240 THE REALITIES OF MODERN SCIENCE 



is that of a pure liquid (a solvent) and a solid (the 

 solute). The pressure exerted by molecules is always 

 proportional to their number per unit volume and to 

 their kinetic energy. Assume N+n molecules per c. c. 

 of the pure solvent. Let the pressure of its saturated 

 vapor be p, then p&N+n. Now suppose n molecules 

 are replaced by those of a dissolved solid, and are in- 

 capable of existing in a vapor state at this temperature. 

 The molecules available for producing the vapor pres- 

 sure are now N per c. c. and the resulting pressure 

 Hence p^p'&N+n N and 



(1) 



This equation gives the fractional reduction in vapor 

 pressure in terms of the molecules per c. c. of the pure 

 solvent and of the solute. The reduction is seen to be 

 independent of the substance which is dissolved, pro- 

 vided the number of molecules per c. c. is the same. If 

 the vapor pressure is reduced the solution cannot boil 

 against a given external pressure at the same tempera- 

 ture as would the pure solvent, and so its temperature 

 must be raised 1 above the normal boiling temperature 

 for that pressure. 



The molecules of the solute exert an osmotic pres- 

 sure, and Van't Hoff showed that it was possible to 

 calculate the lowering of the vapor pressure from a 



1 This phenomenon has only recently entered quantitatively into 

 the daily life of the home in the matter of making jellies and candies. 

 While water will boil at 212 F. under atmospheric pressure, the 

 boiling point of a solution of sugar is higher, depending upon its con- 

 centration. Since other substances like fruit acids also enter into 

 the solution, boiling temperatures furnish a convenient test of the 

 concentration, and recipes are to-day expressed in such terms. 



