine, 
 from which they may be freed by washing first with a 
 dilute sodium carbonate solution and then with water. 
 
 119. As has been pointed out by Kahlenberg, 2 this dif- 
 ference in behavior need not necessarily be a consequence 
 of electrolytic dissociation. In fact, it has been proved 
 that copper oleate dissolved in benzene or toluene gives 
 an immediate precipitate of copper chloride with hydro- 
 chloric acid, phosphorous trichloride, tin tetrachloride, 
 antimony trichloride, etc., dissolved in these same hydro- 
 
 1 Noyes and Blanchard, 1. c. p. 728. 
 
 2 Outlines of Chemistry, revised ed., New York, p. 453, (1916) ; Journ. of 
 phys. Chem. 6, p. i, (1902). 
 
IO6 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 carbons, notwithstanding the fact that all these solutions 
 are non-electrolytes. In the same way, as may be shown 
 in the lecture, sodium alcoholate and ferric chloride (su- 
 blimed) both dissolved in absolute ethyl alcohol, react with 
 the formation of ferric, alcoholate and the direct precip- 
 itation of sodium chloride. Recent investigations by Cady 
 and Lichtenwalter, 1 however, lead to the conclusion that 
 these "exceptional" cases are in agreement with the dis- 
 sociation theory. 
 
 VIII. lonization and Color of Solutions. 
 
 120. All salts of a metal, whose ion is colored, show the 
 same color in dilute aqueous solutions, independent of the 
 anion, present in the solution (provided the latter is col- 
 orless), whatever the colors of the undissociated salts 
 may be. 
 
 Thus dilute aqueous solutions of copper sulphate, 
 acetate, chloride, and nitrate all show the same blue color 
 due to the Cu"-ion. On the other hand, the same salt, 
 when dissolved in a small amount of water or dissolved in 
 solvents other than water, very often shows a different 
 color : 
 
 Twenty-seven grams of anhydrous copper chlor- 
 ide, dissolved in 100 cc. of absolute alcohol gives a dark 
 green solution ; about the same color is exhibited by a con- 
 centrated aqueous solution. On diluting with water, both 
 show the same shade of blue. 
 
 121. Characteristic changes in color occur with cobalt 
 solutions. Alcoholic solutions of cobalt chloride and ni- 
 
 1 Journ. Am. Chem. Soc., 35, p. 1434, (1913). 
 
ELECTROCHEMISTRY AND IONIC THEORY IO/ 
 
 trate are deep violet-blue and purplish-red respectively. 
 The solutions are made up 1 by dissolving 60 grams of 
 CoCL 2 .6H 2 O and 73 grams of Co(NO 3 ) 2 .6H 2 O in 100 cc. 
 of ethyl alcohol (96 per cent.). On adding water to the 
 solutions, they both turn pink. The reverse change : pink 
 -* blue is observed when a concentrated hydrochloric 
 acid solution is added to a pink-colored aqueous solution 
 of cobalt chloride. 
 
 The original idea of Ostwald, that the change in color 
 in this case is solely due to a change in dissociation, ac- 
 cording to the equation : 
 
 CoCl, ^ Co" -f 2C1', 
 
 must be abandoned in the light of recent investigations. 2 
 It is highly probable that in this and similar other cases 
 the change in color is mainly caused by hydratation of the 
 salts, dissociation being of minor importance. 
 
 1 Noyes and Blanchard, 1. c. p. 727. 
 
 2 cf. Bottger, Qualitative Analyse, 3 Aufl. p. 223, (1913). Hantzsch, Zeit- 
 schr. f. anorg. Chem., 73, p. 309, (1912). 
 
CHAPTER VIII. 
 
 SOLUBILITY AND ITS CHANGES, 
 
 In this chapter, which needs no special description of 
 apparatus, most experiments being conveniently carried 
 out in large test-tubes, only a few suitable combinations 
 of liquids and solids are given, in order to illustrate the 
 more common cases of solubility. These may be divided 
 in three groups, demonstrating : 
 
 I. Solubility of liquids. 
 II. Solubility of one solute in two solvents. 
 III. Solubility of salts in water. 
 
 I. Solubility of Liquids. 
 
 122. A case of miscibility in all proportions is that of 
 alcohol and water (consolute liquids). 
 
 123. Partially miscible are water and butyl alcohol. 
 Equal volumes of both liquids are shaken in a test-tube 
 and equilibrium established at 25. On inserting the 
 tube in a beaker of hot water (about 40) without any 
 further shaking, the lower layer, being an aqueous solu- 
 tion of butyl alcohol, becomes turbid. On putting the 
 tube in a beaker of cold water (about 10) the reverse 
 happens: the lower layer clears up and the supernatant 
 liquid turns milky by the separation of water drops. 1 
 
 124. A mixture of 36 per cent, phenol and 64 per cent, 
 water forms a two-layer system, but on raising the tem- 
 perature the mutual solubility increases, so that at 68.8, 
 
 1 Miiller and Abegg, Zeitschr. f. Electrochemie, n, p. 3, (1905). 
 
SOLUBILITY AND ITS CHANGES IOO, 
 
 where the "upper" critical temperature of solution is 
 reached, a homogeneous liquid forms. 1 
 
 125. A "lower" critical temperature of solution is ex- 
 hibited at 1 8.6, by a mixture of 52 per cent tri-ethyl 
 amine and 48 per cent, water. 2 On account of the sharp 
 odor of the amine the mixture should be kept in a sealed 
 tube. Newth 3 recommends this couple as an extremely 
 sharp indicator for slight changes in temperature, a low- 
 ering of the temperature below 18.6 being immediately 
 shown by a milky appearance, followed by the formation 
 of two layers from the homogeneous liquid. 
 
 126. Nicotine and water is a most interesting combina- 
 tion because it is characterized by the occurrence of an 
 upper and lower critical temperature of solubility at 210 
 and 61 respectively for a mixture of 32 per cent, nicotine 
 and 68 per cent, water.* This may be shown by heating 
 the mixture in a small strong well sealed glass tube (in- 
 ner bore 0.5 centimeter, length 10 centimeters) taking 
 proper precautions in case the tube should crack. If 
 the experiment is to be shown to a large audience, a 
 projection apparatus may be used. 
 
 II. Solubility of One Solute in Two Solvents. 
 
 127. The relative decrease in solubility of one solvent 
 as compared with a second by the addition of a solid, 
 soluble in both solvents, may be demonstrated by the 
 couple water and ethyl ether on adding naphthalene. For 
 
 1 Rothmund, Zeitschr. f. phys. Chem., 26, p. 452, (1898). 
 
 2 Rothmund, Ibidem, p 459. 
 
 s Chemical lecture experiments, Condon, p. 69, (1899). 
 * Hudson, Zeitschr. f. phys. Chem., 47, p. 113, (1904). 
 
IIO DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 this experiment a flask with a long calibrated bulb- 
 neck, as devised by Tolloczko, 1 is used (Fig. 52). The 
 flask is filled to the mark (a) with water 
 saturated with ether, and then enough ether 
 saturated with water is added to fill the 
 neck up to the mark ( b ) . In order to get a 
 visible boundary line, the ether is slightly 
 colored with a dye, insoluble in water. On 
 pouring successively equal weights (0.25 
 gram) of naphthalene into the flask, it will 
 be found, that each time after vigorous 
 shaking , the boundary is displaced down- 
 ward for about the same number of scale 
 divisions, showing that the addition of 
 naphthalene causes a decrease of the solu- 
 bility of ether in water proportional to the 
 Fig. 52. weight of the dissolved solid. 
 
 128. The solubility of a solid may be decreased by the 
 addition of another solvent which is consolute with the 
 first, like water and ethyl alcohol. On adding alcohol to 
 a saturated solution of cane sugar in water, part of the 
 sugar is precipitated. 
 
 129. A homogeneous mixture of two, wholly or parti- 
 ally, miscible liquids may be separated into two layers 
 by the addition of a salt. Thus, on adding some solid 
 potassium sulphate to an aqueous solution of phenol, the 
 latter separates in the form of a milky emulsion which 
 changes into a liquid layer after a while. This is the 
 process, well-known in organic chemistry, of "salting 
 
 i Zeitschr. f. phys. Chetn., 20, p. 389, (1896). 
 
SOUJBIUTY AND ITS CHANGES III 
 
 out." It is by this method, that compounds like alcohol, 
 aceton, etc., which are very soluble in water, are easily 
 separated out. 
 
 130. As an exception to the rule that the solubility of 
 a non-electrolyte in water, diminishes by the addition of a 
 salt, the case of iodine and water may be noted. To a 
 saturated solution of iodine in water, with an excess of 
 iodine at the bottom of the test-tube, some crystals of 
 potassium iodide are added. On vigorously shaking the 
 mixture, the iodine goes in solution, forming as is well 
 known a dark brown solution with the salt. The in- 
 crease in solubility is accounted for by the formation of 
 complex ions. 1 
 
 m. Solubility of Salts in Water. 
 
 131. The solubility of potassium nitrate increases very 
 rapidly as the temperature rises, while that of sodium 
 chloride is nearly independent of the temperature; a 
 decrease of solubility on raising the temperature occurs 
 in the case of lithium sulphate. The heat absorption 
 and the resulting fall in temperature on dissolving the 
 potassium nitrate and conversely the rise in temperature, 
 due to heat evolution, when lithium sulphate is dissolved 
 (at room temperature) may be demonstrated in addition, 
 with the aid of a large air or liquid thermometer, as used 
 for lecture experiments. 
 
 132. A decrease in the solubility of salts is generally 
 observed on addition of salts with common ions. 
 
 Thus addition of concentrated hydrochloric acid to sat- 
 
 1 I^eblanc and Noyes, Zeitschr. f. phys. Chem., 6, p. 401, (1890). 
 
112 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 urated solutions of sodium chloride, potassium chloride, 
 barium chloride, etc., causes an immediate precipitation 
 of the salt in question. 1 
 
 In the same way lead chloride is precipitated from its 
 saturated solution by the addition of concentrated sodium 
 chloride solution. 2 
 
 Nernst was the first who showed that an excess of 
 either the anion or the cation of the salt in solution causes 
 precipitation from its saturated solution. He found that 
 an addition of concentrated potassium hydroxide solution 
 or potassium chloride solution to 10 cc. of a saturated 
 solution of potassium chlorate gave in the former case 
 directly, in the latter after a few minutes, an abundant 
 precipitate of potassium chlorate. The same result was 
 obtained by the addition of a very concentrated solution 
 of sodium chlorate. 3 
 
 133. The precipitation of silver acetate from its satur- 
 ated solution by the addition of either silver nitrate or 
 sodium acetate solution is another instance, described in 
 detail by A. A. Noyes and Blanchard. 4 
 
 Five hundred cubic centimeters of a saturated solution 
 of silver acetate is prepared by shaking an excess of the 
 salt with warm water, cooling and filtering. In each of 
 two lecture jars is placed 200 cc. of this solution and then 
 5 cc. of a 4N. solution of sodium acetate added to one jar 
 and 5 cc. of a 4N solution of silver nitrate to the second. 
 On vigorously stirring for a few moments, in both jars a 
 feathery, crystalline precipitate is produced. 
 
 1 Engel, Ann. de chim. et phys., (6) 13, p. 132, (1888). 
 2 Kiister I.e. p. in. 
 
 3 Nernst, Zeitschr. f. phys. Chem., 4, p. 372, (1887). 
 M. c. p. 750. 
 
SOUJBIUTY AND ITS CHANGES 113 
 
 134. However convincing these experiments may be as 
 viewed from the theory given by Nernst, more recent in- 
 vestigations by A. A. Noyes and his pupils have proved 
 that the principle of the constancy of the solubility- 
 product and of the concentration of the non-ionized salt 
 in the saturated solution as derived from the law of 
 mass-action, cannot be universally applied. To cite one 
 example : the solubility of thallous sulphate (T1 2 SO 4 ) is 
 increased by the addition of solutions of sodium sulphate, 
 sodium hydrosulphate and even more of sulphuric acid. 1 
 Some exceptions find a suitable explanation in the as- 
 sumption of the formation of complex ions, e. g. by 
 bringing together solutions of potassium nitrate and lead 
 nitrate. 
 
 135. An increase in solubility of salts, hydroxides and 
 oxides, that are slightly soluble, is usually observed, in 
 accordance with a rule, formulated by A. A. Noyes, 2 on 
 the addition of salts without a common ion, and especi- 
 ally when there is an opportunity for forming non- (or 
 slightly) ionized salts by double decomposition. 
 
 Thus mercuric oxide, which is hardly soluble in water, 
 goes into solution by shaking vigorously with an alkali 
 chloride solution and, still better, on applying a solution 
 of potassium cyanide, mercuric cyanide being scarcely 
 ionized. 3 
 
 The case, well-known in analytical chemistry, of mag- 
 nesium hydroxide, dissolving on the addition of ammon- 
 ium salts is another example belonging to this class. 4 
 
 1 Harkins, Tourn. Am. Chem. Soc., 33, p. 1836, (1911). 
 
 2 Zeitschr. f. phys. Chem., 6, p. 262, (1890). 
 * Bersch, Ibidem, 8. p. 383, (1891). 
 
 4 I^oven, Zeitschr. f. anorg. Chem., 11, p. 404, (1896). 
 
CHAPTER IX. 
 
 COLLOIDS AND ADSORPTION. 
 
 The importance of colloid chemistry need not be em- 
 phasized, since it has been recognized that in nature 
 crystalloid behavior is the exception, colloid behavior the 
 rule. Since the time when Thomas Graham, the father 
 of colloid chemistry, made his famous discovery which 
 led to the distinction of crystalloids and colloids 1 (1861), 
 this subject has been invariably dealt with in almost every 
 textbook of inorganic chemistry in the chapter on silicon, 
 as if the colloid state was primarily characteristic of this 
 substance. During the past 20 years, however, it has 
 been realized that colloid properties are not connected 
 with some definite chemical composition, but that all ma- 
 terials may occur in the colloid state. Chemistry is largely 
 indebted to Wolfgang Ostwald, the editor of the Kolloid- 
 Zeitschrift, for the assumption of the universality of the 
 colloid state and the recognition of the essential independ- 
 ence of the latter in regard to chemical composition. The 
 chief characteristic of a colloid substance lies in its 
 specific surface (expressed by the quotient of absolute 
 surface by the total volume of the disperse phase, accord- 
 ing to W Ostwald). It is therefore, readily understood, 
 that ad-(ab) sorption is intimately connected with colloids 
 and for this reason is included in this chapter. Speaking 
 of colloids in general, colloid solutions are usually meant, 
 as distinguished from colloid precipitates or gels. In 
 the following experiments colloid solutions only will be 
 
 i Phil. Trans. 151, p. 184, (1861). 
 
COLLOIDS AND ADSORPTION 115 
 
 considered, and from the latter only colloids, in which 
 water is the dispersion medium, while the finely sub- 
 divided discontinuous phase, the disperse phase is 
 either a solid or a liquid (perhaps of complex nature). 
 The whole system is conveniently called : dispersoid. 
 
 This leads to the following rough classification: 
 (W Ostwald.) 1 
 
 Dispersion medium (WATER) 
 
 
 V 
 Disperse phase : SOLID 
 
 I 
 V 
 
 1 MOLECULAR 
 
 DISPERSOIDS 
 
 (crystalloids. Graham, > 
 size about i /u/u. or less 
 specific surface >io 7 . ; 
 
 2 SUSPENSOIDS 
 
 (colloid solutions) 
 
 A A 
 ' N ' > 
 
 V 
 Disperse phase : LIQUID 
 
 ( or a complex 
 phase=mixture of 
 mutually soluble 
 compounds). W. 
 Ostwald. 
 
 V 
 
 MOLECULAR 
 
 DISPERSOIDS 
 
 (consolute liquids) 
 
 2 EMULSOIDS 
 
 (colloid solutions) 
 
 3 SUSPENSIONS 
 
 (true dispersions, size of\ .S 4; 
 
 particles >O.I/IA, specific I _O j-i 
 
 surface <6.io& ^ 3 a? 
 
 O Q 
 
 3 EMULSIONS 
 
 (true dispersions) 
 
 As will be seen from this table colloid solutions (sus- 
 pensoids and emulsoids) occupy an intermediate position, 
 
 1 Handbook of Colloid Chemistry, Americ. ed., by M. Fischer (1915), p. 33. 
 
Il6 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 although in reality no sharp boundary line can be drawn 
 between colloid solutions and "real" solutions on one 
 side or between colloid solutions and suspensions or emul- 
 sions on the other side. There is, however, a rather wide 
 gap between typical emulsoids and typical suspensoids, 
 although it must be admitted that transitions between 
 these two groups have been observed in a number of 
 cases. 1 It has been found for instance that some chemi- 
 cal substances like soaps, many dyes, etc., form emuls- 
 oids in water and suspensoids (or molecularly dispersed 
 solutions) in alcohol. Likewise some hydroxides, especi- 
 ally those of iron behave like suspensoids in dilute aqueous 
 solutions and like emulsoids in concentrated solutions. 
 
