HYDRATION IN SOLUTION. 



By Gilbert N. Lewis, Ph. D., PJn/.vcal Chemisf. 



The experiments wliich will be described in this pa])t'r, althoii.ch 

 of the simplest character, have so direct a bearing on the impor- 

 tant question of hydration in solution as to merit a brief notice. 



It is a remarkable fact that while inorganic reactions have been 

 studied chiefly in aciueous solution, we have no knowledge as to 

 whether, or to what extent, the solutes enter into chemical com-, 

 bination with the water. It is true Ave have many independent 

 reasons for suspecting that such union occurs with a considerable 

 number of salts, es])ecially those which dissolve with the evolution 

 of heat and which are noticeably hydroscopic — that is to say, 

 which lower to an abnormal degree the vapor pressure of the 

 solvent. However, no conclusive i)roof of such union has been 

 found, nor any means of determining quantitatively wliat per cent 

 of the solute molecules arc combined or how many molecules of 

 water unite with each. 



The only attempt at such a quantitative determination of the 

 extent of hydration has been made by Jones and Getman.' They 

 base their calculations on the a.^sumption that even in solutions as 

 concentrated as three times molecular-normal there are no devia- 

 tions from the laws of dilute solution, the ajjparent deviations 

 l)eing due to h3'(lratioii, which without changing the totiil number 

 of solute molecules changes the number of free solvent molecules. 



Such an assumption lacks jilausibility in any case, butesp(>cially 

 as it leads the authors to a conclusion which is inconsistent with \\\o 

 fundamental |iiiiiei|iles of chemistry. This conclusion is that in 

 many cases the degree of hydration increases with the concentra- 

 tion of the solute. Sui)pose that the hydration occurs acconling 



^Zeif. Phyn. Chem. 49, 385 (1904). 



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