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).