What does azeotropy mean?
Azeotropy is a phenomenon in chemistry whereby two or more liquids mixed together behave, when boiled, like a single substance. In such a case—called an azeotrope—the vapour produced has exactly the same composition as the liquid mixture, so both components evaporate together and the mixture boils at a constant temperature. As a result, simple or even fractional distillation cannot separate the components beyond that fixed proportion; the classic example is the ethanol–water azeotrope at roughly 95% alcohol. Chemists overcome this limit using techniques such as azeotropic distillation, pressure-swing distillation, or added entrainer agents. The term derives from Greek roots meaning 'unboiling'—that is, not changing upon boiling—and belongs to the same '-tropy' family as isotropy and allotropy. Azeotropy matters chiefly in industrial processes: it constrains the purification of solvents, spirits, and acids, making it a central concept in separation science and chemical engineering design.
nounThe phenomenon in which a liquid mixture forms an azeotrope—a mixture of two or more liquids whose proportions cannot be altered by simple distillation because the vapour has the same composition as the liquid.
- The property or phenomenon whereby a liquid mixture boils at a constant temperature and produces vapour of the same composition as the liquid, so its components cannot be fully separated by simple distillation.
"Because of azeotropy, ethanol and water form a constant-boiling mixture at about 95% alcohol that simple distillation cannot purify further."
"The chemist exploited azeotropy to dry the solvent, distilling off the water–benzene azeotrope."
"Textbooks on separation science devote whole chapters to azeotropy because it limits ordinary fractional distillation."
Rarely pluralised in practice, since the term usually describes a single general phenomenon rather than countable instances.
"The lecture surveyed the various azeotropies encountered in industrial solvent recovery."
Some liquid pairs simply refuse to be separated by boiling—no matter how many times you distil them, they rise together as if bound by chemistry itself.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of azeotropy
The word azeotropy was coined in the early twentieth century by English chemists, combining three Greek elements: 'a-' meaning not, 'zein' meaning to boil, and 'tropē' meaning turning—literally 'not turning (or changing) upon boiling'. It shares its '-tropy' suffix with related scientific terms such as isotropy and allotropy, while the 'zein' element ties it to the act of boiling itself.
Related word forms
How azeotropy is actually used
Strictly technical vocabulary, found almost exclusively in chemistry, chemical engineering, and distillation literature; it carries no figurative use in everyday speech.
Easily confused with azeotropy
Isotropy refers to uniform physical properties in all directions, whereas azeotropy concerns liquid mixtures that boil at constant composition.
An azeotrope is the mixture itself; azeotropy is the phenomenon or property such mixtures exhibit.