When the components are completely immiscible, the vapour pressure of the one is not influenced by the presence of the other.
For immiscible liquids the vapour pressure curve is the horizontal line ab, described so that aP = QB and bQ=AP. For partially miscible liquids the curve is Pa i b i Q.
For simplicity we confine ourselves to mixtures of two components, in which experience shows that three cases are to be recognized according as the components are (I) completely immiscible, (2) partially miscible, (3) miscible in all proportions.
immiscible solutions are easily determined by finding the two minima in the free energy of mixing.