Melting is a physical process that occurs when a solid substance changes into a liquid state by adding heat or pressure. The internal energy of the solid increases, which increases the substances temperature to the melting point. At the melting point, the ordering of ions or molecules in the solid breaks down to a less ordered state, and the solid melts to become a liquid. The temperature at which a solid is changed into a liquid by applying heat is called its melting point.
In a pure crystalline solid, melting occurs at a fixed temperature called the melting point, while an impure solid generally melts over a range of temperatures below the melting point of the principal component. Amorphous 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 behavior of ice, which floats on water, and of a few other substances are notable exceptions to the usual decrease in density upon melting. 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 process of melting 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.
