118 SMITH'S INTERMECIATE CHEMISTRY 



of the hail of molecules leaving the liquid will evidently be reduced 

 by one-tenth. Equilibrium between liquid and vapor over such 

 a solution will be re-established only when the intensity of the 

 hail of vapor molecules returning to the liquid is also reduced by 

 one-tenth, since otherwise more molecules will be returning than 

 leaving. This means that the vapor pressure of the solution must 

 be one-tenth less than that of the pure solvent. 



It is important to note that the nature of the solute is here im- 

 material, the essential factor is the number of molecules it furnishes 

 to the solution. We have here a method of determining the 

 molecular weights of non-volatile substances. By dissolving a known 

 weight of such a substance in a known weight of a suitable sol- 

 vent and determining the relative depression of vapor pressure 

 thereby produced, we learn what fraction of the molecules in the 

 solution belong to the solute, and hence can calculate its molecular 

 weight. 



All aqueous solutions show a lower tension of water vapor than 

 does pure water. With conducting solutes (e.g., sodium chloride), 

 indeed, the vapor pressure depressions obtained are abnormally 

 large, and do not agree with the accepted molecular weights. 

 This is a point to which we shall return later (p. 177). 



If a substance is very soluble in water, the solution may give 

 a vapor pressure of water less even than that commonly present 

 in the atmosphere. Such a solution, placed in an open vessel, 

 will not evaporate. On the contrary, it will take up moisture 

 from the air and increase in bulk. For this reason very soluble 

 substances are commonly moist and, when exposed to the air, 

 extract water from the latter and dissolve in this water. This 

 behavior is called deliquescence, and is shown, for example, by 

 the hydrate of calcium chloride CaCl 2 ,6H 2 0, used to dry gases 

 (p. 59). 



Boiling-Points of Solutions. The boiling-point of a liquid 

 is that temperature at which the vapor pressure reaches 760 mm. 



