melt

Melting is the process by which a solid substance changes into a liquid when heat is supplied. It occurs at a characteristic temperature known as the melting point, which for a pure crystalline substance at a given pressure is a single value; impure substances or nanoscale materials may melt over a range of temperatures.

During melting, energy is absorbed as latent heat of fusion, and the solid and liquid phases are in equilibrium at the melting temperature. The Gibbs free energy of the two phases is equal at this point, and structural changes accompany the transition as the material adopts the more disordered liquid state. The density and volume often change during melting, and the process ends when the material becomes completely liquid.

Several factors influence melting points. Pressure can affect condensed phases, especially at extreme conditions. Impurities and alloying elements typically lower the melting point or create a series of distinct melting temperatures in eutectic systems. Crystal structure and polymorphism determine how stable a solid phase is, and particle size can cause melting-point depression at the nanoscale, broadening the transition into a range rather than a sharp point.

Examples and uses are diverse. Pure ice melts at 0°C at standard atmospheric pressure, while metals melt at higher temperatures according to their elemental properties. In industry, melting enables casting, alloying, and soldering. Phase change materials rely on melting and solidification for thermal energy storage, and culinary processes often involve melting fats, sugars, and chocolate.