What does azeotrope mean?
An azeotrope is a mixture of two or more liquids that boils at a constant temperature and behaves, during distillation, as though it were a single pure substance. Its defining feature is that the vapor produced has exactly the same composition as the liquid mixture itself, which means the components cannot be fully separated by ordinary distillation. A classic example is the ethanol–water azeotrope, which caps simple distillation at roughly 95% alcohol — a fact of great practical importance in industries ranging from spirits production to fuel manufacturing. The word derives from Greek roots meaning 'not boiling over' or 'unchanging on boiling,' coined in the early twentieth century by chemists studying such mixtures. Azeotropes may boil above or below the boiling points of their individual components, and specialized techniques such as azeotropic distillation are required to separate them. The term is technical in register, used mainly in chemistry, chemical engineering, and related industrial contexts.
nounA mixture of two or more liquids that boils at a constant temperature and distills as a single substance whose vapor has the same composition as the liquid, so its components cannot be separated by simple distillation.
- A liquid mixture of two or more components that boils at a constant temperature and produces vapor of the same composition as the liquid, behaving like a single pure substance during distillation.
"The ethanol–water azeotrope limits ordinary distillation to about 95% alcohol."
"Chemists must break the ethanol–water azeotrope using molecular sieves to obtain pure alcohol."
"The hydrochloric acid–water azeotrope boils at just over 108 °C and is often sold as concentrated acid."
Standard English plural; commonly used when referring to multiple constant-boiling mixtures or to the phenomenon across different chemical systems.
"Many binary mixtures form azeotropes at specific compositions and pressures."
Some liquid mixtures refuse to be separated by boiling — no matter how carefully you distill them, they evaporate together in perfect lockstep.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of azeotrope
The word 'azeotrope' was coined in the early twentieth century from Greek elements: 'a-' meaning 'not,' 'zein' meaning 'to boil' (related to 'zeo,' to seethe), and 'tropos' meaning 'turning' or 'change.' It literally suggests something that does not change when boiled — fitting for a mixture whose composition stays constant during distillation. The '-trope' element connects it to words like heliotrope and zoetrope, all built on the same Greek root for turning.
Related word forms
How azeotrope is actually used
A technical term used almost exclusively in chemistry and chemical engineering; it carries no informal register and appears rarely outside scientific writing.
Easily confused with azeotrope
An isotrope relates to uniform properties in all directions (isotropy), whereas an azeotrope is a constant-boiling liquid mixture; they share only the '-trope' ending.