 The experiments, described below, are subdivided for 
 the sake of convenience into the following six groups : 
 
 A. Preparation of suspensoids. 
 
 B. Preparation of emulsoids. 
 
 C. Mechanical properties of dispersoids. 
 
 D. Optical properties of dispersoids. 
 
 E. Electrical properties of dispersoids. 
 
 F. Adsorption. 
 
 A. Preparation of Suspensoids. 
 
 The methods for preparing suspensoids fall in two 
 classes : electrical and (physico-) chemical. 
 
 136. The electrical method of direct disintegration 
 was first introduced by G. Bredig 2 and later successfully 
 applied with a more elaborate arrangement by Th. 
 Svedberg. 3 
 
 1 Ostwald-Fischer, 1. c. pp. 44, 45, 55, 56, 136, 147. 
 
 2 Zeitschr f. phys. Chem., 31, p. 258, (1899). 
 
 3 Ber. d. chetn. Ges., 38, p. 3616, (1905) ; 39, p. 1705, (1906). 
 
COLLOIDS AND ADSORPTION 
 
 117 
 
 Following Bredig's directions, two short wires of 
 the metal to be dispersed (usually platinum, gold 
 or silver), 1-3 millimeters in cross-section, are attached 
 to stout copper wires. Each wire is insulated by slipping 
 small capillary glass tubes over it, leaving a free end of 
 i centimeter. Both are connected with the terminals of 
 a no- volt direct lighting circuit, having a suitable re- 
 sistance (a 20 ohm rheostat or a 32 candle-power lamp) 
 in series, in order to secure a current of 4-5 ampere. 
 
 Fig. 53. 
 
 Pure distilled water (to which a trace of hydrochloric 
 acid may be added) is then placed in a crystallizing dish, 
 10 centimeters in diameter, cooled by ice water in a larger 
 surrounding dish. (Fig. 53.) The ends of both wires 
 are then dipped in the water, brought in contact and im- 
 mediately separated 1-2 millimeters, so as to form an 
 electric arc in the water. One of the wires is suitably 
 fastened to a clamp stand ; the glass-insulated part of the 
 other is grasped in the hand. The arc is maintained for 
 about 10 minutes, taking care, each time that the arc dis- 
 appears, to unite and separate the ends of the wires, or 
 
Il8 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 in case of fusing together, to re-form the required dis- 
 tance. The solution, thus obtained is filtered and kept in 
 a stoppered bottle. Its color is greenish-brown for silver, 
 red for gold and black for platinum. 
 
 Chemical methods include reductions, double decom- 
 positions, hydrolysis (being a special case of the preced- 
 ing) peptizations and dilutions. Reduction-methods are, 
 like the disintegration-method, chiefly confined to the 
 noble metals. As reducing agents yellow phosphorus 
 (Faraday, 1857), formaldehyde (Zsigmondy, 1898), hy- 
 droxylamine (Gutbier), phenyl-hydrazine and other, 
 mostly organic, reducing compounds have been used. 
 
 The following preparations of gold- and silver-hydro- 
 sols are easily made by the reduction method. 1 
 
 137. Gold-hydrosol. Four cc. of a I per cent solution of 
 commercial gold chloride are diluted with 100 cc. of dis- 
 tilled water. A solution of 2 grams tannin (acidum 
 tannicum purissimum) in 100 cc. of distilled water serves 
 as a reducing agent. By mixing three parts of the latter 
 solution with one part of the dilute gold chloride solu- 
 tion a blue gold-hydrosol results. By taking equal parts 
 a ruby-red hydrosol is formed. The sols are fairly 
 stable. 
 
 138. Silver-hydrosols are less stable. A useful prep- 
 aration, however, is obtained in the following manner. 
 To 5 cc. of a i per cent, solution of silver nitrate is added, 
 drop by drop, a dilute ammonia solution, until the first 
 formed precipitate exactly disappears, and then diluted 
 with distilled water to a 100 cc. By mixing equal volumes 
 
 1 Hatschek, Physics and Chemistry of Colloids, Condon, p. 8, (1913). 
 
COLLOIDS AND ADSORPTION IIQ 
 
 of this solution and the above mentioned tannin solution 
 a clear and transparent, brown silver-hydrosol results, 
 which sometimes shows a green color in reflected light. 
 
 The third method to be considered is the process of 
 double decomposition. This has to be carried out in the 
 absence of electrolytes, the latter having the tendency to 
 precipitate the suspension colloids. Therefore only very 
 dilute solutions can be used, so that the small quantity of 
 electrolyte, if formed in the reaction, will not do any 
 harm ; on the contrary it is well-known that traces of elec- 
 trolytes increase the stability of suspensoids. 
 
 For lecture experiments the following preparations of 
 this type may be performed. 
 
 139. By mixing equal volumes of N/5O ferric chloride 
 solution and N/5O potassium ferrocyanide a colloid 
 solution of prussian blue is obtained, 1 which is so dense in 
 color, that it is only transparent in thin layers. 
 
 140. Two hundred cubic centimeters of a I per cent, so- 
 lution of arsenious oxide (As 2 O 3 ), (prepared by boiling 
 water, containing 12 grams of the oxide, cooling and ni- 
 trating the solution) are mixed with 200 cc. of a satur- 
 ated solution of hydrogen sulphide. A turbid yellow 
 solution is formed, which can be filtered through a folded 
 filter. 2 
 
 141. Two hundred cubic centimeters of a one-eighth 
 molar solution of mercuric cyanide and an equal volume 
 of a saturated hydrogen sulphide solution are simultane- 
 
 1 A. A. Noyes, Journ. Am. Chem. Soc., 27, 85104, p. 93, (1905). 
 
 2 Noyes, 1. c. p. 93. 
 
 9 
 
120 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 ously poured in a beaker. The resulting black liquid 
 passes almost completely through a folded filter. 1 
 
 142. Two grams of tartar emetic are dissolved in 100 
 cc. of water and mixed with an equal volume of the com- 
 mon strong ammonium sulphide solution diluted to one- 
 twenty-fifth of its strength. The color changes gradually 
 into orange-yellow by the formation of an antimony sul- 
 phide sol. 2 
 
 143. Hydrolysis of salts affords another means for 
 preparing suspensoids. Thus a dilute iron hydroxide 
 sol, which behaves like a sttspensoid , of great stability, 
 is obtained by heating 500 cc. of water to boiling in a 
 large Erlenmeyer flask or a 800 cc. beaker, and adding to 
 the boiling water 5 cc. of a 33 per cent, solution of ferric 
 chloride. A clear sol of a beautiful reddish-brown color 
 is formed. 3 
 
 144. Peptization, a term, first suggested by Graham 
 and meaning an increase in the degree of dispersion is 
 the reverse process of coagulation. It may be shown in 
 the preparation of cadmium sulphide sol. The sulphide 
 is precipitated by hydrogen sulphide from an ammoniacal 
 solution of cadmium sulphate, the precipitate thoroughly 
 washed and suspended in distilled water. Thus far the 
 experiment may be completed before demonstrating the 
 change from a suspension into a suspensoid. By passing 
 hydrogen sulphide through the suspension, the latter be- 
 
 1 Noyes, 1. c. p. 93. 
 
 2 Hatschek, 1. c. p. 8. 
 s Hatschek, 1. c. p. 8. 
 
COLLOIDS AND ADSORPTION 121 
 
 comes milky and turbid, until finally it changes into a 
 clear, transparent suspensoid of a beautiful gold color. 1 
 
 145. The dilution process consists in pouring a few 
 drops of a non-aqueous solution of the substance to be dis- 
 persed in a large excess of water. Thus colloid solu- 
 tions of sulphur and selenium of great stability are 
 formed by dissolving pulverized sulphur and selenium 
 (either the red amorphous or the greyish crystalline modi- 
 fication) in a few cubic centimeters of hydrazine hydrate 
 and pouring 2 or 3 drops of the dark viscous liquid in 
 several liters of water. In this way an intensely red 
 colloid solution of selenium and a yellowish white col- 
 loid solution of sulphur are obtained. 2 
 
 146. In an analogous manner suspensions of mastic, 
 gutta percha, etc., as were used by Perrin, Henry and 
 others in their study of the Brownian movement may be 
 obtained by pouring a few drops of an alcoholic solution 
 into an excess of water. 
 
 B. Preparation of Emulsoids. 
 
 Examples of this class of colloids are the proteins (like 
 egg-albumen), gelatin, agar-agar, starch dextrin, many 
 gums, silicic acid, stannic acid, many hydroxides in 
 concentrated solution (iron for instance) dye-stuffs (like 
 night-blue, benzo-purpurin, azo-blue, etc.). The prep- 
 aration of these colloids does not require any special de- 
 scription. 
 
 147. As characteristic lecture demonstration types, be- 
 sides the classical silicic acid usually gelatin or agar-agar 
 
 1 Prost, Bull, de 1'Ac. des sc. de Brux., (3) 14, p. 312, (1887). 
 - Meyer, Ber. d. chem. Ges. 46, p. 3089, (1913). 
 
122 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 solutions (containing 1-5 grams in 100 cc. of water) are 
 selected. 
 
 C. Mechanical Properties of Dispersoids. 
 Among these will be described : 
 
 I. Diffusion. 
 II. Dialysis. 
 
 III. Viscosity. 
 
 IV. Surface tension. 
 
 I. DIFFUSION EXPERIMENTS. 
 
 148. An easy method for distinguishing between true 
 solutions and colloid solutions is based on diffusion. W 
 Ostwald 1 uses 5 per cent, gelatin solutions or 2 per cent, 
 agar-agar solutions, which, while hot, are poured in test- 
 tubes, until these are half-way rilled and then allowed to 
 congeal. Care must be taken that the gelatin and agar- 
 agar are thoroughly washed and purified. The solid layers 
 are covered with safranin and congo-red solutions re- 
 spectively. The result of the diffusion in both tubes is 
 clearly visible after 24 hours. The congo-red, being a 
 colloid has only slightly spread into the jelly, while the 
 safranin which forms a true solution has passed into it 
 for a considerable distance so that the originally sharp 
 boundary surface is hardly visible. 
 
 149. In a similar way Noyes 2 shows this difference in 
 speed of diffusion between crystalloid and colloid solu- 
 tions. Two cylindrical sticks of agar-jelly, 5 centimeters 
 in diameter and 15 centimeters in height are prepared by 
 
 1 Ostwald-Fischer, 1. c. p. 9. 
 
 2 Noyes, 1. c. p. 90. 
 
COLLOIDS AND ADSORPTION 123 
 
 pouring a hot 4 per cent, solution of agar-agar into large 
 glass tubes, corked at one end. When cold, the solid 
 sticks are pushed out and placed in two lecture jars of 
 which one is half-way filled with a nearly saturated solu- 
 tion of copper sulphate, treated with enough ammonium 
 hydroxide to redissolve the precipitate first formed, 
 while the other contains a colloid solution of prussian 
 blue, made by mixing equal volumes of N/5O solutions 
 of ferric chloride and potassium ferrocyanide. After 
 the diffusion has been in progress for two days, the re- 
 sult is shown to the audience by removing the sticks and 
 cutting them in two. The blue copper solution has pene- 
 trated the stick uniformly to the very center, while the 
 prussian blue has not entered into the stick over any 
 perceptible distance, thus proving, that colloid solu- 
 tions practically do not diffuse at all. Accurate measure- 
 ments by Voigtlander 1 have brought out that the diffusion 
 of crystalloids is not appreciably influenced by jellies, 
 provided these are not present in greater percentage than 
 3-5 per cent. 
 
 II. DIALYSIS EXPERIMENTS. 
 
 The fact established by the foregoing experiments, 
 that one colloid (the solid jellies being nothing else but 
 gelatinized emulsoids) is practically impermeable by an- 
 other, leads to their recognition as dialyzing membranes. 
 Dialysis therefore, is intimately connected with diffusion. 
 Every substance, which does not diffuse appreciably, but 
 allows itself the passage of crystalloids may be used as a 
 dialyzer, and inversely we might say "that any mem- 
 
 1 Zeitschr. f. phys. Chem., 3, p. 316, (1889). 
 
124 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 brane, which permits the passage of a crystalloid and hin- 
 ders the passage of a colloid, is itself a colloid." 1 
 
 Such membranes are parchment, (first used by Graham 
 in 1861) fish-bladder, urinary bladder, egg-membrane, 
 parchment paper and collodion film (in sheet or tube 
 form) as introduced by Malfitano (1904). 
 
 The process of dialysis requires too much time to show 
 marked results in the course of one lecture hour. Be- 
 sides the classical Graham dialyzer some modern types 
 of dialyzer in tube form (made of parchment paper or 
 collodion) and the new "star-dialyzer," devised by 
 Zsigmondy and Heyer 8 may be demonstrated. 
 
 150. To show the extreme facility, with which crystal- 
 loids pass through a parchment membrane, a solution of 
 potassium thiocyanate (KCNS) is put inside a flat Gra- 
 ham dialyzer, which is then left floating in distilled 
 water for I or 2 minutes. On adding ferric chloride to 
 the outside water a red color appears, demonstrating 
 that the inside crystalloid passes readily through the 
 membrane. 3 Placing a colloid solution of prussian blue 
 into a second parchment dialyzer, no perceptible trace of 
 a blue coloration is seen in the outside water, not even 
 after several hours. 
 
 151. The difference in efficiency in using different 
 membranes may be conveniently illustrated 4 by pouring 
 a moderately concentrated solution of fluorescem into a 
 parchment paper tube and into a similar collodion tube. 
 
 1 Bigelow, 1. c. p. 246. 
 
 2 Zeitschr. f. anorg. Chem., 68, p. 169, (1910). 
 
 3 Bigelow, 1. c. p. 244. 
 
 4 Zsigmondy, Kolloidchemie, Leipzig, p. 33 (1912). 
 
COI^OIDS AND ADSORPTION 
 
 125 
 
 Both tubes are inserted in large lecture jars, filled with 
 tap water. The dye-stuff diffuses after a short time 
 through the collodion membrane, as is shown by the rapid 
 appearance of green fluorescent bands in the outside 
 water, while it takes a considerable 
 time to pass through the parchment 
 membrane. 
 
 152. The collodion tubes, as used 
 by many colloid-workers (Malfitano, 
 Henry, Duclaux, Biltz, Bigelow and 
 others) are made as is easily demon- 
 strated in the lecture by sticking 
 large well-cleaned bulb test-tubes 
 (Fig. 54) into solutions of collodion 
 in ether, ether and alcohol, or acetic 
 acid and water, allowing the layer to 
 harden in the air and repeating the 
 process two or three times if neces- 
 sary, finally hardening the whole by 
 washing in water. The collodion 
 coating is then cut off in the middle of 
 the bulb and carefully stripped off. 
 Details may be looked up in Bigelow 
 and Gemberling's article 1 on "collod- 
 ion membranes." 
 
 Fig. 54. 
 
 III. VISCOSITY EXPERIMENTS. 
 
 153. The viscosity of suspensoids does not perceptibly 
 differ from that of pure water ; the viscosity of emulsoids 
 on the other hand, even at small concentration is much 
 
 Trmrn A tn 
 
 (TOO7V 
 
126 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 greater than that of water and increases rapidly on cool- 
 ing, while suspensoids do not assume an oily or even a 
 gelatinous appearance on lowering the temperature. This 
 is most readily shown 1 by allowing 10 cc. of a 2 per cent, 
 gelatin solution (an emulsoid) and 10 cc. of colloid ar- 
 senious sulphide (a suspensoid) to flow simultaneously 
 from two 10 cc. pipettes with capillary tips which are as 
 nearly alike as possible. The time of outflow, which is 
 roughly speaking directly proportional to the fluidity 
 or inversely proportional to the viscosity, is much longer 
 for the emulsoid solution. Test-tubes with both solu- 
 tions, cooled in ice water, show a marked difference; 
 the gelatin solution changes into a thick jelly; the sus- 
 pensoid does not gelatinize at all. 
 
