azeotrope

thumb|upright=1.5|Vapour-liquid equilibrium of 2-propanol/water showing positive azeotropic behaviour

An azeotrope () or a constant heating point mixture is a mixture of two or more liquids whose proportions cannot be changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture. Knowing an azeotrope's behavior is important for distillation.