SOLUTION 119 



(see p. 62), Since the addition of a non-volatile solute lowers the 

 vapor pressure of a pure liquid, it naturally raises the boiling- 

 point to a higher temperature. 



In dilute non-conducting solutions, equal numbers of mole- 

 cules of different solutes raise the boiling-point of a given sol- 

 vent to the same extent. Thus, one molecular weight of sugar 

 (C^H^On = 342 g.) or of glycerine (C 3 H 8 O 3 = 92 g.), dissolved 

 in 1000 g. of water, will each raise the boiling-point from 100 

 to 100.52. 



Molecular weights of non-volatile, non-conducting substances can 

 therefore be determined by finding out what weight of the sub- 

 stance, when dissolved in 1000 g., is required to raise the boiling- 

 point of water from 100 to 100.52. 



Freezing-Points of Solutions. The addition of a solute 

 similarly tends to prevent the freezing of the solution, for freezing 

 means the separation of a part of the pure solvent in the form of 

 ice. Hence solutions can be frozen only at temperatures below 

 those of the pure solvents. Thus, one molecular weight of a 

 substance, such as sugar (342 g.) or glycerine (92 g.), dissolved 

 in 1000 g. of water, will cause the water to freeze at 1.86 in- 

 stead of 0. Molecular weights can be measured by this method 

 also. 



This behavior explains why sea water is frozen in cold weather 

 much less often than fresh water. 



It explains also why salt thrown on ice will cause the latter to melt. 

 Saturated salt solution freezes only at 21, to give a mixture 

 of pure ice and pure salt, both in solid form. Hence, ice and 

 salt can not permanently exist together above that temperature. 

 When the outside temperature is below 21, salt will no longer 

 melt the ice. But calcium chloride, which is more soluble, will 

 do so. A mixture of ice and salt, giving the temperature 21, 

 is called a freezing mixture. Such a mixture is used in freezing 

 ice cream and ices. 