 IV. SURFACE; TENSION. 
 
 154. From careful measurements it has been deduced, 
 that coarse suspensions and suspensoids hardly alter the 
 surface tension (against air) of the dispersion medium 
 (water) ; emulsions and emulsoids on the other hand de- 
 crease the surface tension of their dispersion medium. 
 This difference, therefore, can be used for discriminating 
 between both classes of colloids. 2 The decrease of sur- 
 face tension is manifested by the more or less easy forma- 
 tion of foam. Thus by shaking two glass-stoppered bot- 
 tles, containing arsenious sulphide sol and a dilute solu- 
 tion of Venetian soap (or egg albumen) respectively, only 
 in the latter case an abundant foam formation is ob- 
 
 1 Ostwald-Fischer, 1. c. p. 13. 
 - Ostwald-Fischer, 1. c. p. 183. 
 
COLLOIDS AND ADSORPTION 
 
 127 
 
 served. Instead of shaking the liquids, an indifferent 
 gas, like nitrogen or air may be bubbled through. 
 
 155. Though little is known about the conditions of 
 stability for emulsoids, it can be shown that by lowering 
 the surface tension of two un-miscible liquids against 
 each other, an emulsion is readily obtained. 1 In a beaker 
 with water a thin layer of olive oil is poured. By stirring 
 the mixture, an emulsion 
 
 forms, which disappears rap- 
 idly. If, however, a few 
 drops of potassium or sodium 
 hydroxide are added, a milky 
 emulsion is formed on stir- 
 ring, which does not notice- 
 ably change, even after sev- 
 eral hours. 
 
 156. This decrease in sur- 
 face, (or better, boundary-) 
 tension can be followed on a 
 measuring scale with the aid 
 of Donnan's pipette and is 
 most suitably made visible to 
 a large audience by projec- 
 tion on the screen. 2 
 
 The pipette, used for this 
 purpose (Fig. 55), is pro- 
 vided with a stopcock and a 
 capillary outflow, bent upward. It is filled with olive 
 
 1 Donnan, Zeitschr. f. phys. Chem., 31, p. 42, (1899). 
 
 2 Donnan, 1. c. p. 42, cf. Hatschek, 1. c. p. 38 ; and Kruyt, Chern. Weekbl., 
 10, P-53, (1913)- 
 
 Fig. 55. 
 
128 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 oil (containing free fatty acid) or paraffin oil, to 
 which a small amount of fatty acid (palmitic or stearic 
 acid) has been added. By carefully opening the stop- 
 cock, the oil is allowed to slowly escape in water w r ith 
 the formation of well-shaped spherical drops which rise 
 to the surface (10-20 drops in a minute). If now the 
 water is replaced by a solution containing a few drops 
 of a base (sodium or potassium hydroxide), the number 
 of oil drops, formed in one minute is more than doubled 
 or even changes into a continuous stream of small drop- 
 lets, the rapid succession not allowing the number of 
 drops to be counted. 
 
 Both phenomena may be explained on the basis of 
 Willard Gibbs' theorem, stating that substances, which 
 lower the surface tension of the dispersion medium, tend 
 to collect in its surface. We must, therefore, assume, 
 that the emulsification is caused by the strong superficial 
 adhesion of the soap formed by the oil. 
 
 D. Optical Properties of Dispersoids. 
 
 157. Differences between true solutions and dispersoids 
 become visible by exposing the liquids to be examined to 
 the light of a powerful incandescent lamp (arc-light), or 
 still better, to a beam of sunlight entering the darkened 
 lecture room through a hole in the window shutter 
 Usually a condenser with diaphragm is used to concen- 
 trate the light on the liquid, contained in a beaker or a 
 Dewar tube (in case hot or very cold liquids are tested). 
 The heterogeneity of colloid solutions is then easily rec- 
 ognized by a more or less opaque cone of light, caused 
 
COLLOIDS AND ADSORPTION 129 
 
 by the diffuse reflection of light from the discrete par- 
 ticles present in the liquid. This is the so-called "Tyndall 
 phenomenon." It can be differentiated from fluor- 
 escence by its property of being polarized. Looking at 
 the cone through a Nicol prism the cone disappears, 
 when the prism is turned around its axis over a certain 
 angle. Suitable demonstration liquids are: a ferric 
 chloride solution, a dilute ferric hydroxide sol (prepara- 
 tion, see p. 120), having the same color, a gold sol (pre- 
 paration, see p. 118) and an arsenious sulphide sol. In 
 the case of the ferric chloride solution the cone is hardly 
 visible, in the other cases Tyndall cones of slightly differ- 
 ent turbidity are observed. 1 
 
 It may be remarked here, that some crystalloid solu- 
 tions, as for instance sugar, show a faint turbidity, on ap- 
 plying the Tyndall test. This test, therefore, is not con- 
 clusive in dubious cases. 
 
 E. Electrical Properties of Dispersoids. 
 
 The most striking reactions shown by colloid solutions 
 are those connected with their electric behavior. The 
 fact that many substances in colloid solution assume an 
 electric charge towards the dispersion medium may be 
 illustrated by migration experiments. Taking as ex- 
 amples two typical suspensoids of opposite character such 
 as silver sol and ferric hydroxide sol. With these two 
 colloids (preparation see pp. 117 and 120), suitably 
 dialyzed before use, the following migration experiment 
 may be performed. 2 
 
 1 Noyes, 1. c. p. 96. 
 
 * Forster, die chemische Industrie, 28, p. 733, (1905). 
 
I3O DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 158. A U-shaped tube (length of limbs about 10 centi- 
 meters; diameter 1-1.5 centimeters) is half filled with 
 silver sol and covered in both limbs by a o.oi per cent, 
 sodium hydroxide solution, in order to increase the stabil- 
 ity of the sol. A ( second tube is filled with ferric hy- 
 droxide sol covered by a o.oi per cent, sodium acetate so- 
 lution. For these and similar experiments U-tubes with 
 two stopcocks, having the same bore as the inner cross- 
 section of the tube, as devised by Coehn 1 (Fig. 56), are 
 very useful. These tubes are 
 half filled with the solutions to 
 be used, the stopcocks are then 
 closed, the excess of the liquid 
 poured out, the upper parts of 
 both limbs rinsed with distilled 
 water and filled to the same 
 height with the required solution. 
 Platinum electrodes are then in- 
 serted, the stopcocks opened and 
 the current passed through. In 
 the above mentioned experiment 
 both U-tubes, are connected in 
 series with the terminals of the 
 22O-volt direct current lighting curcuit. After 20-30 
 minutes it will be seen, that the silver sol has moved 
 towards the anode, the iron hydroxide sol towards the 
 cathode. No actual separation on the electrodes occurs, 
 the visible effect being limited to a more or less dense 
 cloud, collecting in the neighborhood of the electrodes. 
 
 Fig. 56. 
 
 1 Zeitschr. f. Electrochemie, 15, p. 653 (1909). 
 
CONOIDS AND ADSORPTION 131 
 
 Disturbing effects like convection currents often affect the 
 phenomenon. 
 
 159. A more elaborate arrangement, insuring very good 
 results, for the demonstration of colloid migration in an 
 electric field sometimes called "electrophoresis" or 
 "cataphoresis," was given by Noyes. 1 
 
 The suspensoids used for this purpose were arsenious 
 sulphide sol and ferric hydroxide sol. The latter had 
 been prepared by adding to a molal ferric chloride solu- 
 tion a molal ammonium carbonate solution until the pre- 
 cipitate on each addition would barely dissolve. Both 
 sols had been dialyzed for a week, first against distilled 
 water and finally against the purest water obtainable 
 (conductivity water). This was done to remove electro- 
 lytes as completely as possible, in order to avoid convec- 
 tion by the heat, produced by the current, causing disturb- 
 ance of the moving surface. Two U-tubes, 15 centimeters 
 in total height with a 3-centimeter bore covered at both 
 ends with goldbeaters' skin, are completely filled by pour- 
 ing the sols through a hole at the bottom of the bend, i 
 centimeter in diameter, closed by slipping a rubber band 
 over it. The limbs of each tube are surrounded by glass 
 tubes of a slightly greater diameter and fitted tightly by 
 means of rubber bands, connecting both glass walls. These 
 tubes extend about 5 centimeters above the goldbeaters' 
 skin and are filled with conductivity water (Fig. 57). 
 After inserting platinum wire electrodes, the tubes are 
 connected in parallel with the terminals of a no-volt (if 
 available 22O-volt) direct current lighting circuit with a 
 
 1 Noyes, 1. c. p. 97. 
 
132 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 copper coulomb-meter in series to indicate the direction 
 of the current. After 5-10 minutes the ferric hydroxide 
 sol is seen moving downward with a sharp boundary sur- 
 face, leaving clear water above, towards the cathode, 
 
 Fig. 57. 
 
 while the arsenious sulphide moves downward towards 
 the anode, thus proving that the former possesses a posi- 
 tive charge, while the latter is negatively charged. 
 
 It should be remembered, that this phenomenon is not 
 limited to suspensoids (and some emulsoids like egg-al- 
 bumen) but is also observed in the case of suspensions 
 
COLLOIDS AND ADSORPTION 
 
 133 
 
 like kaolin, quartz and lamp black. The reverse of this 
 motion is called electrical "endosmosis," and was dis- 
 covered by Reusz (1807). For tne sa ke of completeness 
 two experiments demonstrating this electro-osmosis may 
 be cited. 
 
 160. A plug of cotton is tightly pressed into the bend 
 of a U-tube, and the tube half rilled with distilled water. 1 
 Platinum electrodes are inserted and connected with the 
 terminals of the lighting circuit (direct current). The 
 water is seen moving towards the cathode, as shown by 
 the rise in level on the side of 
 
 the negative electrode. Freund- 
 lich 2 replaces the cotton by a 
 bundle of short capillary tubes, 
 and obtains a like result. 
 
 161. An interesting modifica- 
 tion of the foregoing experiment 
 is the following: An unglazed 
 porcelain plate, covered with dis- 
 tilled water is supported on an 
 iron ring, connected with the 
 negative pole of the lighting cir- 
 cuit (220- volt, direct current), 
 while the positive pole is formed 
 by a lead disk, dipping in the 
 water. (Fig. 58.) As soon as 
 the current is turned on, the 
 
 water is seen dropping from the plate, a flow, which 
 comes to a standstill on disrupting the connection. 
 
 1 Coehn in Muller-Pouillet's Handbook, p. 615. 
 
 2 Kapillarchemie, p. 224, (1909). 
 
 Fig. 58. 
 
134 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 On account of the different behavior in an electric 
 field we distinguish between negative and positive col- 
 loids. 
 
 Examples of the first are, besides arsenious sulphide: 
 antimony sulphide, gold, silver, sulphur, selenium, prus- 
 sian blue, etc. Positive colloids are metallic oxides 
 (iron, aluminium, etc.). * 
 
 162. A simple method to discriminate between these 
 two types of colloids without electrophoresis is based on 
 the capillary analysis, the use of which was introduced 
 about 40 years ago by a Swiss chemist Goppelsroeder, 
 and recently extensively applied in colloid chemistry by 
 Fichter and Sahlbohm. 
 
 The experiment consists in dipping the strips of 
 filter paper into colloid solutions of (i) ferric hydrox- 
 ide and (2) prussian blue. In the latter case the colloid 
 ascends along with the water up the strip of paper over 
 some 10-20 centimeters, depending on the kind of blotting 
 paper used, while the ferric hydroxide shows a marked 
 lag in rising. The dispersion medium (water) rises as 
 high as in the case of the prussian blue sol, but the 
 colloid phase (ferric hydroxide) rises only slightly above 
 the level of the liquid, concentrates then and finally co- 
 agulates. 1 
 
 A third method to determine the charge of an un- 
 known colloid in solution is to test it with two solutions 
 of known character, such as ferric hydroxide sol (+.) 
 and arsenious sulphide sol ( ). 
 
 1 Ostwald-Fischer, 1. c. p. 15. This method has been criticised and con- 
 demned as being " fallacious " by Thomas and Garard, (Journ. Am. Chem. 
 Soc., 40, p. joi, 1918). It is therefore advisable to check the results of the "cap- 
 illary analysis" by at least one other method. 
 
COI^OIDS AND ADSORPTION 135 
 
 This is based on the fact, that colloid solutions of oppo- 
 site charge precipitate each other. If the unknown so- 
 lution is precipitated by arsenious sulphide, it is positive, 
 and in case ferric hydroxide sol is an effective precipitant, 
 the unknown sol is negatively charged. 
 
 163. In the lecture the formation of a precipitate, in 
 bringing together these two typical sols (Fe(OH) 3 and 
 As^Sg) may be carried out. 
 
 About 150 cc. of dialyzed ferric hydroxide sol and 200 
 cc. of dialyzed arsenious sulphide-sol, prepared according 
 to the foregoing directions (see pp. 119 and 120) are 
 simultaneously poured into a lecture jar; a flocculent 
 precipitate is formed, leaving a clear solution above. 1 
 
 164. In order to show the application on a large scale 
 of mutual precipitation of colloid solutions of opposite 
 electric charge, it is interesting to test for this purpose 
 the waste water of some industrial plant, which usually 
 forms a negative colloid solution. 2 The solution is fil- 
 tered to separate particles suspended in the liquid. On 
 addition of the required quantity of a dialyzed colloid 
 solution of ferric hydroxide of known strength, a precipi- 
 tate is formed, which settles after a few minutes, leav- 
 ing a clear solution easily separated by filtration from 
 the precipitate. The quantity of ferric hydroxide sol must 
 correspond with the so-called optimum of precipitation, 3 
 and has to be found out by trial-experiments before the 
 lecture. Taking three different portions of the test so- 
 lution, the second of which represents about the required 
 
 1 Noyes 1. c. p. 101. 
 
 2 Baur, 1. c. p. 104. 
 
 3 Biltz, Ber. d. chem. Ges. 37, 1095 (1904). 
 
 10 
 
136 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 amount, the effect of too much or too little ferric hydrox- 
 ide sol can also be demonstrated. 
 
 165. Two colloid solutions of the same electrical 
 charge do not give a precipitate on mixing. Thus by 
 bringing together 200 cc. of an arsenious sulphide sol 
 and 200 cc. of a gold-sol no precipitate is formed. The 
 gold-sol is previously made by dialyzing a colloid gold 
 solution, obtained by pouring an ethereal solution of dry 
 gold chloride into an aqueous solution of acetylene. 1 
 
 166. Precipitation of colloid solutions is easily brought 
 about by the addition of electrolytes. Here a marked 
 difference between emulsoids and suspensoids must be em- 
 phasized. The latter are most readily precipitated by 
 small quantities of neutral salts, while the former are 
 not precipitated by the addition of salts, unless in exces- 
 sive amounts. 
 
 This is shown by adding 10 cc. of a normal 
 solution of magnesium chloride to each of two test- 
 tubes, half filled with a i per cent, gelatin solution 
 and a colloid solution of arsenious sulphide respec- 
 tively. The first solution is not changed apparently, 
 while the second shows a voluminous precipitate. 2 
 
 167. The influence of the valence of the precipitating 
 ion, as proved by experiments of Freundlicrr and others, 
 is very pronounced; and was demonstrated by Noyes 4 in 
 the following manner : 
 
 1 Noyes, 1. c. p. 101. 
 
 2 Noyes, 1. c. p. 101. 
 
 3 Zeitschr. f. phys Chem., 44, 135, 151, (1903). 
 * Noyes, 1. c. p. 102. 
 
CONOIDS AND ADSORPTION 137 
 
 A colloid solution of (negative) arsenious sulphide 
 is made by mixing equal volumes of a i per cent, solution 
 of arsenious oxide and a saturated hydrogen sulphide 
 solution and filtering the resulting liquid. 
 
 Fifty cubic centimeters of this sol are poured into each 
 of seven conical lecture jars, containing 200 cc. of the 
 following solutions, having in I liter dissolved : 
 
 (1) 0.6 milli-equivalent of A1C1 3 .* 
 
 (2) 1.5 milli-equivalents of MgCl 2 . 
 
 (3) 20.0 milli-equivalents of MgCl,.* 
 
 (4) 60.0 milli-equivalents of NaCl. 
 
 (5) 400.0 milli-equivalents of NaCl* 
 
 (6) 60.0 milli-equivalents of Na 2 SO 4 . 
 
 (7) 400.0 milli-equivalents of Na 2 SO 4 .* 
 
 Coagulation depends here on the cation, the Al*"-ion 
 having the strongest effect and the monovalent Na'-ion 
 the least. The Mg'Mon occupies an intermediate posi- 
 tion. Precipitation only occurs in the cases marked with 
 an asterisk. 
 
