Azeotrope

An azeotrope is a liquid mixture that boils at a constant temperature with vapor composition equal to its liquid composition, so ordinary distillation cannot separate its components.

Vapor–liquid equilibrium: The balance of chemical potentials between vapor and liquid phases at equilibrium. An azeotrope occurs where equilibrium vapor and liquid compositions coincide.
Fractional distillation: A separation method that uses repeated vaporization and condensation to enrich components with different volatilities. Repeated stages cannot cross an azeotropic composition at fixed pressure.
Ethanol–water azeotrope: A minimum-boiling mixture of ethanol and water that contains about 95.6% ethanol by mass at atmospheric pressure. It is the familiar example that prevents ordinary distillation from producing pure ethanol.
Zeotropic mixture: A liquid mixture whose vapor and liquid compositions differ during boiling and change through distillation. Unlike an azeotrope, its components can be enriched by ordinary fractional distillation.
Raoult's law: An ideal-solution relation connecting a component's vapor pressure to its liquid mole fraction. Departures from this ideal relation help produce azeotropic compositions.
Azeotropic distillation: A distillation method that adds an entrainer to change mixture volatility and enable separation. An added component can disrupt the original azeotrope and create a separable mixture.
Hydrochloric acid–water azeotrope: A maximum-boiling mixture of hydrogen chloride and water at a composition dependent on pressure. Its maximum-boiling behavior contrasts with the ethanol–water minimum.
Eutectic system: A mixture of solids that melts or solidifies at a single lowest temperature for its composition range. It is a phase-diagram invariant point often confused with an azeotrope, but concerns solid–liquid equilibrium.
Activity coefficient: A factor that measures how a component's effective thermodynamic concentration differs from its ideal-solution value. Composition-dependent activity coefficients shape the vapor–liquid equilibrium that creates an azeotrope.
Extractive distillation: A distillation method that uses a high-boiling solvent to alter component relative volatilities. A selective solvent can help separate components constrained by azeotropic behavior.

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