CONCENTRATED AQUEOUS SOLUTIONS, AND HYDRATION OF THE SOLUTE. 879 



The curves for which values of percentage hydration have been plotted against 

 grammes of salt added to one hundred grammes of water (fig. 7, p. 875) show how 

 hydration varies with concentration and the limit toward which it approaches at high 

 concentrations. The curves also show the difference in hydration capacity of the 

 several salts. It may be pointed out that some of the curves have a well-defined 

 maximum point, which corresponds to a certain concentration which varies with the 

 salt (Li 2 S0 4 , BaCl 2 , NH 4 C1 , and NH 4 N0 3 ). For higher concentrations than that 

 represented by the maximum point the amount of hydration falls off. It almost 

 vanishes at the highest concentration for ammonium nitrate. 



The diagram shows that for other salts (i.e. CaCL, , CaBr 2 , SrBr 2 , LiBr, and LiCl) 



/y/u> 



PERCENTAOE HYDRATION 



Fig. 9. 



hydration increases up to the highest concentrations observed, and the curves do not 

 contain a maximum point. 



In fig. 8 elevation of the boiling point and depression of the freezing point 

 have been plotted against percentage hydration. The curves show that the 

 percentage hydration per degree of elevation is greater for the dilute solutions than 

 the percentage hydration per degree of depression, but that with increase of concentration 

 this difference diminishes, until for high concentrations, for the salts and acids considered, 

 the degree of hydration is much the same per degree of depression of freezing point 

 and per degree of elevation of the boiling point. The diagram indicates further that at 

 high concentrations the percentage hydration is approximately the same whether viewed 

 from the freezing point, or elevation of the boiling point, point of view. This is also 

 indicated by the curves on p. 880, fig. 10. 



The curves,* fig. 9, show the relation between values of /%//««, and percentage hydra- 

 tion, and indicate the maximum point to which hydration tends for several salts at the 



* The straightness of several such curves I have already pointed out, Trans. Roy. Soc. Edin., xlv., 201, 1906. 

 TRANS. ROY. SOC. EDIN., VOL. XLV. PART IV. (NO. 32). 127 