 168. Taking a positive colloid like ferric hydroxide 
 sol, it is the anion, that causes precipitation, trivalent 
 anions having the greatest effect, monovalent anions the 
 smallest, insofar that then the greatest quantity of salt is 
 required to bring about precipitation. 
 
 To eight conical lecture jars, containing respectively 
 200 cc. of solutions having in I liter : 
 
 (1) 0.02 milli-equivalent of K 3 (FeCy 6 ). 
 
 (2) o.io milli-equivalent of K 3 (FeCy 6 ).* 
 
 (3) o.io milli-equivalent of Na 2 SO 4 . 
 
138 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 (4) i. 60 milli-equivalents of Na 2 SO 4 .* 
 
 (5) 5.00 milli-equivalents of NaCl. 
 
 (6) 50.00 milli-equivalents of NaCl.* 
 
 (7) 5.00 milli-equivalents of MgCl 2 . 
 
 (8) 50.00 milli-equivalents of MgCl 2 .* 
 
 is added 50 cc. of the ferric hydroxide sol, whereupon 
 precipitation is observed in the cases, marked by an 
 asterisk. 
 
 169. "Protective Colloids." The use of emulsoids in 
 preventing the precipitation of dispersoids is demon- 
 strated as follows: 1 Adding first 200 cc. of a N/5O 
 sodium chloride solution to 200 cc. of a N/5O silver ni- 
 trate solution, containing 5 cc. of strong nitric acid 
 (specific gravity 1.42), a white flocculous precipitate im- 
 mediately forms. 
 
 The experiment is then repeated with equally strong 
 solutions of both salts, containing I per cent, of gelatin 
 dissolved. The mixture becomes opalescent, and the 
 turbidity increases after a while, without forming a pre- 
 cipitate. 
 
 170. The deflocculation of suspensions by the addition 
 of a small amount of acid and the stabilizing effect of 
 hydroxyl-ions are readily demonstrated as follows: 
 
 Ordinary China clay is stirred up in water, so as 
 to form a suspension, which settles out rather quickly, 
 leaving a clear liquid above and a sharply defined sedi- 
 ment below. If, however, a little alkali, or a salt with 
 alkaline reaction is added, it will be observed that the 
 
 1 Noyes, 1. c. p. 91. 
 
CONOIDS AND ADSORPTION 139 
 
 settling takes place much more slowly, the smallest par- 
 ticles not settling out at all, or if so only very gradually. 
 
 171. The mobility of a clay suspension containing a 
 little acid is very much less than that of the same sus- 
 pension with a trace of alkali as may be shown by allow- 
 ing the suspensions (which must be rather concentrated) 
 to flow down an inclined glass plate. 
 
 172. With a suspension of colophony (rosin) the de- 
 flocculation by one drop of acid is a very striking 
 phenomenon. An opaque suspension of a milky ap- 
 pearance is obtained by dissolving 0.5 gram rosin in 10 
 cc. of alcohol and pouring the solution in 90 cc. water. 
 On adding one drop of 5N hydrochloric acid an immedi- 
 ate deflocculation takes place. A small amount of alkali 
 dissolves the flocks with the formation of a soap. 
 
 F. Adsorption. 
 
 Adsorption includes a number of closely related 
 phenomena, sometimes distinguished as (i) adsorption, 
 (2) absorption, occlusion or solution and (3) formation 
 of absorption compounds. A sharp demarcation between 
 these groups is impossible. In some cases, e. g., that of 
 palladium, taking up hydrogen, it is likely that all three 
 phenomena occur. In order to avoid these cumbrous 
 distinctions some authors speak of "sorption." The fol- 
 lowing mostly well-known experiments on sorption or, 
 using the more familiar term adsorption as a general 
 designation on adsorption refer to the condensation of 
 (a) gases, (b) liquids, and (c) dissolved substances on 
 different solids. 
 
140 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 173. Coses. Twenty to thirty cubic centimeters of 
 dry ammonia gas are collected over mercury in a eudiom- 
 eter tube. A small piece of charcoal, preferably co- 
 coanut charcoal, previously heated over a Bunsen flame 
 to expel adsorbed gases, on coming in contact with the 
 gas, immediately takes up several cubic centimeters, 
 thereby causing a considerable rise of the mercury 
 column. 
 
 174. The usefulness of charcoal as a deodorant is 
 demonstrated by passing a slow stream of hydrogen sul- 
 phide, washed with distilled water and dried over granu- 
 lated calcium chloride, through a tube (length 50-100 
 centimeters, diameter 2 centimeters) filled with previously 
 ignited wood charcoal. The tube is connected by means 
 of an~|-shaped delivery tube with a lecture jar containing 
 a lead acetate solution. No blackening is seen. 
 
 175. The fact that adsorption is accompanied by heat 
 evolution, accounts (partly) for the following phenom- 
 enon. A piece of platinum foil, heated over a Bunsen 
 flame, is allowed to cool by turning off. the gas, until the 
 foil is no longer red hot. The gas is then turned on again, 
 causing the platinum to glow stronger and stronger, until 
 finally the gas is relighted. Adsorption and combination 
 heat accumulate here in raising the temperature to the 
 ignition point. 
 
 If adsorption involves heat evolution, lowering of the 
 temperature must increase the quantity of absorbed gas. 
 Numerous experiments by Dewar and others have cor- 
 roborated this conclusion and an ingenious method of 
 creating a high vacuum was based hereon. (See Chap- 
 ter XII.) 
 
COLLOIDS AND ADSORPTION 14! 
 
 176. Liquids. The adsorption of water by charcoal, 
 powdered clay, kaolin, silica and other finely divided ma- 
 terials is illustrated by heating 5-10 grams of the sub- 
 
 nee in a test-tube. Water will be seen to condense 
 nst the upper walls of the tube. 
 
 177. Dissolved Substances. The adsorptive power of 
 amorphous carbon in the form of wood charcoal, bone 
 black, blood charcoal, etc., has been so universally rec- 
 ognized, that only quite recently other equally effective and 
 
 a per substances have come into use. In the sugar in- 
 dustry animal charcoal has been replaced by mixtures 
 of wood meal and Fuller's earth: in the oil industries the 
 last named substance has been lately introduced for de- 
 colorizing oils ; iu purifying potable waters use is made of 
 the flocculent precipitates formed by aluminum and iron 
 salts, etc. The difference in adsorptive power of various 
 adsorbentia may be exemplified by adding to five 2OO cc. 
 Erlenmeyer flasks, each containing 100 cc. of a dilute con- 
 go-red solution (o.i gram in i liter water) : i gram of 
 talcum powder, i -ram of (English) Fuller's earth, I 
 gram of finely divided bone black, 10 cc. of alumina cream 
 and 10 cc. of ferric hydroxide cream respectively. The 
 aluminium and iron hydroxide paste are made by precipi- 
 
 ing dilute solutions of aluminium and ferric chloride 
 with an CXCCSS of ammonia, decanting the supernatant 
 liquid and frequent washing of the flocculent precipitates 
 with distilled water. The pastes should contain in IO cc. 
 about 0.6 O.8 grain of the anhydrous oxides. On heating 
 the flasks over a Bunsen flame until the liquids boil and 
 subsequent filtering, it will be seen that the live filtered so- 
 
142 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lutions show different degrees of decoloration, compared 
 with the original solution. The talcum is only slightly 
 colored, and the shade of color of the liquid is only little 
 lighter than that of the original solution; the Fuller's 
 earth shows a better result, while the solution, treated 
 with bone black retains a faint red color. The fourth 
 and the fifth solution, however, are completely decolor- 
 ized, and the alumina cream precipitate on the filter shows 
 the color of the congo-red very distinctly. Similar re- 
 sults may be obtained by using other organic dye-stuff 
 solutions (e.g. litmus, indigo, etc.). 
 
 178. With filter paper the two following interesting ad- 
 sorption experiments can be performed. A few drops of 
 a barium hydroxide solution are allowed to fall on one 
 spot of a piece of filter paper. At 2-3 centimeters dis- 
 tance from this spot, outside the wet ring is put 0.2-0.5 cc - 
 of a i per cent, alcoholic phenolphthalein solution. The 
 red color does not appear until the wet rings have over- 
 lapped each other over some distance, thus clearly show- 
 ing that the dissolved substances are absorbed by the 
 paper. 1 Therefore, the first 5-10 cc. of a filtrate should 
 be rejected, when solutions of a definite strength are re- 
 quired. 
 
 179. Differences in the degree of adsorption are shown 
 by allowing solutions of hydrochloric acid and barium hy- 
 droxide of the same strength to creep along strips of filter 
 paper, dipped with their lower end into the solutions. 
 After the liquids have been sucked up as high as 5-10 
 centimeters, the wet portions of both strips are tested by 
 
 1 Bigelow, 1. c. p. 241. 
 
AND ADSORPTION 143 
 
 touching at different heights with glass rods, moistened 
 with methyl orange and phenolphthalein respectively. It 
 will be seen, that the base has travelled only one-third as 
 far into the paper as the acid which has gone up almost 
 as far as the water itself. 1 
 
 180. The process of dyeing is largely one of adsorption 
 by the animal or vegetable fiber. On bringing 150 
 grams of woolen yarn into a large beaker, containing 30 
 milligrams of crystal violet in two liters of distilled 
 water, the solution is practically decolorized, the dye-stuff 
 having been completely adsorbed by the wool. 
 
 181. A piece of cotton fabric, dipped in a dilute solu- 
 tion of purified congo-red, which is a direct dyeing cot- 
 tion dye-stuff, no mordant being required for "fixing" 
 the color, takes on a fairly light shade of red color. On 
 adding sodium chloride, or still better Glauber's salt to 
 the solution, and dipping another piece of cotton into the 
 liquid, the fabric takes on a much deeper tinge of red, 
 thus showing the marked effect of salt in driving the 
 color, uniformly distributed (German: "egalisiert") on to 
 the fabric. The dye, being a sodium salt of a complex 
 organic acid (Na 2 C 32 H 22 N 2 S 2 O 6 ), is of colloid nature, 
 and "salted out" within the fibers of the fabric by the inor- 
 ganic salt, thus materially assisting in the process of ad- 
 sorption by the cotton. 
 
 On bringing the fabric in a beaker with hot water, part 
 of the color is lost; the cotton "bleeds." 
 
 182. An interesting phenomenon is the "selective ad- 
 sorption" of fine powders, suspensions and suspensoids. 
 
 1 Ostwald-McGowan, 1. c. p. 229. 
 
144 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 It has been found for instance that, if a solution of po- 
 tassium chloride is shaken with clay and poured on a 
 filter, part of the potassium is missing in the nitrate, 
 while all the chlorine passes through the filter. 1 
 
 Van Bemmelen's well-known experiment, showing the 
 strong selective absorption power, which Fremy's vol- 
 uminous manganese peroxide exerts on potassium sul- 
 phate is a typical instance. 2 
 
 The manganese peroxide is made, according to Fre- 
 my's directions 3 by adding a cold mixture of 150 grams 
 of water and 500 grams of concentrated sulphuric acid 
 to 100 grams of potassium permanganate. The result- 
 ing acid is slowly decomposed, in the course of 2-3 days, 
 with evolution of oxygen. After frequent shaking with 
 fresh quantities of distilled water, a powder results, 
 which when dried in air has the average composition of 
 MnO 2 2H 2 O, and does not impart an acid reaction to 
 water. Twenty grams of the powder are suspended in 
 100 cc. of water and the suspension, mixed with 100 cc. 
 of a normal solution of potassium sulphate (neutral 
 towards litmus), shaken for some time. The man- 
 ganese peroxide is allowed to settle and the supernatant 
 liquid filtered and tested with blue litmus. The solution 
 shows a distinct acid reaction. 
 
 183. An analogous result is obtained, when a sus- 
 pensoid like colloid arsenious sulphide is precipitated 
 by a potassium chloride solution, as was first observed 
 by Whitney and Ober. 4 
 
 1 H. W. Wiley, Agricultural analysis, Vol. i, p. 127, (1906). 
 
 2 Journ. f. prakt. Chemie, N. F. 23, p. 342, (1881). 
 * Comptes rendus 82, p. 1232, (1876). 
 
 4 Journ. Am. Chem. Soc., 23, p. 852, (1901). 
 
CONOIDS AND ADSORPTION 
 
 145 
 
 TO 
 
 On adding a sufficient amount of potassium chloride 
 (20 cc. of a normal solution) to 100 cc. of dialyzed 
 arsenious sulphide sol (with no appreciable acid reac- 
 tion, the sol is precipitated, absorbing the cation, and 
 the supernatant liquid becomes acid. 
 
 184. The selective absorption power of soil is conven- 
 iently demonstrated with the aid 
 of an apparatus, devised by 
 Miiller. 1 A vertical glass cylin- 
 der (Fig. 59), 75 centimeters 
 long, with an internal diameter 
 of 4.5 centimeters is closed at 
 each end by a perforated rubber 
 stopper, provided with L,-shaped 
 glass trbes for the passage of the 
 solution to be used. The lower 
 part of the cylinder is filled with 
 broken glass or glass beads, cov- 
 ered by a layer of glass wool, 
 about i centimeter thick. The 
 cylinder is then filled up with 
 soil, carefully sampled, air-dry 
 and previously passed through a 
 sieve. The soil is covered with 
 glass wool. The standard so- 
 lution of N/io potassium car- Fig- 59. 
 bonate, contained in a 2 liter bottle, is allowed to rise 
 slowly in the soil and is gradually deprived from its 
 potassium, the latter being absorbed by the soil. The so- 
 
 1 Zeitschr. f. angew. Chem. 13, p. 501, (1889). 
 
146 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 lution, finally collected in the lecture jar is compared with 
 a sample of the original solution, collected in another jar 
 by opening a pinchcock (P^) in a connecting T-piece. 
 The flow of the solution through the soil is regulated by 
 a screw pinchcock (P^. If the experiment is properly 
 carried out, nearly all the potassium is adsorbed, so that 
 the final solution is neutral towards red litmus, while the 
 original solution is distinctly alkaline. 
 
 As this test is not quite satisfactory for showing the loss 
 of potassium, a solution of picric acid, saturated at room 
 temperature (not exceeding 17) may be used as in- 
 dicator. Taking N/5, N/io, and N/2O solutions of po- 
 tassium salts (K^COg, K 2 SO 4 , KC1), it will be seen that 
 on shaking 10 cc. of the solution with 10 cc. of the picric 
 acid solution, a heavy precipitate is formed with the N/5 
 solution, a slight precipitate with the N/io solution, and 
 no precipitate at all with the N/2O solution. A N/5 so- 
 lution of a potassium salt may be suitably used, since 
 more than half of the potassium will be adsorbed. 
 
 The following data, obtained by Huston, 1 may be in- 
 cluded, to show the selective action of the soil : Two 
 hundred and fifty cubic centimeters of N/io solutions of 
 sodium phosphate, potassium chloride, potassium sulphate, 
 ammonium sulphate and sodium nitrate, when treated for 
 48 hours with 100 grams of soil lost by adsorption respec- 
 tively: 0.259 gram P 2 O 5 , 0.316 gram K 2 O, 0.332 gram 
 K 2 O, 0.096 gram N and o.ooo gram N. 
 
 185. Finally one out of many cases of reciprocal ad- 
 sorption of colloids may be mentioned. Mutual adsorp- 
 
 1 Experim. Station, Purdue Univ., Bull. 33, p. 50. 
 
CONOIDS AND ADSORPTION 147 
 
 tion of two suspensoids has as yet not been observed ; on 
 the other hand a number of cases, in which either one or 
 both colloids are emulsoids have been studied. The ac- 
 tion of protective colloids is probably nothing but an ad- 
 sorption of the emulsoid by the suspensoid. As an ex- 
 ample of reciprocal adsorption compounds Cassius' gold 
 purple, may be prepared. This substance, long consid- 
 ered as a chemical compound of tin oxide and aurous 
 oxide is really a mixture of (suspensoid) colloid gold 
 and (emulsoid) colloid stannic acid, as has been proved 
 by the investigations of Zsigmondy 1 and his pupils. 
 
 It is usually obtained, as is easily shown in a lecture 
 demonstration, by adding a solution of stannic and stan- 
 nous chloride to a very dilute solution of gold chloride. 
 The gold purple can, however, also be prepared by mixing 
 colloid solutions of gold and stannic acid. These so- 
 lutions have to be made up beforehand and are obtained 
 as follows : The gold solution is made according to Zsig- 
 mondy's directions, 2 by starting with 100 cc. of pure 
 water, (redistilled from a quartz or pyrex flask, using a 
 silver or tin condenser) to which are added 25 cc. of a 
 solution containing 0.6 gram auric acid. The latter is 
 obtained by evaporating a solution of gold in aqua regia. 
 The mixture is then treated with 3 cc. of a N/5 solution 
 of potassium carbonate and boiled. Four cubic centimeters 
 of a solution, containing one part of freshly distilled for- 
 maldehyde in one hundred parts of water is poured grad- 
 ually and with frequent stirring in the boiling liquid. In 
 this manner a deep red or purple red gold solution of 
 
 1 lyieb. Ann. 301, p. 361, (1898). 
 
