Physical equilibrium

A system is said to be in physical equilibrium when the amount of matter making up each of its different phases stays constant with time, with no perceptible physical change taking place overall. Three classic phase-change equilibria illustrate this.

Solid-liquid equilibrium. Consider ice melting inside a closed container held at 273 K. Left long enough, the system settles into a physical equilibrium in which the amount of water present as solid ice and the amount present as liquid water no longer change with time -- at any instant, exactly as many water molecules are leaving the solid phase (melting) as are returning to it (freezing). A thermos flask holding ice cubes and water together at 273 K and 1 atm shows this directly: the mass of ice and the mass of water both stay steady. At equilibrium, the rate of melting of ice equals the rate of freezing of water:

This equilibrium exists only at one particular temperature and pressure for a given substance -- the temperature at which the solid and liquid phases of a substance coexist in equilibrium is called its melting point (equivalently, its freezing point).

Liquid-vapour equilibrium. In just the same way, a liquid can be in equilibrium with its own vapour. Liquid water in a closed vessel at 373 K and 1 atm is in equilibrium with water vapour:

Here the rate of evaporation equals the rate of condensation. The temperature at which the liquid and vapour phases of a substance are in equilibrium is called the boiling point (equivalently, the condensation point) of that liquid. …