Phase transitions play an important theoretical and practical role in the study of heat flow. In melting or fusion, a solid turns into a liquid; the opposite process is freezing. In evaporation, a liquid turns into a gas; the opposite process is condensation.

A substance melts or freezes at a temperature called its melting point and boils or condenses at its boiling point. These temperatures depend on pressure. High pressure favors the denser form of the substance, so typically, high pressure raises the melting point and boiling point, and low pressure lowers them.

For example, the boiling point of water is 100 °C at 1.00 atm. At higher pressures, the boiling point is higher, and at lower pressures, it is lower. The main exception is the melting and freezing of water.

The energy involved in a phase change depends on the number of bonds or force pairs and their strength. The number of bonds is proportional to the number of molecules and, thus, to the mass of the sample. The energy per unit mass required to change a substance from the solid phase to the liquid phase, or released when the substance changes from liquid to solid, is known as the heat of fusion. The energy per unit mass required to change a substance from the liquid phase to the vapor phase is known as the heat of vaporization. Further, the latent heat of fusion and the latent heat of vaporization are material constants that can be determined experimentally. These constants are "latent,"or hidden, because, in phase changes, energy enters or leaves a system without causing a temperature change in the system; so, in effect, the energy is hidden.