Vapor–liquid equilibrium

Vapor–liquid equilibrium is the thermodynamic condition in which liquid and vapor coexist without net transfer of matter between phases. Their compositions are set by equality of each component’s chemical potential across the phases.

Fugacity: A thermodynamic quantity that represents a species’ effective pressure in a nonideal system. Equilibrium requires each component’s fugacity to match across liquid and vapor.
Phase: A region of matter with uniform physical and chemical properties, separated from other regions by an interface. Vapor–liquid equilibrium concerns two distinct phases sharing an interface.
Distillation: A separation process that exploits differences in component volatility through repeated vaporization and condensation. Its separation performance depends on vapor and liquid compositions at equilibrium.
Ideal solution: A solution whose components obey Raoult’s law across the full composition range. It is the simplifying reference case for nonideal vapor–liquid mixtures.
Chemical potential: The change in a system’s Gibbs energy when the amount of a component changes at fixed conditions. Equality of chemical potential is the fundamental condition for phase equilibrium.
Gibbs free energy: A thermodynamic potential that determines equilibrium at constant temperature and pressure. Stable coexistence minimizes Gibbs free energy subject to material balance.
Flash evaporation: A process in which a liquid mixture partially vaporizes after a pressure or temperature change. Flash calculations determine the coexisting phase amounts and compositions.
Azeotrope: A mixture that boils at a fixed composition, with vapor and liquid compositions equal at a given pressure. It marks a composition where ordinary distillation cannot enrich either component.
Raoult's law: An ideal-solution relation stating that a component’s partial pressure equals its liquid mole fraction times its pure-component vapor pressure. It approximates vapor composition when liquid interactions resemble those of the pure components.
Phase rule: A relation that counts the independent intensive variables of a system at equilibrium. It predicts how many conditions can vary in a specified vapor–liquid system.

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