MOLECULAR MIXTURES 243 



is evidently at a point corresponding to a lower tem- 

 perature, that is, the freezing point of the solution is 

 less than that for the pure solvent. 



Let us now summarize as to molecular pressures in 

 liquids. The lowering of the freezing temperature and 

 the raising of the boiling temperature are both the re- 

 sult of the reduction in the vapor pressure of a liquid 

 which occurs when some of the molecules are not avail- 

 able for producing vapor. Within the liquid those 

 molecules which are not available for producing vapor 

 exert a partial pressure known as the osmotic pressure 

 of the solution. These phenomena depend upon the 

 number of such molecules per unit of volume. 



It is however more accurate to say that they depend 

 upon particles of essentially molecular size. This 

 allows for cases where the number of particles entering 

 into the reaction differs from the number of molecules 

 of the substances involved either because of a disso- 

 ciation or of a polymerization of the molecules. Van't 

 Hoff's equation, therefore, is usually written as 

 P = RTic, where the factor i is unity for those cases 

 like sugar solutions where there is a normal molecular 

 condition. 



