SOLUTION 



123 



place between simple H 2 O molecules to form more complex 

 molecules with the general formula (H 2 0) n . Dissolving any sub- 

 stance in water must upset the equilibrium amongst these different 

 kinds of molecules: 



(H 2 0) n <=nH 2 



and produce more of one and less of the other kind. This is the 

 extent of the chemical change in the water. 



The dissolved substance probably combines also, in the ma- 

 jority of cases, with part of the water. The nature of the com- 

 pounds is hard to determine, and no simple statement can as yet 

 be made about them. But the compounds, whatever they are, 

 are physically dissolved in the rest of the water. 



Dissolving, therefore, is partly a chemical, and only partly 

 a pure physical process. The striking differences in solubility 

 already mentioned (p. 110) may consequently be accounted for 

 partly on a chemical, and partly on a physical basis. The main 

 chemical factor is compound formation between the components 

 of the solution. The more extensive this is, the greater, in general, 

 is the solubility. Thus substances which form definite hydrates 

 with water are mostly extremely soluble, while substances which 

 are only difficultly soluble in water invariably crystallize out 

 from an aqueous solution in an anhydrous state. The main 

 physical factor is the relative magnitude of the cohesive forces 

 between the various types of molecules present in the solution. 

 Thus in the case of water and benzene, the water molecules at- 

 tract one another much more strongly than they do the benzene 

 molecules. Molecules of benzene endeavoring to intermingle 

 with water molecules encounter, therefore, very considerable 

 resistance, and are almost certain to be squeezed out. The two 

 liquids, indeed, are found to be practically immiscible. 



Exercises. 1. Give two ways of separating a mixture, con- 

 sisting of a suspended solid and a liquid (p. 109). 

 2. If you had a spot on your clothing consisting of: (a) grease, 



