the vapor of an azeotrope has the same composition as the unboiled mixture
An azeotrope (UK /əˈziːəˌtrəʊp/, US /əˈziəˌtroʊp/) or a constant boiling point mixture is a mixture of two or more liquids whose proportions cannot be altered or changed by simple distillation. This happens because when an azeotrope is boiled, the vapour has the same proportions of constituents as the unboiled mixture.
Azeotrope