Phase equilibrium

The states of matter – solid, liquid, and gas – are referred to as phases. A transition from one state to another is called a phase change. The freezing and melting temperature for a given material at any pressure, is the same. At this temperature the liquid and solid phases coexist in phase equilibrium. The same also happens in phase transitions between liquid and gas. The heat transfer in a phase change is given by:

where L is the heat of fusion in case of transition between liquid and solid, or heat of vaporization in case of transition between liquid and gas.