What is the definition of "phase equilibrium"?

Phase equilibrium is the state of balance between two phases of a molecule. It occurs when the transfer of matter or heat energy from one phase to another phase is equal to the rate of transfer of matter or heat energy in the reverse direction.

The driving forces of phase equilibrium are equal during phase equilibrium. The three different phases of compounds are solid, liquid and gas. To change phases, a compound must gain or lose energy in the form of matter or heat. Compounds can change from one phase to another if enough energy is applied to the system.