THERMODYNAMICAL SYSTEM OF GIBBS 



113 



Thus, at constant temperature, the pressure is in general a 

 maximum or a minimum when the composition (i.e., the pro- 

 portions of the two components) of the two phases is identical. 

 Similarly, it can be shown that, at constant pressure, the tem- 

 perature of the two coexistent phases is in general a maximum 

 or a minimum when the composition of the two phases is 

 identical. 



Applying these relations to the equilibrium between a binary 

 liquid and its vapor, we see that (1) at constant temperature 

 the vapor pressure is a maximum or a minimum when the 

 vapor has the same composition as the liquid, and (2) the 



% Benzene 

 Fig. 2 



100 



temperature at which the two phases are in equilibrium at 

 constant pressure, i.e., the boiling point, is a maximum or a 

 minimum when the composition of the two phases is identical. 

 These rules were arrived at independently by Konowalow.* 

 As an example of this behavior. Figure 2 shows the boiling 

 points and compositions of the liquid and vapor phases of 

 ethyl alcohol and benzene. 



Similarly, the temperature at which a binary liquid is in 

 equilibrium with a solid phase, which may be a solid solution of 

 the same components (mixed crystals) or a compound of 



* Wied. Annalen, 14, 48, (1881). 



