 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.

Vapor-liquid equilibrium (VLE) refers to the condition where a liquid and its vapor phase coexist at a specific temperature and pressure, such that the rates of evaporation and condensation are equal. At this equilibrium state, the vapor is in a saturated state, meaning it contains the maximum amount of vapor that can exist at that temperature and pressure without condensing further.