Molecular Theory of Solution. G99 



body which have a momentum sufficient to overcome the sur- 

 face forces will escape as vapour. But, as the molecular 

 cohesion of solid substances must usually be greater 

 than that o£ liquids, the vapour tension of solids must be 

 less in the same degree. Certain solids such as camphor 

 and iodine have appreciable volatility. Usually solids are 

 non-volatile. However, this appears to be due in part 

 also to the aggregation of the molecules in the solid state. 

 Any cause which tends to balance the molecular forces at 

 the surface will increase the number of molecules which are 

 able to escape. The surface energy of two solid bodies can 

 be considerably reduced by bringing them into such close 

 contact that the particles at the two surfaces come within 

 the range of molecular attraction. Under such circum- 

 stances the compensation of the surface forces allows the 

 migration of particles from one substance to the other. 

 A familiar instance is where pieces of lead and gold are 

 pressed together. 



In exactly the same way the surface tension of a solid 

 must be reduced by introduction into a liquid. Although 

 the adhesion of the liquid and solid molecules will usually 

 be less than in the case of two solids pressed together, the 

 contact at the boundary surface is the more complete, and 

 particles which cross the surface are able to move away, so- 

 that solution in a liquid is more rapid than in a solid. 



Thus it is evident that solution must be of the same 

 nature as vaporization. The presence of a solvent causes a 

 diminution in the surface forces, so that many more particles 

 have sufficient energy to cross the surface layer. 



Representing the force between two molecules as 



* = ^#- B 0) 



where M A and Mb are the resultants of the moments of 

 the atomic doublets in each molecule, it follows that the 

 adhesion of a molecule of solid to a molecule of liquid will 

 usually be less than the cohesion of the solid molecules- 

 themselves and greater than the cohesion of the liquid mole- 

 cules. Therefore, the particles at the surface of a solid bodv 

 in contact with a liquid will still experience a resultant 

 force acting towards the solid, although this force will be 

 less than in the absence of the liquid. Only such particles 

 at the surface of the solid will be able to escape into the liquid 

 as acquire a velocity normal to the surface which is equal to 

 or greater than, a given value s, such that their momentum 



