= Vapor-liquid equilibrium
Vapor-liquid equilibrium is coexistence of a <liquid> and its vapor with no net macroscopic transfer of matter between them. The two phases have equal <temperature>, equal <pressure> for a flat interface without external stress, and equal <chemical potential> for each exchanging species.
For a pure substance, fixing the <temperature> below its <liquid-gas critical point> selects the saturation <vapor pressure>. Molecules still cross the interface in both directions; equilibrium means equal average opposing rates, rather than halted molecular motion. In a mixture, the vapor and liquid generally have different compositions, while each component separately satisfies the chemical-potential equality. This distinction underlies separation by evaporation and condensation and explains why equal total pressure alone is insufficient to determine phase compositions.
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