Consider a binary mixture of cyclohexane (BP ) and benzene (BP ) at . This mixture presents a minimum-boiling azeotrope. It is possible to break this azeotrope using aniline. The vapor-liquid equilibrium (VLE) behavior of the ternary mixture is described by a modified form of Raoult's law with activity coefficients predicted by the Wilson model [1]. This Demonstration plots the pseudo-binary phase diagram for user-set values of the aniline mole fraction. We plot the benzene liquid mole fraction versus the benzene vapor mole fraction with both mole fractions calculated on an aniline-free basis. It is clear from the first snapshot that, when no aniline is present, the binary mixture exhibits a positive azeotrope. On the other hand, when enough aniline is used (see snapshots 3 and 4), the azeotrope disappears and there is an inversion of volatility, so that the heavy-boiling cyclohexane distills overhead in an extractive distillation process [2, 3]. In this case, the VLE data is below the diagonal black line.