<P> 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 . </P> <P> Because their composition is unchanged by distillation, azeotropes are also called (especially in older texts) constant boiling - point mixtures . The word azeotrope is derived from the Greek words ζέειν (boil) and τρόπος (turning) combined with the prefix α - (no) to give the overall meaning, "no change on boiling". The term "azeotrope" was coined in 1911 by English chemist John Wade (1864--1912) and Richard William Merriman . </P>

The vapor of an azeotrope has the same composition as the unboiled mixture