Chapter III 



27 



Solutions 



Dissolved molecules may be converted into inactive closed groups : 



H H 



/ \ 



RX + XR ±^ RXXR + 2H2O 



\ / 



OH HO 



(iJ) 



Dilution would increase the number of active molecules of the type 



OH H 



HCi J whereas in concentrated solutions the hydrolyzed solute HoQ 



\ 



H 



\ 



CI 



would increase. 



Most of the earlier workers on osmotic properties of solutions postu- 

 lated hydration of the sucrose molecules to explain the fact that observed 

 values of osmotic pressure were higher than calculated values. Calcula- 

 tions were based on the assumption that a hexahydrate existed in solution 

 (FiNDLAY, 1919) ; Haldane (1918) postulated a pentahydrate ; for the 

 highest concentrations lower hydration values were used (Findlay, Tables 

 XXV and XXXI). 



Pennycuick (1928) pictures the reaction between H2O and HCl as 

 follows : 



HCl + HoO ^ H":C1; 



H:0:- 

 H 



(i^) 



This compound would react with ammonia in this manner 



H 



NHs + HCI-H20 ^ H : N ; 



H 



which in turn would break down thus 



H 

 H:N:H:C1: 



H:C1: 



H:0: 

 H 



{15) 



H : O : ±5 NH4CI + H2O 

 H 



(id) 



Because the water molecule has both the plus charges of the hydrogen 

 nuclei and the negative charges of the lone oxygen electron pairs it will 

 hydrate any substance having a force field, 



Bernal and Fowler (1933) conclude that all strongly polarized ions, 

 all divalent and trivalent positive ions, and the negative ions OH" and F~ 

 are hydrated {see, however, page 7). The effect of hydration of ions is 

 to lower the refractive index of water by the coordination of water mole- 

 cules around these ions. This would tend toward an increase in the regu- 

 larity of arrangement {i.e., a shift in the direction Water III -^ Water II 

 -^ Water I) . (See page 12) . 



In the past there has been a tendency to draw a distinction between 

 hydration and imbibition, the former term being applied to the association 

 of water with ions and molecules, the latter to colloidal materials. Modern 

 views would favor the definition of hydration as the resultant of any inter- 

 action between solute and water molecules tending to reduce the activity 

 of the latter. Such a definition erases any line that has been arbitrarily 

 drawn between imbibition and hydration. Though the extreme examples 

 of crystalloidal and colloidal hydration may be readily distinguished the 



