Vapor–liquid equilibrium

Vapor–liquid equilibrium (VLE) is a condition in which a liquid and its vapor (gas phase) are in equilibrium with each other, a condition or state where the rate of evaporation (liquid changing to vapor) equals the rate of condensation (vapor changing to liquid) on a molecular level such that there is no net (overall) vapor–liquid interconversion. A substance at vapor–liquid equilibrium is generally referred to as a saturated fluid. For a pure chemical substance, this implies that it is at its boiling point. The notion of "saturated fluid" includes saturated liquid (about to vaporize), saturated liquid–vapor mixture, and saturated vapor (about to condense).