20 HYDRATES IN AQUEOUS SOLUTION. 



RELATION BETWEEN WATER OF CRYSTALLIZATION AND LOWERING OF 



FREEZING-POINT. 



Jones and Getman* pointed out a relation between the lowering of the 

 freezing-point produced by electrolytes and their water of crystallization. This 

 relation was shown to hold for a large number of chlorides, bromides, iodides, 

 and nitrates indeed, for about 40 electrolytes with larger and smaller quan- 

 tities of water of crystallization. The discovery of this relation confirmed 

 the conclusion reached much earlier by one of them,f that the abnormal 

 freezing-point results produced by the electrolytes, especially in concentrated 

 solutions, were due to a combination between the solvent and the substance 

 dissolved in it. 



Indeed, this conclusion seems to be almost a necessary consequence of 

 the above relation. // hydrates are present in such solutions, those substances 

 that form the most complex hydrates in solution would be the substances that 

 would crystallize from solution with the largest amounts of water. This is the 

 same as to say that those substances which, in solution, have the greatest 

 power to combine with water, would be the ones that would bring with 

 them the largest amounts of water out of solution. 



We should, however, not expect the two phenomena to be exactly pro- 

 portional to one another, because the actual shape of the molecules which 

 build up the crystal would also be a conditioning factor in determining 

 the exact number of molecules of water with which any given substance 

 would crystallize. The best we could hope to discover would be a general 

 qualitative relation between the lowering of the freezing-point and water 

 of crystallization. That such a relation actually exists can be seen by 

 examining figs. 14 to 17. 



The data incorporated in these curves have already been published. J 



In fig. 14 the results that have been obtained for the various chlorides 

 are plotted on one sheet, as far as could be done, without undue crowding. 

 The abscissse are concentrations expressed as gram-molecular normal; the 

 ordinates are molecular lowerings of the freezing-point. The chlorides of 

 sodium, potassium, and ammonium, which crystallize without water, give 

 the smallest lowering of the freezing-point. 



The chloride of lithium, with 2 molecules of water of crystallization, gives 

 considerably greater lowering of the freezing-point of water than the chlo- 

 rides which have no water of crystallization. 



When we pass to barium chloride, which contains 2 molecules of water 

 of crystallization, it gives a greater lowering of the freezing-point than 

 lithium chloride, with the same amount of water of crystallization. It 



*Ztschr. phys. Chem., 49, 385 (1904). 



t Jones: Amer. Chem. Journ., 23. 89 (1900). 



JZtschr. phys. Chem., 31, 303 (1904); 46, 244 (1903) ; 49, 385 (1904). 



