Azeotrope
A liquid mixture that boils at constant temperature and distils without any change in composition.
Unit
°C
In depth
At an azeotropic composition the vapour has exactly the same composition as the liquid, so further distillation achieves no separation. Minimum-boiling azeotropes arise from large positive deviations and maximum-boiling ones from negative deviations. Separation requires a third component or a change of pressure.
Examples in the real world
Ethanol and water form a 95.6% azeotrope boiling at 78.2 °C, which is why absolute ethanol needs a drying agent.