Schmelzpunkt

Schmelzpunkt, in English melting point, is the temperature at which a solid changes into a liquid under a specified pressure, typically one atmosphere. For a pure crystalline substance, this occurs at a single temperature. Impure substances or mixtures do not melt at a single point; they exhibit a melting range, beginning at the melting onset and ending at the temperature of complete liquefaction.

Measurement and terminology: The melting point is determined by observing heat-induced phase change, using methods such as capillary tube melting point, differential scanning calorimetry (DSC), differential thermal analysis (DTA), or hot-stage microscopy. In quality control, the observed melting point helps identify substances and assess purity; a narrow melting range indicates high purity, while a broad range indicates impurities.

Pressure dependence and exceptions: The melting point can shift with pressure, as described by the Clapeyron relation. For most solids near standard pressure, the change is small, but some cases are notable: ice melts under higher pressure to liquid water, exhibiting a negative slope, while most materials have a positive slope. In practice, 1 bar is commonly used as a reference pressure, with small differences from 1 atmosphere in many sources.

Applications: The melting point is a basic physical property used in chemistry, materials science, and pharmaceuticals to characterize substances, verify identity, and assess purity. It informs processing temperatures for manufacturing, formulation, and quality control.