 2 Ibidem 301, p. 30, (1898). 
 
148 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 great stability is obtained. The colloid stannic acid is 
 easily prepared by dissolving 2 grams of anhydrous stan- 
 nic chloride in 3 liters of distilled water. On mixing both 
 solutions, no change in color is observed, not even after 
 the addition of a few drops of dilute nitric or sulphuric 
 acid, but on boiling the same gold purple is obtained as In 
 the usual procedure of reducing gold chloride with stan- 
 nous chloride. 
 
CHAPTER X. 
 
 ACTING-CHEMISTRY. 
 
 Although a great many reactions are known, which are 
 influenced by light, our knowledge of radiant energy as 
 such is still very limited. No theory connecting a multi- 
 tude of observations and forecasting unknown phenom- 
 ena, thereby stimulating further researches in this im- 
 portant branch of physical chemistry has been put for- 
 ward. In spite of persistent investigations, especially in 
 organic chemistry, where "light" reactions are most ob- 
 vious, the work of Ciamician and Silber, Benrath, 
 Plotnikov and others has not led to any far-reaching gen- 
 eralization. 
 
 186 The most typical case of photo-synthesis, which 
 has formed the subject of exhaustive researches by some 
 of the most famous chemists of the igth century (Ber- 
 thollet, Draper, Bunsen and Roscoe), is the combination 
 of hydrogen and chlorine. The reaction takes place with 
 explosive rapidity under the influence of bright sunlight 
 or the light of burning magnesium ribbon. The experi- 
 ment, shown in first courses in chemistry, may be safely 
 carried out by filling, in diffuse light, small thick- 
 walled medicine bottles of 100 cc. contents over brine 
 with the mixture of the gases in equal volumes, keeping 
 the bottles corked in the dark until needed. Using glass 
 screens of various color (yellow and red), the absorption 
 of the actinic rays may be shown in addition. 
 
 187. A reversal of the photochemical union of chlorine 
 and hydrogen is the decomposition of hydrochloric acid 
 
150 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 under the influence of light. Coehn and Wassiljewa 1 per- 
 form this experiment by passing the gas (prepared from 
 fused sodium chloride and sulphuric acid), free from 
 air, through a quartz tube 20 centimeters long and 0.5 
 centimeters in diameter, illuminated by a Heraeus' mer- 
 cury quartz lamp at a distance of about 2 centimeters, 
 into a narrow glass tube, blackened on the outside, which 
 is inserted in a flask with potassium iodide solution. The 
 hydrogen gas, not absorbed in the solution, is collected 
 in an explosion eudiometer and exploded with oxygen in 
 the usual way. The operator in the immediate neigh- 
 borhood of the quartz lamp should not forget to protect 
 the eyes with blue glasses. 
 
 188. As a common type of actinometer, Eder's mer- 
 curic oxalate actinometer may be mentioned. The light 
 activity is measured here by the chemical transformation, 
 which mercuric oxalate undergoes when exposed to light. 
 
 A solution of 4 grams of crystallized ammonium oxa- 
 late in loo cc. of distilled water is added to a 5 per cent 
 mercuric chloride solution and the clear liquid is then ex- 
 posed to arc light. 2 The separation of white crystals 
 of calomel soon becomes visible ; at the same time carbon 
 dioxide is liberated: 
 2 HgCl 2 + (NH 4 ) 2 C 2 O 4 = 2HgCl + 2NH 4 C1 + 2 CO 2 
 
 By measuring the gas volume or by weighing the pre- 
 cipitate the light intensity may be quantitatively deter- 
 mined. 
 
 189. The same solution may be used for illustrating 
 photochemical extinction. This phenomenon, also 
 
 1 Berichte d. chem. Ges. 42, p. 3183, (1909). 
 
 2 Meldola, the Chemistry of Photography, Condon, p. 32, (189:). 
 
ACTING-CHEMISTRY !$! 
 
 called, after its discoverer, the law of Draper (1841), 
 serves to demonstrate, that photochemical decomposition 
 implies absorption of the chemically active rays. For 
 demonstration purposes, 1 two glass troughs are taken, 
 with parallel sides, at least I inch apart, each divided by 
 a vertical septum, and strapped together by means of rub- 
 ber bands. A mixture of mercuric chloride and ammo- 
 nium oxalate solutions, made up as above mentioned is 
 poured in three of the four cells, the second: B (Fig. 60), 
 being filled with distilled water. The whole system is 
 then exposed to the arc light, A-B being nearest to the 
 light. As soon as the contents of A becomes opalescent, 
 
 A 
 
 a 
 
 C 
 
 D 
 
 Fig. 60. 
 
 the cells are disconnected and on exhibiting the results it 
 will be observed, that while D has become opalescent C 
 has not appreciably been affected, no opalescence being 
 visible. 
 
 190. An electro-chemical actinometer is described by 
 Coehn, 2 adapted for a demonstration from experiments 
 by Gouy and Rigollot. 3 Into a U-tube, filled with a I 
 per cent, sodium chloride solution, two strips of copper 
 foil, I centimeter wide, previously heated over a Bunsen 
 flame until the clean surface has taken on a uniform 
 brown color, are inserted, and connected by means of a 
 
 1 Meldola, 1. c. p. 327. 
 
 2 Miiller-Pouillet's Handbook of Physics, p. 598. 
 
 3 Journal de Physique (3) 6, p. 520, (1897). 
 
 II 
 
152 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 copper wire with a lecture galvanometer and a contact 
 key. Both limbs are covered by black cardboard caps. 
 On closing the circuit no deviation of the pointer is vis- 
 ible, but on removing one of the caps a deviation is ob- 
 served. On lowering the cap the pointer moves back 
 again. Raising of the other cap reverts the current. 
 
 191. The chemistry of photography covers a large field 
 of highly interesting phenomena, offering a number of 
 unsolved scientific problems. One of the most important 
 reactions, which has been and is, up to the present time, 
 a matter of controversy among chemists, is the well- 
 known photo-decomposition of silver chlo- 
 ride and the accompanying change in color. 
 This may be illustrated by placing some 
 moist silver chloride, freshly prepared, on 
 the bottom of a cylindrical vessel, closed by 
 a rubber stopper, through which passes a 
 glass rod, carrying a strip of starch iodide 
 paper. 1 The chloride is exposed for about 
 10 minutes to the electric light. (Fig. 61.) 
 It will be seen that the chloride rapidly 
 darkens, while at the same time the paper 
 becomes intensely blue. 
 
 Fig. 61. 
 
 192, The retarding effect of mercuric chloride may be 
 shown at the same time by exposing, in another vessel, 
 freshly prepared and washed silver chloride, to which a 
 few drops of mercuric chloride have been added. Ex- 
 posure to the light produces no visible change in the salt. 2 
 
 1 Meldola, 1. c. p. 66. 
 
 2 Meldola, 1. c. p. 67. 
 
ACTING-CHEMISTRY 153 
 
 193. Other inorganic salts, which are readily affected 
 by light, are cuprous and thallous chloride. The follow- 
 ing experiment with cuprous chloride, due to Priwoznick, 1 
 is easily performed. A sheet of polished copper with a 
 perfectly clean surface, is immersed in a photographic 
 disk, filled with a concentrated copper chloride solution 
 (made by boiling hydrochloric acid with an excess of 
 cupric oxide), until it is uniformly covered with a thin 
 grey film. After 5 minutes the plate is removed, washed, 
 drained on blotting paper, and when still moist, exposed 
 under a design, cut in black rjaper, for about 10 minutes 
 or longer, to the electric light. The design appears photo- 
 graphed on the plate, the exposed portions being much 
 darker than the protected parts. 
 
 194. Apart from the pre-eminent value of the silver 
 haloids for reproduction purposes, two other processes 
 deserve to be mentioned : the "blue print" and the "pig- 
 ment" process. The former may be carried out in the 
 following manner: A sheet of drawing paper is coated 
 with a 10 per cent, ferric ammonium citrate solution by 
 floating on the liquid for a few minutes and dried in the 
 dark. It is then covered with a piece of black paper in 
 which a design has been cut out and exposed to direct 
 sunlight or to arc light, concentrated by means of a lens, 
 for 5 or 10 minutes. Under the influence of the light the 
 ferric salt is reduced by the organic material of the paper 
 and a faint image becomes visible. By brushing a so- 
 lution of potassium ferric cyanide over the exposed sur- 
 
 1 Dingler's Pol. Journ. 221, p. 38, (1877.) 
 
154 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 face, the pattern is developed in Turnbull's blue. Finally 
 the non-exposed ferric salt is washed out with tap water. 
 
 195. The pigment process depends on the fact, that 
 gelatin, containing some potassium bichromate, is sensi- 
 tive to light, when dry, but hardly sensitive when wet. 
 The process may be illustrated by exposing a sheet of 
 drawing paper, previously coated with a mixture of gela- 
 tin and potassium bichromate together with finely divided 
 carbon (or any other pigment used in oil painting) and 
 dried in the dark, under a negative to direct sunlight or to 
 arc light for several minutes. On the exposed parts, the 
 gelatin is rendered almost insoluble. Consequently, on 
 washing the paper in warm water, a picture appears in 
 the pigment, held by the undissolved gelatin. 1 
 
 Actino-chemistry does not only treat of reactions, in 
 which light causes chemical changes, but also includes the 
 converse processes of chemical reactions, producing radi- 
 ant energy. Here we are with regard to a deeper under- 
 standing of these transformations almost completely ig- 
 norant, since apart from the phenomena and the names 
 customarily given to them, little or nothing is known 
 about the fundamental principles governing these differ- 
 ent cases of so-called luminescence. Next to thermo- 
 luminescence and electro-luminescence (the light emitted 
 by rarified gases with the aid of the alternating current 
 of an induction coil), of which no instances need to be 
 mentioned, we distinguish : tribo-luminescence, crystal- 
 lization luminescence, fluorescence and phosphorescence. 
 
 1 Bigelow, 1. c. p. 515. 
 
ACTING-CHEMISTRY 
 
 155 
 
 196. Tribo-luminescence may be observed, when a 
 bottle, containing uranium nitrate crystals, is shaken vig- 
 orously in the dark. 
 
 197. Another substance showing a marked tribo-lumi- 
 nescence is salophen (acetyl para-amidophenyl salicylate 
 or C 6 H 4 OHCOOC 6 H 4 NHCOCH 3 ). For a demonstration 
 in the lecture room two test-tubes of slightly 
 
 different diameter are used, so that the 
 one with smaller bore can be pushed in the 
 larger tube (Fig. 62). If about i gram 
 powdered salophen is placed in the annular 
 space between the tubes and crushed by 
 rotating one tube within the other, an in- 
 tense glow is observed in the dark. 1 For 
 individual observation a number of these 
 tubes, filled with salophen are circulated 
 among the audience. 
 
 198. Crystallization luminescence is more 
 difficult to observe. It is usually shown, by 
 shaking in the darkened lecture room a 
 supersaturated solution of arsenious acid 
 or sodium fluoride. As soon as crystalliza- 
 tion sets in flashes are seen, but the light being very faint, 
 the phenomenon is difficult to observe from a distance. 
 
 199. Fluorescence, first discovered with fluorspar, from 
 which mineral the phenomenon derives its name, is 
 characteristic of several mineral oils and is exceedingly 
 marked with dilute solutions of fluorescein or cosin. 
 
 1 Plotnikov, Photochemische Versuchstechnik, Leipzig, p. 235, (1912), 
 which contains a large number of lecture experiments on the subject of 
 actino-chemistry (p. 190279). 
 
 Fig. 62. 
 
156 DEMONSTRATIONS IN PHYSICAL, CHEMISTRY 
 
 200. It may also be seen, by exposing a card, moistened 
 with a quinine sulphate solution in the violet and ultra- 
 violet region of the arc-light spectrum obtained with a 
 quartz or flint-glass prism. 
 
 201. Phosphorescence derives its name from the glow 
 which phosphorus emits in contact with oxygen. As is 
 well known, no glowing is seen, when the element is ex- 
 posed to pure oxygen, under atmospheric or higher press- 
 ure. On reducing the pressure below a certain limit, the 
 glow becomes visible. This is particularly well shown by 
 using the arrangement, given by Newth, 1 consisting of a 
 glass tube (length 30-50 centimeters; diameter 2.5 centi- 
 meters), bent upward at both ends and provided with two 
 stopcocks (Fig. 63). A solution of yellow phosphorus is 
 
 Fig. 63. 
 
 made by gently warming a few pieces, the size of a pea, 
 in a conical flask with olive oil. The bottom of the tube 
 is then covered with a layer of this solution, and after 
 expelling the air, filled with oxygen. No glowing is seen, 
 but on reducing the pressure with the water-suction 
 pump, the tube becomes luminous over its entire length. 
 The experiment requires darkening of the lecture room. 
 202. The glowing of phosphorus can be observed even 
 in diffuse daylight, by following the directions, given by 
 Marino and Porlezza. 2 The authors pass carbon dioxide 
 through a saturated sodium bicarbonate solution, dry it 
 
 1 Newth, 1. c. p. 243. 
 
 2 Gazz. chim. ital. 41, (II), p. 420, (1911). 
 
ACTING-CHEMISTRY 
 
 157 
 
 in a calcium chloride tower and then introduce it in a 
 hard glass tube (2 centimeters diameter), in which red 
 phosphorus is heated over three wing-top Bunsen burn- 
 ers (Fig. 64). When starting the experiment the phos- 
 phorus is heated very gradually, while at the same time 
 the gas current passes over the phosphorus very slowly, 
 
 Fig. 64. 
 
 until all traces of moisture have been expelled. An L- 
 shaped delivery tube is then connected with the open end 
 of the tube, leading down to the bottom of a large 2- 
 liter Florence flask. The combustion tube is then heated 
 up, until the phosphorus condenses in yellow droplets in 
 the delivery tube. The gas stream, which has been kept 
 slow for a while, is then suddenly increased. Imme- 
 diately a beautiful greenish flame appears, while the flask 
 
158 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 itself shows, in its lower part a splendid phosphores- 
 cence. 
 
 203. What is sometimes called "chemiluminescence" 
 may be seen in certain chemical reactions, where lumines- 
 cence accompanies the reaction. Thus, on adding rapidly 
 50 cc. of a 30 per cent, hydrogen peroxide solution to a 
 mixture of 35 cc. of a 50 per cent, potassium carbonate 
 solution, 35 cc. of a 10 per cent, pyrogallol solution, and 
 35 cc. of a 35 per cent, formaldehyde solution, 1 vigorous 
 foaming accompanied by a reddish glow, results. 
 
 1 Trautz, Zeitschr. f. Electrochemie 10, p. 593, (1904); Zeitschr. f. phys. 
 Chem., 53, p. i, (1905). 
 
CHAPTER XL 
 
 FLAME, COMBUSTION AND EXPLOSION. 
 
 From the time, when the phlogiston hypothesis was 
 universally accepted by Priestley, Scheele, Bergmann and 
 other prominent chemists of the eighteenth century, up 
 to the recent flame gas investigations by Haber, Bone and 
 their co-workers, many attempts have been made to ar- 
 rive at a clear insight into the nature of flames and the 
 causes of their luminosity. Numerous experiments are 
 known, the more important are given in almost any text- 
 book of elementary inorganic chemistry, but up to the 
 present time a general explanation, covering all the re- 
 search work, that has been done by Davy, Frankland, 
 Heumann, Smithells, Lewes, Bone and others cannot be 
 given. 
 
 In the following a number of experiments will be men- 
 tioned and briefly described, illustrating: 
 
 I. Combustion of gases in general. 
 II. The structure and chemical reactions of 
 flames. 
 
 III. Luminosity in the presence of solid particles. 
 
 IV. The separation of solids from flames. 
 V. Luminosity without solid particles. 
 
 VI. Changes in luminosity. 
 VII. Explosion and its prevention. 
 
l6o DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 I. Combustion of Gases in General. 
 
