Miscibility in binary liquid systems

When two different pure liquids are unable to mix in all proportions, they are said to be partially miscible. When these Uquids are placed in contact with one another and allowed to come to thermal, mechanical, and transfer equilibrium, the result is two coexisting liquid mixtures of different compositions. 

Liquids are never actually completely immiscible. To take an extreme case, liquid mercury, when equilibrated with water, has some H2O dissolved in it, and some mercury dissolves in the water, although the amounts may be too small to measure. 

The Gibbs phase rule for a multicomponent system to be described in Sec. 13.1 shows that a two-component, two-phase system at equilibrium has only two independent intensive variables. Thus at a given temperature and pressure, the mole fraction compositions of both phases are fixed the compositions depend only on the identity of the substances and the temperature and pressure. 

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