equilibrium liquid line

In chemistry, an equilibrium liquid line represents the conditions of temperature and pressure where a substance exists as both a liquid and a gas simultaneously. This line is a critical component of a phase diagram, which graphically illustrates the physical states of a substance under varying conditions.12

At any point along this equilibrium line, the liquid and its vapor are in balance, meaning the rate at which liquid turns into vapor is equal to the rate at which vapor turns back into liquid. As temperature or pressure changes, this equilibrium shifts. The line extends to a point called the critical point, beyond which the liquid and gas phases become indistinguishable. Surface tension, the force that causes a liquid's surface to contract, also diminishes with increasing temperature and disappears at the critical point.1324