 204. A flame, defined as a mass of glowing gas, re- 
 quires a medium in which it can "burn," that is the com- 
 bustion must be supported by another gas, in order to pro- 
 duce a combination of the two gases with evolution of 
 heat and light. The term "combustible" and "supporter 
 
 of combustion" are interchange- 
 able, however, as shown by the 
 experiment of the "reversed 
 flame." 1 
 
 A lamp-glass (Fig. 65) is 
 closed at its lower end by a 
 cork stopper, carrying a central 
 tube of metal or glass, I centi- 
 meter in diameter and a smaller 
 side tube (inner bore 2-3 milli- 
 meters), through which coal gas 
 is admitted. The lamp chimney 
 is closed at the top by a per- 
 forated asbestos disk, and the 
 hole in this cover closed until 
 the lamp is filled with gas. 
 After 2-3 minutes the gas is ig- 
 nited at the bottom of the central 
 Fig> 65 - tube and the hole in the asbestos 
 
 disk slowly opened. The flame is drawn up into the tube 
 and the reversed flame appears in an atmosphere of coal 
 gas. The gas, issuing from the hole at the top is ignited 
 and represents the ordinary coal gas flame. By introduc- 
 
 1 Waitha, Ber. d. chem. Ges. 4, p. 91, (1871). 
 
FI<AMF,, COMBUSTION AND EXPLOSION l6l 
 
 ing a small tube, from which coal gas is burned, into the 
 central tube, both flames are close together, the one en- 
 veloping the other. 
 
 205. The same inference is reached by passing a jet of 
 oxygen into an inverted glass cylinder, filled with hydro- 
 gen and ignited at the lower end. The ordinary oxy- 
 hydrogen flame is obtained by lowering a jet of burning 
 hydrogen into a glass cylinder filled with oxygen. 
 
 206. Substances, that give off oxygen readily, may be 
 used to burn oxygen in a coal gas atmosphere. In the flame 
 of coal gas, burning from a lamp-glass, covered by an 
 asbestos disk with a hole in the middle, dry chlorates 
 of potassium, barium and strontium are fused successive- 
 ly on a deflagrating spoon until oxygen is given off. 
 The spoon is then lowered and the liberated oxygen burns 
 with a brilliantly colored light. 
 
 207. That oxygen may be replaced, either as a "sup- 
 porter of combustion" or as a "combustible" by another 
 gas, chlorine for instance is illustrated by introduc- 
 ing a jet of burning hydrogen into a glass jar, filled with 
 chlorine. In a similar way chlorine can be made to 
 burn in an atmosphere of hydrogen by the use of the ap- 
 paratus for the "reversed" flame. Replacing the short 
 central tube by a long tube, which reaches the hole in 
 the asbestos disk, from which hydrogen burns, the chlor- 
 ine is introduced through this central tube in a slow 
 stream and ignites on issuing from the opening. The 
 tube is then drawn down carefully. It will be seen that 
 the chlorine continues to burn with a copious evolution 
 of hydrogen chloride vapor. 
 
l62 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 II. The Structure and Chemical Reactions of Flames. 
 
 Taking the flame of illuminating gas, obtained with a 
 Bunsen or Teclu burner as an instance, it will be noticed, 
 that the non-luminous flame represents an inner and an 
 outer cone. The inner cone burns with a bright green 
 color, which must be ascribed to luminescence since the 
 temperature of this cone, under a strong draught, accord- 
 ing to Haber and Richardt 1 is only about 1550. The 
 mixture of gas and air burns in the inner cone to carbon 
 monoxide, carbon dioxide, hydrogen and water vapor, 
 mixed with uncombined nitrogen and is, therefore, noth- 
 ing but "a water gas, diluted with nitrogen" (Haber). 
 
 In the outer cone (temperature ca 1800) the carbon 
 monoxide and hydrogen form carbon dioxide and water 
 vapor. The gas mixture between the two cones does not 
 contain any oxygen, consequently it is inferred, that 
 "nothing burns in the flame." Inside the inner cone we 
 have the unburned gas mixture; the flame, therefore, is 
 hollow. From the experiments, devised to prove this, 
 the following may be quoted : 
 
 208. Well glazed writing paper, dusted on the upper 
 side with mercuric iodide, asbestos paper or thin copper 
 foil, when depressed for a short time upon the flame, at 
 various angles, show the well-known flame figures. (Fig. 
 66.) 
 
 209. A glass tube, about 10-15 centimeters long, with 
 an inner bore of 5 millimeters, cut off at an angle of 45 
 at its lower end, is held in the flame, as indicated in 
 
 1 Haber-I^amb, Thermodynamics of Technical Gas Reactions p. 301, (1908). 
 
FLAME, COMBUSTION AND EXPLOSION 
 
 the figure (Fig. 67), and the unburned gas lighted at the 
 upper end. 
 
 /' ,// 
 
 5 
 
 Fig. 66. 
 
 Fig. 67. 
 
 210. A pin is pushed at right angles through a match, 
 about i or 2 centimeters from its head and the latter thus 
 supported vertically on the jet of the burner. 1 On light- 
 ing the gas, the match-head does not catch fire. The ex- 
 periment can be repeated by thrusting a match quickly 
 into the center of the flame ; only the middle part burns 
 off directly. 
 
 211. A modification of the foregoing experiment 
 is obtained by connecting the gas supply with the stem 
 of a small funnel (5-7 centimeters in diameter) covered 
 with a piece of fine copper wire gauze. Right in the 
 center of the gauze a number of match-heads or a small 
 heap of gunpowder may be placed. On turning on the 
 gas and igniting the latter with a burning taper, held over 
 
 1 Hofmann, Ber. d. chem. Ges. 2, p. 254, (1869). 
 
164 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the gauze, the inflammable material in the center remains 
 unconsumed. 
 
 212. The device which made it possible to investigate 
 the chemical composition of the inter-conal gas in the 
 Bunsen flame is the flame- or cone-separator, found by 
 Smithells and Ingle 1 and simultaneously by Teclu. 2 
 The apparatus (Fig. 68) consists of a glass tube, 
 40 centimeters long and 15 millimeters 
 wide, fitted at its lower end either 
 with a T-piece for the inlet of gas and 
 air, the supply of each being regulated 
 by stopcocks (or screw clips) or with 
 a one-hole rubber stopper, through 
 which passes the mouth of a Bunsen 
 or Teclu burner. This glass tube is 
 surrounded by a second glass tube, 20 
 centimeters long and 3 centimeters 
 wide, fitted with a rubber union and 
 one (or two) asbestos or cork rings, 
 to permit the outer tube to be easily 
 slid up and down. The inner tube is 
 held by a clamp in a vertical position. 
 The upper ends of both tubes are pro- 
 vided with metal ends (preferably 
 platinum, but aluminium or copper foil will do just as 
 well). In order to make both cones visible to a large 
 audience, the metal ends are moistened with a sodium 
 chloride solution. The outer tube may be pro- 
 
 1 Journ. Chem. Soc., 61, p. 204, (1892); Brit. Ass. Rep., lyeicester, (1907) p. 469. 
 
 2 Journ. f. prakt. Chemie 44, p. 246, (1891). 
 
 Fig. 68. 
 
COMBUSTION AND EXPLOSION 165 
 
 vided with a small side tube (closed by a cork stopper) 
 to show how in practice the interconal gas can be drawn 
 off for analysis (Smithells, Haber and others). At the 
 beginning of the experiment both tubes are adjusted to 
 the same level. The gas is turned on, the air being com- 
 pletely shut off ; the air is then gradually admitted and the 
 luminous flame changed into the ordinary non-luminous 
 flame. The outer tube is then raised and the flame split 
 in two cones, the outer cone ascending with the wide 
 tube, the inner cone continuing to burn at the opening of 
 the narrow tube. By increasing or decreasing the sup- 
 ply of air the inner cone can be made to "strike back" 
 or to ascend to the mouth of the outer tube, in which 
 case the original Bunsen flame is restored. 
 
 213. The presence of carbon in the luminous flame of 
 illuminating gas is indicated also by Soret's optical test. 
 The flame placed between a screen and a strong light 
 causes a shadow to be formed on the screen. No shadow 
 is formed by the flames of carbon disulphide and burn- 
 ing phosphorus. 
 
 214. The view of a preferential combustion of hydro- 
 gen, liberating carbon, which should cause the luminosity 
 in luminous flames of hydrocarbons has been given up in 
 favor of the conception of a gradual dissociation, pre- 
 ceding combustion. In fact, intermediate formation of 
 acetylene can be shown in certain cases, for instance by 
 drawing off the gases formed, when a Bunsen burner 
 strikes back and passing the gas mixture through an am- 
 moniacal solution of cuprous chloride (Fig. 69). 
 
l66 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 Fig. 69. 
 
 Another case is that of air burning in an excess of 
 illuminating gas ; here again acetylene is formed and the 
 formation of this gas shown in the same way as before. 
 
 III. Luminosity in the Presence of Solid Particles. 
 
 215. That non-luminous flames become luminous in the 
 presence of solids is well-known. The easiest illustration 
 is holding a thin platinum wire in the colorless flame of 
 hydrogen, burning from a platinum jet. Other instances 
 are "Drummond's lime-light" being an oxy-hydrogen 
 flame, directed on a piece of quicklime, and the Welsbach 
 lamp, which is an ordinary Bunsen burner, giving a non- 
 luminous flame, made luminous by the incandescent 
 mantle of thorium oxide (ThO 2 ) mixed with about I per 
 cent, of its weight cerium oxide (CeO 2 ). 
 
FLAME, COMBUSTION AND EXPLOSION 
 
 i6 7 
 
 216. The flame of burning alcohol which is almost in- 
 visible, can be made luminous by passing a jet of chlorine 
 into an Erlenmeyer flask, in which alcohol is boiled. The 
 flame, burning from the neck becomes luminous by the 
 separation of carbon, formed by the decomposition of the 
 alcohol by the chlorine. 
 
 217. A number of experiments have been devised show- 
 ing the "carburetion" of non-luminous 
 
 flames by the introduction of carbon in 
 the flame. This is usually done by mix- 
 ing the gas with unsaturated hydro- 
 carbons, rich in carbon, as benzene or 
 acetylene. The hydrogen from a Kipp 
 generator is passed through a calcium 
 chloride tower, connected with a U-tube, 
 both legs of which are provided with fish 
 tail tips (Fig. 70). A plug of cotton 
 soaked with benzene is inserted in the 
 left limb, and after having expelled the 
 air completely, both tips are lighted. The 
 luminous flame appears on the left side, 
 while the right flame is almost colorless. 
 
 218. The enrichment of illuminating 
 gas with acetylene makes the colorless 
 flame of the Bunsen burner quite lumi- 
 nous and is conveniently carried out by 
 
 passing the gas into a wide-mouthed bottle, introduced 
 between the gas outlet and the Bunsen burner, and half- 
 way filled with water. A piece of calcium carbide is held 
 12 
 
 70 - 
 
1 68 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 by a copper wire, as shown in the figure (Fig. 71 ). 1 
 After shaking off the carbide into 
 the water, the luminosity of the 
 flame will be considerably increased 
 for several minutes. 
 IV. The Separation of Solids 
 
 from Flames. 
 
 An interesting way of separating 
 solid particles from a flame, is by 
 the formation of vortices, of which 
 two instances are given by 
 Newth. 2 
 
 219. Coal gas is passed through a 
 T-shaped tube, connected with two 
 
 glass tubes (6-8 millimeters bore) drawn out in capillary 
 jets, held by clamps and inclined at such an angle, that 
 by carefully regulating the gas stream in each tube, two 
 
 Fig. 71. 
 
 Fig. 72. 
 
 horn-shaped wings of carbon vortices are formed (Fig. 
 72). Draught must be avoided, as the phenomenon is 
 affected by the slightest air currents. 
 
 1 Baker, Journ. Am. Chem. Soc., 39 p. 646, (1917). 
 
 2 Newth, 1. c. p. 216. 
 
FLAME, COMBUSTION AND EXPLOSION 
 
 169 
 
 220. Much easier to set up is another arrangement, in 
 which an Argand burner and a blowpipe are used. The 
 chimney of the burner (about 4 inches high) is covered 
 with a piece of twenty-five-mesh wire gauze and the jet 
 of a blowpipe pressed against the gauze (Fig. 73). The 
 burner is lighted and the flame turned down as far as 
 possible. Coal gas is then admitted through the blow- 
 
 gas 
 
 Fig. 73. 
 
 pipe, and a beautiful vortex ring of sparkling carbon par- 
 ticles will be formed. 
 
 221. In many cases the substance, to which the lumin- 
 osity is due can be separated by cooling the flame with a 
 cold object. Thus by holding a porcelain dish over the 
 flame of illuminating gas, arsine, stibine and nickel car- 
 bonyl respectively, the separation of carbon, arsenic, 
 antimony and nickel will be observed. 
 
I/O DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 222. In the following experiment the separation of 
 heavy metals from the flames may be demonstrated, 
 by following the example given by Ste. Claire Deville 1 in 
 his cold-warm tube. The arrangement, as proposed by 
 Bancroft and Weiser 2 consists of a Bunsen burner, pro- 
 vided with an asbestos chimney (Fig. 74). The latter 
 
 has a hole, through which 
 passes a platinum wire, bent 
 in a loop, covered with a piece 
 D of asbestos soaked in a con- 
 centrated solution of an easily 
 volatilized metal salt (like 
 cadmium chloride, bismuth 
 nitrate, lead nitrate). After 
 introducing the salt in the 
 Fig. 74. hottest part of the flame, a 
 
 porcelain tube of about I centimeter outer diameter, 
 cooled by a rapid stream of cold tap water, is held for a 
 few minutes in the upper part of the flame. The result 
 is a separation of the metal, very often in the form of 
 an extremely lustrous mirror. 
 
 V. Luminosity without Solid Particles. 
 
 223. Vapor of carbon bisulphide, burning in nitric ox- 
 ide, produces an intense blue light, although no solid par- 
 ticles are present. In a jar, filled with nitric oxide (over 
 water), a few drops of carbon disulphide are introduced 
 from a dropping funnel, and after thorough shaking, the 
 mixture is ignited. 
 
 1 Ieons sur la dissociation, Paris, pp. 45-63, (1864). 
 
 2 Jour n. of phys. Chem., 18, p. 213, (1914). 
 
COMBUSTION AND EXPLOSION 
 
 171 
 
 8 
 
 224. On burning phosphine in pure oxygen (Fig. 75), 
 the gas burns with a dazzl- 
 ing white flame. 
 
 VI. Changes in Luminosity, 
 
 The luminosity of flames 
 can be changed either by in- 
 creasing or decreasing the 
 pressure or by raising or 
 lowering the temperature. 
 The following set of ex- 
 periments illustrates this "nnr"" Of 
 point : 
 
 225. Cooling of the flame 
 diminishes the luminosity: 
 
 the luminous flame of a Bunsen or a fish-tail burner be- 
 comes practically colorless by holding a platinum disk or 
 a sheet of nickel or iron plate against the flame. The 
 luminosity can be restored by heating the disk or sheet 
 by means of a blast lamp on the opposite side. 
 
 226. Liipke 1 illustrates the effect of a cooling in the 
 following manner. A 10 per cent, solution of ether in 
 water is poured in a test-tube and the tube closed with 
 a cork stopper, through which a needle is run down to 
 the bottom. The solution is solidified in a freezing mix- 
 ture, the test-tube stripped off and the frozen cylinder 
 fixed upside down in a candlestick. The ether, when lit 
 with a match burns with an almost colorless flame. 
 
 227. The chilling of the flame can also be brought 
 
 1 Riidorff-Iyiipke, 1. c. p. 372. 
 
172 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 about by diluting the gas with an inert gas like nitrogen 
 or carbon dioxide. Thus by introducing, through a T- 
 tube, dry carbon dioxide in coal gas, the luminous flame 
 of a Bunsen burner becomes colorless. 
 
 228. The difference in luminosity of phosphorus and 
 sulphur, burning in oxygen and in air also demonstrates 
 strikingly the effect of temperature change. 
 
 229. Diminished pressure reduces the luminosity of 
 flames. Thus the light of a candle, burning in the re- 
 ceiver of an air pump, will become almost invisible on 
 quickly reducing the air pressure. The carbon dioxide, 
 formed in the combustion is conveniently removed by 
 placing a dish with quicklime in the receiver. 
 
 230. Detonating gas ("Knall-gas") when exploded in 
 an eudiometer or in an explosion pipette forms water 
 with a luminous flash, owing to the enormously increased 
 pressure, while in case the gas mixture is bubbled through 
 a soap solution and a taper held near the froth hardly 
 any flash is visible. 
 
 VH. Explosion and Its Prevention. 
 
