154 HYDRATES IN AQUEOUS SOLUTION. 



the ion decreases with rise in temperature. This does not refer to the charged 

 atom or group of atoms which we usually term the ion, but to this charged 

 nucleus plus a larger or smaller number of molecules of water that are 

 attached to it, and which it must drag along with it in its motion through 

 the remainder of the solvent. 



That ions are hydrated has been shown beyond question by Jones and his 

 co-workers. That these hydrates are relatively unstable compounds has 

 also been demonstrated; the higher the temperature, the less complex the 

 hydrates existing in the solution. This can be seen from one example. 

 In a solution of a certain definite concentration every molecule of calcium 

 chloride, or the ions resulting from it, holds about 30 molecules of water. 

 From such a solution practically all of the water can be removed by simply 

 boiling it, except six molecules of water to one of calcium chloride; this num- 

 ber being brought out of the solution by the salt as water of crystalliza- 

 tion. The higher the temperature, then, the less complex the hydrate formed 

 by the ion. The less the number of molecules of water combined with the 

 ion, the smaller the mass of the ion and the less its resistance when moving 

 through the solvent; consequently, the ion will move faster at the higher 

 temperature. This conclusion can be tested by the results of experiment. 

 If this factor of diminishing complexity of the hydrate formed by the ion 

 with rise in temperature, plays any prominent role in determining the large 

 temperature coefficient of conductivity, then we should expect to find those 

 ions with the largest hydrating power having the largest temperature coefficients 

 of conductivity. This will readily be seen to be the case. The more com- 

 plex the hydrate, i. e., the greater the number of molecules of water combined 

 with an ion, the greater the change in the complexity of the hydrate with 

 rise in temperature. We can readily test this conclusion by the results of 

 the experimental work of Jones and West.* Let us compare the temperature 

 coefficients of conductivity, per degree rise in temperature, for some of 

 those substances that have slight hydrating power, with the corresponding 

 coefficients for a few of the substances that have much greater power to 

 combine with water. (See table 101, page 154.) 



The volumes for which the comparisons are made are 2 and 1024, and 

 the temperatures from 25 to 35. A comparison of the two sections of 

 the table will show that the above conclusion is confirmed by the experimen- 

 tal results. The substances included in the first section of the table have 

 very slight hydrating power. Those in the second section have very much 

 greater hydrating power. It will be remembered that hydrating power is 

 a function of water of crystallization the larger the number of molecules 

 of water of crystallization the greater, in general, is the hydrating power 

 of the substance. It will be seen that the substances in the first section of 



*Amer. Chem. Journ., 3-4, 357 (1905). 



