change of a solid into a liquid when heat is applied. In a pure crystalline solid, this process occurs at a fixed temperature called the melting point (q.v.); an impure solid generally melts over a range of temperatures below the melting point of the principal component. Amorphous (non-crystalline) substances such as glass or pitch melt by gradually decreasing in viscosity as temperature is raised, with no sharp transition from solid to liquid.

The structure of a liquid is always less ordered than that of the crystalline solid and, therefore, the liquid commonly occupies a larger volume. The behaviour of ice, which floats on water, and of a few other substances are notable exceptions to the usual decrease in density upon melting.

Thermal fusion of a given mass of a solid requires the addition of a characteristic amount of heat, the heat of fusion. In the reverse process, the freezing of the liquid to form the solid, the same quantity of heat must be removed. The heat of fusion of ice, the heat required to melt one gram, is about 80 calories; this amount of heat would raise the temperature of a gram of liquid water from the freezing point (0° C, or 32° F) to 80° C (176° F).