160 



DISCUSSION OF EVIDENCE. 



the number of molecules of the dissolved substance and of the solvent. 

 If one-fourth of the water present is combined with the dissolved 

 substance, the freezing-point lowering would be one-fourth greater 

 than if all the water were present as free water and therefore acting 

 as solvent water. Freezing-point lowering would thus be affected pro- 

 portionally by hydration. Dissociation of concentrated solutions 

 calculated from the freezing-point lowering would therefore be much 

 too high. 



The conductivity of a solution depends upon the number of ions 

 present and their velocities. The number of ions would probably not 

 be affected greatly by the hydration, but their velocities would be. 

 The hydrated ions, would, of course, move more slowly than the 

 unhydrated. 



The effect of hydration would obviously be more pronounced on 

 freezing-point lowering, which is proportional to the amount of solvent 

 present, than on conductivity. The following results taken from the 

 work of Pearce 1 will show that this conclusion is justified: 



TABLE 50. Dissociation from freezing-paint lowering and from conductivity. 



Concentrated solutions were also studied in the above work; but 

 on account of very large hydration it was impossible to calculate dis- 

 sociation from the freezing-point results. 



An examination of the above table will show that the dissociation 

 of dilute solutions, as measured by the freezing-point method, is uni- 

 formly greater than as measured by the conductivity method. This 

 seems to admit of reasonable explanation in terms of hydration in 

 aqueous solution. 



Carnegie Inst. Wash., Pub. 180; Amer. Chem. Journ., 39, 313 (1908). 



