meltingpoint

Melting point is the temperature at which a solid substance changes into a liquid when heat is applied at a specified pressure, most commonly at 1 atmosphere of pressure. For a pure crystalline substance, this transition occurs at a single, well-defined temperature. Impure or imperfect samples do not melt at a single point; instead they exhibit a melting range, with a solidus temperature where melting begins and a liquidus temperature where melting completes.

Thermodynamically, melting occurs when the solid’s Gibbs free energy equals that of the liquid, and it involves the absorption of latent heat of fusion. The temperature dependence on pressure is described by the Clapeyron equation; for most substances, increasing pressure raises the melting point, but water is an exception because ice is less dense than liquid water, giving a negative slope.

Factors that influence the observed melting point include impurities, crystallite size, defects, and polymorphism. Impurities disrupt the crystal lattice and typically depress and broad­­en the melting range. Different crystalline forms (polymorphs) of the same substance can have different melting points. Temperature rate during measurement and ambient pressure also affect the observed value.

Melting points are determined by methods such as capillary melting point techniques or differential scanning calorimetry (DSC). In the capillary method, a small sample is heated gradually in a capillary tube and the onset and completion of melting are recorded. In DSC, the heat flow to a sample is monitored to identify the melting peak and the latent heat of fusion.

Applications include material characterization, quality control in pharmaceuticals and polymers, and the study of phase behavior in alloys and minerals. Common reference points include the melting of ice at 0°C at 1 atm and various substances with characteristic melting temperatures.