126 CHEMICAL STATICS 



form "solvates," and that these hydrates or "solvates" are 

 readily decomposed at temperatures which approach the boiling 

 point of the solvent and by the presence of other agents in the 

 solution which compete for the solvent.* An interesting attempt 

 has been made, upon the basis of this hypothesis, to explain the 

 color changes which many salts undergo in the presence of vary- 

 ing amounts of water or of dehydrating agents (37) (33) (36) 

 (42) (34). As is well known, anhydrous cobalt chloride is blue, 

 but on taking up water it becomes violet or red. Ostwald (50) 

 believed that the undissociated cobalt chloride is blue, while the 

 cobalt ion is red. Since, however, the color of a concentrated 

 solution of cobalt chloride can be changed from purplish red to 

 blue by the addition of small amounts of calcium, or still smaller 

 amounts of aluminium chlorides, or by the addition of a few 

 drops of alcohol (1) Lewis concludes that this change in color 

 is due to dehydration of the cobalt chloride molecule in solution, 

 by the abstraction of water from it by the added substance, 

 similar conclusions had previously been reached by other observ- 

 ers (70) (63) (81) (40) (27) (28). Similarly the progressive 

 change in color of cupric chloride solutions, from blue to greenish 

 brown, on concentration or dehydration is attributed to the loss 

 of water on the part of cupric chloride-water complexes. Lewis 

 finds that if various bromides be added to concentrated solutions 

 of cupric bromide the copper salt is dehydrated (turned brown) 

 by the salts of monovalent metals in the order Li > (Na, NH4) > K. 

 For the chlorides the order was Li > Na > NH4 > K. Di- 

 valent metals dehydrate more strongly, the order being Mg > 



* A very striking experiment illustrating the formation of "solvates" is 

 that cited by Pickering (6). If a mixture of propyl alcohol and water be 

 placed in a semipermeable vessel and surrounded with water, it is found that 

 water enters the cell, but that no propyl alcohol escapes. If, however, the 

 same semipermeable vessel, containing the same mixture of alcohol and water 

 be immersed in propyl alcohol, propyl alcohol enters the cell and water does 

 not leave it. In other words the vessel is permeable to either propyl alcohol 

 or water when these are pure, but it is impermeable to mixtures of the two, 

 the inference being that large molecular complexes are formed on mixing these 

 reagents, which cannot pass through the pores of the vessel. From these and 

 similar experiments Poynting (65) concludes that osmotic pressure is an ex- 

 pression of the diminution in the active mass of the solvent due to the formation 

 of compounds with the dissolved substance. For a fuller discussion of these 

 and similar hypotheses the reader is referred to cm-rent works on physical 

 chemistry. 



