HETEROGENEOUS EQUILIBRIUM 



263 



additional empirical restrictions that there can be but one vapor 

 phase, and, in a binary system, but two liquid phases. The 

 possible types, representing vapor, liquid, and soUd by V, 

 L, and S, are as follows: 



Types of Invariant Points; Four Coexisting Phases 



Types of Univariant Systems; Three Coexisting Phases, and the 

 Invariant Types from Which They May Be Derived 



In these various types of univariant systems, one, two, or 

 three of the phases may be of variable composition. Type 1, 

 S1S2S3, is only of interest where there is solid solution. Type 2, 

 S1S2L, is the "condensed" equilibrium, giving the change with 

 pressure of the temperature and composition of a eutectic or an 

 incongruent melting point. The most common example of 

 type 3 is the "dissociation pressure" curve of a salt hydrate; 

 and of type 5, the solubility curve of a salt in water, or the 

 melting-point curve of a fused salt or metal system. Examples 

 of all of the types have been discussed, except those containing 

 two hquid layers, types 4 and 6. Systems in which two Uquid 

 layers are formed are of both theoretical and practical interest, 

 and water-phenol is an excellent example. 



23. Equilibrium Involving Two Immiscible Liquids. Water- 

 phenol. In the discussion of the system, water-phenol,* the 



* F. H. Rhodes and A. L. Markley, J. Phys. Chem., 25, 527 (1921). 



