204 GENERAL DISCUSSION OF RESULTS. 



water are the same for any two salts, such as the above, which contain a common 

 cation. From these results it therefore appears probable that the hydrating power 

 of a salt is dependent chiefly upon the cation. 



The quaternary electrolyte aluminium chloride and the binary electrolyte sodium 

 bromide were also brought within the scope of this work. It was also extended to 

 the strong mineral acids, hydrochloric, nitric, and sulphuric. This work included 

 fifteen salts and three strong acids, over a range of dilution from 0.01 to 2.0 normal. 

 The results seem to justify the following conclusions: 



In the more dilute solutions we have mainly ions present; in the more concentrated, 

 largely molecules. We could thus study the effects of ions and molecules on freezing- 

 point lowering. 



The hydration, or number of molecules of water combined with one molecule of 

 the salt, or the ions resulting from it, increases with the dilution. This, in very 

 dilute solutions, becomes of a large order of magnitude. The ions, thus largely 

 hydrated in very dilute solutions, move more slowly than unhydrated ions, and the 

 dissociation as measured by the conductivity method would thus be too small. 



The decrease in the number of molecules of water combined with one of the 

 dissolved substances is in keeping with the work on the absorption spectra of solu- 

 tions which has been in progress in this laboratory for the past seven years, and upon 

 which several monographs 1 have already been published by the Carnegie Institution 

 of Washington. The resonators in the more concentrated solutions are much freer 

 to vibrate than in the more dilute they are less hydrated. 



The most interesting and important point established in this investigation is 

 that the hydrating power of a salt is a function chiefly of the cation, and the relation 

 between hydrating power and atomic or ionic volume. Anions may have some 

 hydrating power, but it seems to be very slight. 



At the highest maxima of the atomic-volume curve are the elements potassium, 

 rubidium, and caesium. These elements with the largest atomic volumes form salts 

 with small amounts of Avater of crystallization. These elements have very small 

 hydrating power. Sodium and lithium also occupy maxima on the atomic volume 

 curve, but these maxima are much lower than those of the three elements previously 

 mentioned. Salts of sodium and lithium crystallize with two or three molecules of 

 water, and these salts have some hydrating power. 



Barium (with the largest atomic volume of the alkaline earths) forms salts which 

 crystallize with two molecules of water. It has the smallest hydrating power of the 

 alkaline earths. Calcium, strontium, and magnesium have smaller atomic volumes, 

 form salts that crystallize with six molecules of water, and, therefore, have much 

 larger hydrating power than barium. Magnesium, having the smallest atomic vol- 

 ume of this group, has the largest hydrating power. 



Iron, cobalt, copper, and aluminium have very small atomic volumes and very 

 large hydrating power. 



A study of all the data obtained in this work shows not only that hydration is 

 primarily a function of the cation, but varies inversely as the atomic volume of the 

 cation; the smaller the cation the greater its hydrating power. 



'Carnegie Institution of Washington Publications Nos. 60, 1 10, 130, and 160. 