 The question of explosion and the means of preventing 
 gas mixtures from exploding may be treated in connec- 
 tion with the familiar phenomenon of the "striking back" 
 of the Bunsen flame. The principle on which this burner, 
 invented by Bunsen in 1855, is built, is as follows: The 
 illuminating gas, escaping from a narrow jet in the base 
 of the burner, creates a partial vacuum around the jet 
 and consequently air is drawn through the air holes in the 
 
COMBUSTION AND EXPLOSION 
 
 173 
 
 burner tube. By turning the air regulator, fitted with 
 two opposite holes, corresponding to the draught holes in 
 the tube, the air supply can be varied at will. 
 
 231. The reduction of the pressure can be made visible 
 by closing one air-hole and connecting the other with a 
 small manometer (Fig. 76), containing an indigo-solu- 
 tion. As soon as the gas is turned on, the liquid in the 
 gauge tube moves towards the burner, and moves back 
 
 Fig. 76. 
 
 again on turning the gas off. If necessary, the movement 
 of the liquid can be projected on the screen. 
 
 232. On fully opening the air holes, it will be seen, 
 that the inner cone, distinguished by its green color, 
 burns with a loud noise and moves constantly up and 
 down. This is due to cross-currents and incomplete mix- 
 ing of the gases, as can be proved by lengthening the tube 
 with another piece of copper tubing, 20-30 centimeters 
 long, after which the inner cone burns quietly and be- 
 comes stationary. It represents a state of dynamic equi- 
 librium, which is only disturbed by decreased gas supply. 
 
174 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 On gradually turning off the gas, the flame first begins to 
 flicker and finally strikes back. 
 
 233. More striking and directly visible to a large aud- 
 ience, is the following modification of the preceding ex- 
 periment i 1 
 
 A glass tube, 4 to 6 feet long and i to 1^2 
 inches wide, is clamped vertically over a Bunsen or Teclu 
 burner and the space between burner and tube closed by 
 a plug of cotton wool. The top of the tube is fitted with 
 a piece of platinum or nickel foil to prevent the glass 
 from cracking. The gas is lighted as it issues from the 
 top and burns with a luminous flame when the air suppl} 
 at the base of the burner is shut off. The cotton plug 
 is then gradually removed and the blue Bunsen flame is 
 obtained. On admitting more air by opening the air 
 holes gradually, or by reducing the gas supply the blue 
 cone will recede down the tube. By careful adjustment 
 it can be made to travel up and down slowly at will. 
 From this it may be concluded that the flame of the 
 Bunsen burner represents a "stationary" explosion, in 
 which the speed of the combustion wave is just held in 
 equilibrium by the speed of the gas mixture moving up- 
 ward. 
 
 234. In order to show the propagation of the combus- 
 tion wave, L,e Chatelier 2 passes a mixture of nitric oxide 
 and carbon bisulphide, [made by allowing nitric oxide 
 (from nitric acid, specific gravity 1.2 and copper turn- 
 ings and dried by concentrated sulphuric acid) to move 
 
 1 Mellor, 1. c. p. 758; Newth, 1. c. p. 230. 
 
 2 1. c. p. 273. 
 
COMBUSTION AND EXPLOSION 
 
 175 
 
 over the surface of a layer of carbon disulphide, con- 
 tained in a small flask] through a glass tube, 3-4 meters 
 long and 3 centimeters wide, slightly inclined under an 
 angle of 5-10, until the tube becomes colorless again, 
 thereby indicating that the nitrogen peroxide, first formed, 
 has been completely expelled. The gas generator is then 
 put aside and the mixture lit at the upper end of the 
 tube. The route of the dazzling white flame can be fol- 
 lowed for about 1-2 meters, until a sudden report indi- 
 cates the transition of the combustion wave into the ex- 
 plosion wave. The latter travels at a speed of several 
 (two or more) kilometers per second. 
 
 235. H. Erdmann 1 uses a 
 three-necked Woulfe-bottle 
 of 1-1.5 liters contents, to 
 show that not every gas 
 mixture is explosive. The 
 middle neck is provided 
 with a one-hole rubber or 
 cork stopper, through 
 which passes a glass tube, 
 about i meter long and 
 10-15 millimeters wide. 
 Through the "~~j shaped de- 
 livery tube on the left (Fig. 
 77), illuminating gas is 
 passed into the bottle, until 
 all air has been removed, 
 the third neck being closed 
 by a stopper. The gas is lit at the mouth of the outlet 
 
 gas 
 
 Fig. 77. 
 
 1 I^ehrbuch der anorg. Chemie, 5e Aufl., p. 433, (i9 10 )- 
 
176 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 tube and a luminous flame results. On removing the 
 stopper, the flame becomes non-luminous, blue, and al- 
 though the gas is now mixed with air, there is no ex- 
 plosion. On gradually turning off the gas supply the blue 
 flame cone flattens and recedes with a concave surface 
 first slowly, then with increased speed, until after a few 
 seconds a loud report is heard, resulting from the ex- 
 plosion of the gas-air mixture in the bottle. A possible 
 collapse of the bottle may be counteracted by wrapping 
 the bottle in strong copper wire gauze, although this may 
 seem to many chemists an unnecessary precautionary 
 measure. 
 
 236. An analogous experiment is the following, 1 per- 
 formed with a glass tube, 60 centimeters long and 3 centi- 
 meters wide, held in a clamp at an angle of about 30. 
 The upper end is fitted with a one-hole cork stopper, 
 through which passes a copper tube 6 millimeters in diam- 
 eter and 30 centimeters long; the lower end is closed 
 by a perforated stopper, carrying the delivery tube, con- 
 nected with the gas supply. The gas is turned on and 
 lighted at the orifice of the copper tube. On cutting off 
 the gas and removing the stopper at the lower end, air 
 enters the tube, forming an explosive mixture; the flame 
 retreats down the copper tube and explodes the mixture 
 in the glass tube. The experiment is then repeated, re- 
 placing the copper tube by another copper tube, with a 3 
 millimeter bore. If properly adjusted, the flame is ex- 
 tinguished, before reaching the explosive mixture in the 
 glass tube, which is then lit at the lower end by applying 
 a match. 
 
 1 Mellor, 1. c. p. 743. 
 
COMBUSTION AND EXPLOSION 177 
 
 237. The following lecture experiment, due to Dixon 1 
 illustrates the contrast between the quiet burning of car- 
 bon monoxide and oxygen in a short tube, where no ex- 
 plosion wave can be set up and the violent explosion 
 which takes place when a wave is formed. First a thin- 
 walled test-tube rilled with the gaseous mixture is ignited 
 by a taper. The quiet passage down the tube of the blue 
 flame of the burning mixture can be followed by the eye. 
 The tube is then refilled and fastened to the end of a 
 piece of lead tubing a few feet long, rilled with the mix- 
 ture. The test-tube is inclosed in copper gauze and sur- 
 rounded by a thick glass cylinder. When a flame is ap- 
 plied at the open end of the pipe, a loud report is heard 
 and the test-tube is shattered to pieces. 
 
 238. The cooling effect of metallic surfaces in keep- 
 ing the temperature below the ignition point, may be 
 further illustrated by suddenly depressing a piece of cop- 
 per wire gauze on a Bunsen flame. The flame remains 
 for a while entirely below the gauze. As the latter be- 
 comes heated, the gas above the wire catches fire after 
 a few moments. 
 
 238. The Davy safety lamp, the best known practical 
 application of the metal wire gauze as a means of pre- 
 venting explosion, may be demonstrated and its useful- 
 ness illustrated by lowering it into a highly explosive 
 mixture of ethyl ether and air, contained in a 2-liter 
 beaker. The flame inside the lamp is extinguished after 
 a few moments, but no explosion results. 
 
 1 Mellor, Chemical Statics and Dynamics, I^ondon, p. 485, (1909). 
 
178 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 240. The same principle, slightly modified, is also ap- 
 plied in the construction of the burner, invented by 
 Meker. On taking the burner apart, it will be seen that 
 the air holes (four or five, instead of two) are unusually 
 large, so that really an explosive mixture of gas and air 
 is formed. No striking back is observed, however, on 
 lighting the gas, since a deep nickel grid, closing the en- 
 larged outlet of the burner tube, exerts its cooling effect, 
 thereby preventing the flame from striking back. 
 
CHAPTER XII. 
 
 LIQUID AIR EXPERIMENTS. 
 
 The experiments which can be carried out with the aid 
 of liquid air are among the most striking that can be 
 performed in chemistry courses. Since 
 liquid air is on the market nowadays 
 at reasonable prices, 1 there is no 
 obstacle in the way of performing a 
 number of interesting low-temperature 
 experiments which can all be shown 
 in a i- or 2-hour period. 
 
 Liquid air can be stored for quite a 
 while when kept in the double-walled 
 vessels with an evacuated space be- 
 tween, first introduced by Dewar. 2 Re- 
 cently Weinhold has devised vessels 
 with four walls, having three air-free 
 layers between the liquid air and the 
 outside atmosphere (Fig. 78). A 
 further improvement for the conser- 
 vation of liquid air has been made by 
 silvering the inner walls in order to 
 keep off radiant heat. In this way, 
 it has been made possible to store 
 quantities of 1-2 liters of liquid air for 
 about 8-14 days, the daily loss being about 100 cc. Liquid 
 air should never be poured from one vessel into another 
 
 1 Not long ago (March, 1915) the firm of A. R. Ahrendt & Co., in Berlin, 
 offered liquid air for sale at the price of 50-60 f per liter. 
 
 2 Proc. Royal Inst. 14, p. i, (1893). 
 
 Fig. 78. 
 
l8o DEMONSTRATIONS IN PHYSICAL, CHEMISTRY 
 
 since it happens very often that the vessel from which 
 the air is taken cracks at the junction of both walls. 
 For this reason it is necessary to transfer the liquid air 
 by blowing it from one vessel into another by means of 
 a rubber balloon, as shown in Fig. 79. Care should be 
 
 Fig. 79. 
 
 taken, that the two-hole cork stopper, carrying the glass 
 tubes, is not pressed too tightly into the neck of the bal- 
 loon or jar, containing the air. 
 
 The experiments to be performed are conveniently di- 
 vided in two sets : First those illustrating purely phys- 
 ical properties of matter at very low temperature (liquid 
 
LIQUID AIR EXPERIMENTS 181 
 
 air boils at about 190) and in the second place those 
 illustrating chemical reactions at extreme temperatures, 
 depending on the fact that liquid air is a source of oxygen. 
 Liquid air when freshly prepared contains about 30 per 
 cent, of oxygen, but on standing it gradually becomes 
 richer in oxygen (up to 55 per cent.) due to the fact, that 
 nitrogen having a lower boiling point ( 195) evap- 
 orates more rapidly than oxygen (boiling point 182). 
 
 I. Physical properties of matter at 190. 
 
 241. Most of the familiar gases change into almost 
 colorless solids when cooled to 
 the temperature of liquid air. 
 Two 500 cc. glass balloons, with 
 hollow bottoms, (Fig. 80), con- 
 taining dry chlorine and dry 
 bromine gas respectively, are 
 sealed and then held upside down 
 by clamps between two thick- 
 walled glass plates placed par- 
 allel to each other, so that in the 
 event of an explosion the lectur- 
 er and his auditory are sufficient- 
 ly protected. Liquid air is cau- 
 tiously poured in the cavities of 
 the bottoms, with the result that 
 both gases instantly solidify 
 (freezing point of chlorine 
 
 102, of bromine 7). An almost perfect vacuum 
 is created, no gas being left in the balloons which appear 
 absolutely colorless. 
 
l82 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 Z42. That a high vacuum can be effected in this way 
 is clearly shown by solidifying carefully dried carbon di- 
 oxide gas in a T-shaped tube (length of horizontal limb 
 20 centimeters, of vertical limb 22 centimeters, diameter 
 3 centimeters). The tube is provided with two platinum 
 
 Fig. 81. 
 
 electrodes to which are soldered two short aluminium 
 wires 2 centimeters long and 3 millimeters thick, (Fig. 
 81). The electrodes are connected with the secondary 
 poles of an inductorium. The distance between the alu- 
 minium wires (about 15 centimeters) is too large to allow 
 
UQUID AIR EXPERIMENTS 
 
 the passage of sparks, but on slowly lowering the vertical 
 limb of the tube into a Dewar vessel, half filled with 
 liquid air (a preliminary cooling is effected by the escap- 
 ing air vapor, before the tube is lowered in the liquid 
 air) the gas immediately changes into a white snow, de- 
 posited against the walls of the vertical limb. At the 
 same time a brilliant yellowish green spark light be- 
 comes visible, increasing in intensity the more the tube 
 cools. Darkening of the lecture room helps to demon- 
 strate this striking phenomenon in all its splendor. 
 
 243. The freezing of water by its own evaporation is 
 
 A 
 
 Fig. 82. 
 
 brought about instantly in liquid air. Two glass bulbs, 
 about 4 centimeters in diameter are connected by a glass 
 tube, 30 centimeters long and 0.8 centimeter wide, twice 
 bent at right angles (Fig. 82). One of them is partly 
 filled with water. The air is completely driven out, so 
 that the whole apparatus contains nothing but water and 
 water vapor. On plunging the vapor-bulb in liquid air, 
 13 
 
184 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 the water in the second bulb freezes at once. It may be 
 remarked that this apparatus, Wollaston's "cryophorus," 1 
 gives good results even at much higher temperatures 
 ( 20) although not as readily as in liquid air. 
 
 244. The freezing of liquids, like ether and alcohol, 
 may be shown next. An alcoholic solution of iodine is 
 transformed into a viscid glass-like material which con- 
 tracts on solidifying. At the same time it becomes yel- 
 lowish-orange in color while in the liquid state the solu- 
 tion is distinctly brown. 
 
 245. Many solid substances change color when cooled 
 in liquid air, proving that the power of absorbing light 
 is considerably modified at low temperature. 2 Crystals of 
 sulphur, potassium bichromate and cinnabar may be used 
 as examples. 
 
 246. Rubber, when brought to the temperature of liq- 
 uid air becomes as brittle as glass and may be ground in 
 a mortar. The same is true for grapes, meat, lard, etc. 
 On allowing the temperature to rise, all these substances 
 regain their original properties. 
 
 247. Metals become stronger towards a steady pull. A 
 coil of lead wire, 3 millimeters in diameter, when cooled 
 in liquid air is able to hold a weight of 300 grams with- 
 out considerable stretching. Rise of temperature causes 
 the wire to stretch and finally to become almost straight 
 on returning to room temperature. 
 
 i Graham, Elements of Chemistry, ed. Watts and Bridges, p. 75, (1866.) 
 * Kreuz, Phil. Mag., (5) 39, p. 209, (1895). 
 
LIQUID AIR EXPERIMENTS 185 
 
 248. On striking a small lead disk (or bell) cooled to 
 i 80 with a wood hammer a clear metallic sound is 
 
 heard. 
 
 249. Liquid air, when kept for several hours in a 
 Dewar vessel is heavier than water. This is due to the 
 fact that it contains then about 40-50 per cent, oxygen, 
 with a specific gravity of 1,135 ( at 182) against 60- 
 50 per cent, nitrogen with a specific gravity of 0.885 ( at 
 195). On pouring 20-30 cc. of liquid air in a beaker 
 
 of water, a few drops of air are seen floating on the 
 water, separated by a layer of gaseous air (phenomenon 
 of L,eidenfrost), but occasionally some drops sink down 
 and rise again by the continuous formation of gas. 
 
 250. The usefulness of liquid air for the creation of 
 high vacua is due to the fact, that the power of char- 
 coal to absorb gases is greatly increased at low tempera- 
 tures. Thus Dewar found, 1 that i cc. of charcoal at o 
 takes up 4 cc. of hydrogen or 18 cc. of oxygen, at 185 
 it takes up 135 cc. of hydrogen or 239 cc. of oxygen. The 
 charcoal obtained from the shells of the cocoa-nut is es- 
 pecially adapted for this purpose. A tube, filled with this 
 charcoal (previously heated to drive off absorbed gases 
 and moisture), is sealed to another tube (dimensions 20 
 by 3.5 centimeters) containing dry nitrogen, and provided 
 at both ends with platinum wires to which are soldered 
 short aluminium electrodes. When connection is made 
 with the secondary poles of an inductorium (Fig. 83), 
 nothing except an occasional flash is observed, showing 
 
 i Proc. Royal Soc., 74, p. 126 (1904) ; Chem. News, 94, p. 173 (1906). 
 14 
 
l86 DEMONSTRATIONS IN PHYSICAL CHEMISTRY 
 
 that absorption of nitrogen has not taken place to any 
 marked extent. On slowly lowering the limb containing 
 the charcoal into a Dewar vessel filled with liquid air, 
 forked brush-like reddish sparks begin to leap from elec- 
 trode to electrode. As the gas absorption proceeds there 
 appears a luminous band breaking up (at a pressure of 3 
 millimeters) into a number of striae, while at the same 
 time a dark space, Faraday's dark space, forms around the 
 
 Fig. 83. 
 
 cathode. Finally after about 10 minutes, when the press- 
 ure has sunk to about 0.03 millimeter of mercury the 
 striae disappear, and a dark space, starting from the ca- 
 thode, "Crookes' dark space," expands and fills the tube. 
 A green fluorescent light is emitted from the walls around 
 the cathode which, on reverting the primary current, 
 switches over to the other end of the tube. 
 
