schmelzen

Schmelzen is the process by which a solid substance changes into a liquid when heat is supplied. It occurs at a characteristic temperature called the melting point. For pure crystalline substances at a given pressure, melting happens at a single temperature; for impure substances, mixtures, or polymers, melting occurs over a range of temperatures. Pressure can influence the melting point, with different materials showing different dependencies; ice, for example, exhibits a nonmonotonic relationship between pressure and melting point due to its phase diagram.

In thermodynamic terms, melting involves the absorption of latent heat of fusion. The amount of energy required to convert a unit mass of solid to liquid at the melting point is a measure of the strength of the solid’s lattice. At the melting point, solid and liquid phases are in equilibrium. If heat is added beyond the melting point, the temperature of the liquid rises; if heat is removed, the solid re-forms.

At the microscopic level, melting starts with disruption of the ordered crystal lattice, aided by thermal fluctuations and the presence of defects that act as nuclei for the liquid phase. In homogeneous nucleation, nuclei form within the bulk; in heterogeneous nucleation, surfaces and interfaces lower the energy barrier.

Materials vary in their melting behavior. Pure crystalline metals and minerals typically have sharp melting points; impure substances and alloys tend to melt over a range. Some polymers soften gradually rather than melt sharply, and certain compounds decompose before melting. Melting is central to many processes, including metal casting, welding, soldering, glass formation, and geological partial melting that generates magmas.