Melting

Melting is the transition of a substance from a solid to a liquid when heat is added. In pure crystalline materials, melting occurs at a definite temperature called the melting point. Impure substances, mixtures, or materials that decompose upon heating generally melt over a range of temperatures or may not melt at all in a conventional sense.

When melting occurs, energy is absorbed without an immediate rise in temperature; this is the latent heat of fusion. The temperature remains near the melting point until the solid has converted to liquid. For most substances, pressure has a modest effect on the melting point, but exceptional cases exist, such as water ice, where pressure changes can shift the melting point.

Impurities disrupt orderly crystal lattices and lower or broaden the melting range. Some materials exhibit congruent melting, where the compound melts to a liquid of the same composition; others exhibit incongruent melting, or decompose on heating. In mixtures, melting behavior forms eutectic reactions at specific compositions and temperatures, producing mixtures that melt at temperatures lower than either constituent.

Not all solids melt at a single temperature. Glass, for example, does not have a true melting point but becomes progressively softer through the glass transition. Metals melt at characteristic temperatures, enabling processes such as casting, welding, and soldering.

Applications and relevance: melting is central to metallurgy, ceramics, and food science; it governs crystal growth, phase diagrams, and thermal processing. Understanding melting points aids material design and quality control.