meltingtemperature

Melting temperature, denoted Tm, is the temperature at which a solid becomes a liquid when heated at a specified pressure, typically one atmosphere. For a pure crystalline substance, melting occurs at a definite temperature with a sharp transition and involves the latent heat of fusion (ΔHfus). Impure substances or polymers generally melt over a temperature range, between a solidus and a liquidus, or a melting range, due to the presence of different phases or molecular weights.

Tm is defined on phase diagrams; for most materials, it rises with pressure. Water is unusual: its melting point decreases with increasing pressure due to the negative slope of the ice-water boundary.

Factors that influence melting temperature include purity, crystal structure, polymorphism, molecular weight (in polymers), heating rate, pressure, the presence of additives, and nanoscale effects. Impurities or defects typically lower and broaden the observed melting temperature, while different crystal forms can yield distinct Tm values.

Measurement of melting temperature is commonly performed by differential scanning calorimetry (DSC), differential thermal analysis (DTA), or dedicated melting-point apparatus. Observations can report onset, peak, and endset temperatures, and the measured value can depend on the heating rate and sample preparation. For pure crystals, the melting point is sharp; for mixtures or polymers, a melting range is typical.

Applications of melting temperature include material characterization, quality control, and process design in metallurgy and polymer science. It serves as a basic thermodynamic property used to assess purity, predict processing behavior, and calibrate temperature measurement instruments.