LIQUID AIR EXPERIMENTS 187 
 
 II. Chemical properties of matter at 190. 
 
 251. Chemical action at 190 is hardly perceptible; 
 the molecules are, according to Dewar, near to 
 the "death of matter." When a small piece of sodium 
 and a few cubic centimeters of strong hydrochloric acid 
 are cooled separately in two test-tubes in liquid air and 
 then brought in contact with each other, no reaction takes 
 place. 1 
 
 252. Liquid air, on standing, becomes rich in oxygen. 
 This accounts for the fact that a burning taper, when 
 plunged into some liquid air, contained in a beaker, is not 
 extinguished, but burns vigorously notwithstanding the 
 extremely low temperature in the beaker. In performing 
 this experiment, proper precautions must be taken, the 
 beaker in most cases being reduced to pieces. 
 
 253. A plug of cotton wool is treated with powdered 
 charcoal so that the latter becomes finely divided in the 
 cotton. The preparation held in the loop of platinum 
 wire, is then dipped into liquid air for a few minutes. 
 On igniting the cotton-charcoal' in a flame the mixture 
 burns like gun cotton, 2 with an intensely bright flame. 
 
 1 Pictet, Comptes rendus 115, p. 814 (1892). 
 8 Erdmanii, 1. c. p. 243. 
 
BIBLIOGRAPHY. 
 
 The following is a list of the literature references, frequently 
 quoted in the text: 
 
 Emil Baur, Themen der physikalischen Chemie, Leipzig (1910). 
 S. Lawrence Bigelow, Theoretical and Physical Chemistry, New 
 York (1914). 
 
 H. und W. Biltz, t)bungsbeispiele aus der unorganischen Experi- 
 
 mental-Chemie, Hamburg (1907). 
 Alfred Coehn, Electro-Chemie, in Miiller-Pouillet's Handbuch 
 
 der Physik, IVer End., Leipzig (1909). 
 H. Erdmann, Lehrbuch der anorganischen Chemie, 5e Aufl., 
 
 Braunschweig (1910). 
 
 E. Hatschek, Physics and Chemistry of Colloids, London (1913). 
 Karl Heumann, Anleitung zum Experimentieren, 36 Aufl., von 
 
 O. Kuhling, Braunschweig (1904). 
 
 Louis Kahlenberg, Outlines of Chemistry, revised ed., New York 
 (1916). 
 
 F. W. Kiister, Zeitschr. f. Electrochemie, 4, pp. 105-113 (1898). 
 H. Le Chatelier, Lemons sur le carbone, Paris (1908). 
 
 R. Liipke, Grundziige der Electro-Chemie, 5e Aufl., von E. Bose, 
 
 Leipzig (1907). 
 R. Luther, Die chemischen Vorgange in der Photographic, Halle 
 
 (1899). 
 
 R. Meldola, The Chemistry of Photography, London (1891). 
 J. W. Mellor, Modern Inorganic Chemistry, new impression, 
 
 London (1016). 
 
 G. S. Newth, Chemical Lecture Experiments, revised ed.. London 
 
 A. A. Noyes, Journ. Am. Chem. Soc., 27, pp. 85-104 (1905). 
 A. A. Noyes and A. A. Blanchard, Ibidem, 22, pp. 726-752 (1900). 
 A. A. Noyes and G. V. Sammet, Ibidem, 24, pp. 498-515 (1902). 
 
BIBLIOGRAPHY 189 
 
 Wilhelm Ostwald, Grundlinien der anorganischen Chemie, 36 
 
 Aufl., Leipzig (1912). 
 Wilhelm Ostwald, Grundrisz der allgemeinen Chemie, 46 Aufl., 
 
 Leipzig (1909). 
 Wilhelm Ostwald, Die wissenschaftlichen Grundlagen der analy- 
 
 tischen Chemie, 5e Aufl., Leipzig (1910). Translated by 
 
 G. McGowan : The Scientific Foundations of Analytical 
 
 Chemistry, 3rd ed., London (1908). 
 Wolfgang Ostwald, Handbook of Colloid-Chemistry; English 
 
 translation by Martin H. Fischer, Philadelphia (1915). 
 E. Riidorff, Grundrisz der Chemie, I2e Aufl. von R. Lupke, 
 
 Leipzig (1902). 
 Alexander Smith, Introduction to Inorganic Chemistry, 3rd ed., 
 
 New York (1917). 
 Julius Stieglitz, Elements of Qualitative Chemical Analysis, vol. 
 
 I, New York (1916). 
 
INDEX TO AUTHORS.* 
 
 Arrhenius, 67. 
 
 Bakhuis Roozeboom, 2, 54. 
 
 Bancroft, 2, 54, 170. 
 
 Baur, 44, 63. 
 
 Benrath, 149. 
 
 Bergmann, 159. 
 
 Berthollet, 149. 
 
 Berzelius, 55. 
 
 Bigelow, S. L., 125. 
 
 Biltz, H, 16. 
 
 Biltz, W., 125. 
 
 Bingham, 13. 
 
 Blanchard, 50, 52, 75, 76, 93, 
 
 95, H2. 
 Bone, 159. 
 Brann, 43. 
 
 Bredig, 61, 65, 116, 117. 
 Bucholz, 89. 
 Bunsen, 149, 172. 
 
 Cady, 106. 
 Ciamician, 36, 149. 
 Coehn, 130, 150, 151. 
 Cohen, E., 8. 
 Crafts, 56. 
 Crookes, 186. 
 Crum Brown, 28, 98. 
 
 Daniell, 88. 
 
 Davy, 159. 
 
 Deville, 41, 170. 
 
 Dewar, 140, 179, 185, 187. 
 
 * Text references only. 
 
 Dixon, 177. 
 Dolezalek, 89. 
 Donnan, 127. 
 Draper, 149, 151. 
 Duclaux, 125. 
 Dulong, 3, 12. 
 
 Eder, 150. 
 Erdmann, H., 175. 
 Eykman, 34, 36. 
 
 Faraday, n, 12, 118, 186. 
 Fenton, 6. 
 Fichter, 134. 
 Fischer, E., 40. 
 Frankland, 159. 
 Freer, 20. 
 Fremy, 144. 
 Freundlich, 133, 136. 
 Friedel, 56. 
 
 Gattermann, 8. 
 Gay-Lussac, 54. 
 Gemberling, 125. 
 Gibbs, J. W., 2, 128. 
 Gladstone, 42. 
 Goppelsroeder, 134. 
 Gouy, 151. 
 Graham, 15, 18, 20, 21, 114, 
 
 120, 124. 
 Gutbier, 118. 
 
 Haber, 159, 162, 165. 
 
INDEX TO AUTHORS 
 
 191 
 
 Handa, 51. 
 Henry, V., 121, 125. 
 Heumann, 159. 
 Heydweiller, 100, 102. 
 Heyer, 124. 
 Hofmann, A. W. t 67. 
 Huston, 146. 
 
 Ingle, 164. 
 
 Kahlenberg, 29, 105. 
 Keir, II. 
 Kenrick, 42. 
 Kohlrausch, 100, 102. 
 Kruger, 89. 
 Kruyt, 10. 
 Ktister, 73, 78, 82. 
 
 Landolt, 48, 49. 
 Landsberger, 34. 
 Lash Miller, 42. 
 Le Chatelier, 174. 
 Leidenfrost, 185. 
 Lermontoff, 82. 
 Leveing, 44. 
 Lewes, 159. 
 Lhermite, 29. 
 Lichtenwalter, 106. 
 Lodge, 70, 74. 
 Liipke, 25, 78, 80, 84, 85, 
 
 171. 
 Luther, 44, 65. 
 
 Malfitano, 124, 125. 
 Marino, 156. 
 McCoy, 34. 
 Meker, 178. 
 
 Mellor, 54. 
 Meyer, V., 35. 
 Morse, H. N., 26. 
 Miiller, 145. 
 
 Nernst, 27, 51, 54, 68, 71, 73, 
 
 74, 112, 113. 
 Newth, 109, 156, 168. 
 Nollet, 25. 
 Noyes, A. A., 43, 50, 52, 55, 
 
 56, 57, 60, 75, 76, 93, 95, 
 
 112, 113, 122, 131, 136. 
 
 Ober, 144. 
 
 Ostwald, Wilhelm, 18, 54, 
 
 55, 97, 99, 107- 
 
 Ostwald, Wolfgang, 10, 114, 
 115, 122. 
 
 Palmaer, 69. 
 Pebal, 6. 
 Perrin, 121. 
 Petit, 3, 12. 
 Pfeffer, 26. 
 Plotnikov, 149. 
 Porlezza, 156. 
 Priestley, 159. 
 Priwoznick, 153. 
 
 Quartaroli, 62. 
 
 Ramsay, 24. 
 Reusz, 133. 
 Richardt, 162. 
 Rigollot, 151. 
 Roberts Austen, 22. 
 Roscoe, 149. 
 
192 INDEX TO AUTHORS 
 
 Sahlbohm, 134. Treadwell, 100. 
 
 vSammet, 55, 56, 57, 60. Tyndall, 129. 
 Scheele, 159. 
 
 Schonbein, n. Van Bemmelen, 144. 
 
 Schoorl, 103. Van 't Hoff, 2, 38, 44. 
 
 Scriba, 92. Voigtlander, 123. 
 
 Silber, 149. Volta, 91. 
 
 Smithells, 159, 164, 165. Von Dieterich, 45. 
 Smith, A., 10, 54. 
 
 Smits, 12. Wassiljewa, 150. 
 
 Soret, 165. Weinhold, 179. 
 
 Spitalski, 65. Weiser, 170. 
 
 Stieglitz, 47, 93. Whitney, 95, 144. 
 
 Svedberg, 116. Wilke, 65. 
 
 Wohler, F., 16. 
 
 Tammann, 2, 3, 26. Wohler, L,., 45. 
 
 Teclu, 164. Wollaston, 184. 
 Than, 6. 
 
 Tolloczko, no. Zsigmondy, 118, 124, 147. 
 Traube, M., 25. 
 
INDEX OF SUBJECTS, 
 
 Actinometers, 150, 151. 
 
 Adsorption, of gases, 140; of liquids, 141; of dissolved sub- 
 stances, 141-143; selective , 143-146; mutual of colloids, 
 146-148 . 
 
 AUotropy, of tin, 8 ; dynamical of sulphur, 10. 
 
 Atmolysis, 6, 18. 
 
 Atomic heat, of tin, 13; of lead, 13. 
 
 Autocatalysis, 61-63. 
 
 Blue print process, 153. 
 
 Capillary analysis, 134. 
 
 Carburetion of non-luminous flames, 167, 168. 
 
 Catalysis, 55-66. 
 
 Cataphoresis, 131. 
 
 Chemical properties of matter at 190, 187. 
 
 Collodion tubes, use of for dialysis, 125. 
 
 Colloid solutions, preparation of , 116-122. 
 
 Combination cells, 83. 
 
 Combustion of gases, 160, 161. 
 
 Common ion effect, 98-100. 
 
 Concentration cells, 88, 89. 
 
 Conductivity, of water, 91 ; of hydrochloric acid in toluene, 92 ; 
 
 in water, 93, 94; of sodium chloride, 92. 
 Crystallization luminescence, of arsenious acid, 155; of sodium 
 
 fluoride, 155. 
 
 Davy's safety lamp, 177. 
 
 Degree of dissociation, 94-97. 
 
 Depolarizers, 88. 
 
 Devitrification, of sodium silicate glass, 7. 
 
 Dialysis, 123-125. 
 
194 INDEX OF SUBJECTS 
 
 Diffusion, of bromine, 16; of carbon dioxide, 16; of copper 
 nitrate, 21; of hydrogen, 15; in gelatine plates, 22; speed 
 of , 122, 123. 
 
 Disintegration of noble metals in water, 117, 118. 
 
 Dispersoids, mechanical properties of, 122-128; optical proper- 
 ties of, 128, 129; electrical properties of, 129-139. 
 
 Displacement cells, 78-83. 
 
 Dissociation, of ammonium chloride, 5. 
 
 Draper's law, 151. 
 
 Dulong and Petit's law, 12. 
 
 Effusion, of hydrogen, 20; of oxygen, 20. 
 
 Electrolysis, of stannous chloride, 68; of potassium chloride 
 
 68. 
 
 Electromotive chemistry, 78-91. 
 Electrophoresis, 131. 
 Emulsoids, preparation of , 121. 
 Endosmosis, 133. 
 Equilibrium displacement, 46, 47. 
 Explosion and its prevention, 172-178. 
 
 Fever reactions, 63. 
 
 Flames, structure of, 162, 163; chemical composition of, 
 
 164-166. 
 Fluorescence, of fluorescein, 155; of eosin, 155; of quinine 
 
 sulphate solutions, 156. 
 
 Galvanic liquid cells, 89, 90. 
 Hydrolysis, 100-104, 120. 
 
 lonization, and chemical activity, 104-106; and color of solu- 
 tions, 106, 107. 
 
 Liquid crystals, 8. 
 
 Liquid air experiments, 179-187. 
 
INDEX OF SUBJECTS IQ5 
 
 Luminosity of flames, in the presence of solid particles, 166-168; 
 without solid particles, 170, 171; changes in , 171, 172. 
 
 Mass action law, 42-45. 
 
 Migration of ions, 69-78. 
 
 Molecular weight determination, by the ebullioscopic method, 
 
 34; by the vapor density method, 35; by the cryoscopic 
 
 method, 36, 37. 
 Mutual precipitation of colloid solutions, 135. 
 
 Negative catalyzers, 63. 
 
 Osmotic experiments, with ammonia-air mixtures, 24; with 
 
 sugar solutions, 25, 27; with three liquids, 26-29. 
 Oxidation cells, 83. 
 
 Passivity, of iron, 12. 
 
 Peptization, 120. 
 
 Phase transitions, 2. 
 
 Phosphorescence, of phosphorus, 156-158. 
 
 Photochemical decomposition, of hydrogen chloride, 150; of 
 silver chloride, 152; of cuprous chloride, 153. 
 
 Photochemical extinction, 150, 151. 
 
 Photochemical synthesis of hydrogen chloride, 150. 
 
 Physical properties of matter at 190, 181-186. 
 
 Pigment process, 153. 
 
 Plasticity of clay, I. 
 
 Polarization, 84-86. 
 
 Polymorphic transformation, of cuprous mercuric iodide, 3 ; of 
 mercuric iodide, 4 ; of potassium bichromate, 5 ; of potas- 
 sium tungstate, 4. 
 
 Precipitation membranes, of various silicates, 26. 
 
 Protective colloids, 138. 
 
 Reduction methods for preparing hydrosols, 118. 
 Reversible reactions, 39-41. 
 
196 INDEX OF SUBJECTS 
 
 Semipermeable membrane of copper ferrocyanide, 25. 
 
 Separation of solids from flames, 168-170. 
 
 Solubility, of liquids, 108, 109; of one solute in two solvents, 
 
 109-111; of salts in water, 111-113. 
 Soret's optical test, 165. 
 Strength of acids, 98. 
 Successive reactions, 54. 
 Supercooling, 6. 
 
 Surface tension, of emulsoids, 126-128. 
 
 Suspensions, preparation of , 121 ; deflocculation of , 138. 
 Suspensoids, preparation of , 116-121. 
 
 Time reactions, 48-49. 
 
 Tribe-luminescence, of uranium nitrate, 155; of salophen, 155. 
 
 Tyndall phenomenon, 129. 
 
 Undercooling, of sodium thiosulphate, 6. 
 
 Vapor pressure, of water, alcohol and ether, 31 ; of solutions 
 of benzoic acid in ether, 32; lowering of by dissolved sub- 
 stances, 31-33- 
 
 Velocity of chemical reactions, 50-53. 
 
 Viscous character, of pitch, I. 
 
 Viscosity, of suspensoids, 125, 126; of emulsoids, 125, 126. 
 
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