156 HYDRATES IN AQUEOUS SOLUTION. 



special comment. Barium chloride crystallizes with only 2 molecules of 

 water, yet it forms hydrates comparable with those substances with larger 

 amounts of water of crystallization. It is, therefore, perfectly in keeping 

 with the above relation that its temperature coefficients of conductivity 

 should be of the order of magnitude that they are in the above table. 



Manganese chloride crystallizes with only 4 molecules of water, but the 

 work of Jones and Bassett shows that it forms hydrates nearly as complex as 

 the other salts in the second part of table 101. Its temperature coefficients 

 of conductivity are of the same order of magnitude as the other substances 

 in this table. 



Of the substances recorded in the second part of table 101, the one that 

 apparently presents the most pronounced exception to the relation that we 

 are now considering is copper chloride. This salt crystallizes with only 2 

 molecules of water, and yet has a temperature coefficient of conductivity 

 that is nearly as large as the salts with 6 molecules of water of crystalliza- 

 tion. It might be inferred that this salt has much less hydrating power than 

 the others in the second section of table 101. The work of Jones and Bassett 

 shows that this is not the case. Copper chloride has a comparatively large 

 hydrating power; indeed, larger than would be expected from the amount 

 of water with which it crystallizes. Its temperature coefficient of conduc- 

 tivity is, therefore, not surprisingly great. 



A third point that is brought out by the results in the above tables is the 

 following: At the higher dilution, the temperature coefficient of conductivity 

 for any given substance is greater than at the lower dilution. 



That this is a general relation will be seen by reference to the work of Jones 

 and West.* This is explained very satisfactorily on the basis of the sugges- 

 tion made above. The complexity of the hydrate at the higher dilution is 

 greater than at the lower dilution, as is shown by the work recorded in the 

 earlier part of this monograph, on the composition of the hydrates formed 

 by different substances at different dilutions. 



The hydrate being more complex at the higher dilution, the change in 

 the composition of the hydrate with change in temperature would be greater 

 at the higher dilution, and, consequently, the temperature coefficient of 

 conductivity is greater the more dilute the solution. 



The three points that are established in this connection are: 



1. The temperature coefficients of conductivity of aqueous solutions of 

 electrolytes are greater the greater the hydrating power of the electrolyte. 



2. The temperature coefficients of conductivity of aqueous solutions of 

 electrolytes are of the same order of magnitude for those substances having 

 approximately the same hydrating power. 



*Amer. Chem. Journ., 34, 357 (1905). 



