THE NEW THEORY OF SOLUTIONS. 

 [Continued from vol. ?'.,/>. 419.) 



FREEZING-POINT. 



SINCE solid substance and liquid substance are in 

 equilibrium at the freezing-point, in order that this 

 state of equilibrium may remain undisturbed, the vapour- 

 pressure of the solid must be equal to that of the liquid. 



Now, when a solution freezes, experiment indicates 

 that, in general, the solvent alone solidifies. The fact that 

 drinkable water is produced by melting sea-ice supports 

 this conclusion, and evidence of the same nature is afforded 

 by the results of Fritzsche, who on freezing aqueous solu- 

 tions of coloured substances obtained colourless ice, and by 

 the striking observation made by Rudorff, who found that 

 although solid magnesium platinocyanide had an intensely 

 deep-red colour, yet the ice which separated from its 

 aqueous solutions was colourless. 



It may therefore be assumed that at the freezing-point 

 of a solution the solid solvent and the solution are in 

 equilibrium, and the freezing-point may be defined as the 

 temperature at which the solution has the same vapour- 

 pressure as the solid solvent. 



It can be shown thermodynamically (and experiment 

 justifies the conclusion) that at any temperature below the 

 freezing-point the vapour-pressure of the solid solvent must 

 be lower than that of the supercooled liquid solvent. In 

 fig. 4, CF and AB may thus be taken to represent the 

 vapour-pressure curves of the solid and liquid solvents 

 respectively, and the temperature f , corresponding with the 

 point F at which the curves cut, will be the freezing-point 

 of the solvent. It has been already seen that the vapour- 

 pressure of a solution is lower than that of the pure solvent 

 at the same temperature ; DF' may therefore be taken as 



