The formation of azeotropes due to deviations from Raoult's law was discussed in Section 1.3. An azeotrope is a mixture that, at a given pressure (the azeotropic pressure), boils at a constant temperature (the azeotropic temperature), and has the same composition (the azeotropic composition) in the equilibrium vapor and liquid phases. Homogeneous azeotropes are those that form one liquid phase at equilibrium with the vapor; heterogeneous azeotropes are those that form two liquid phases at equilibrium with each other and the vapor.