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Melting (sometimes called fusion) is a physical process that results in the phase change of a substance from a solid to a liquid. The internal energy of a solid substance is increased, typically by the application of heat or pressure, resulting in a rise of its temperature to the melting point, at which the rigid ordering of molecular entities in the solid breaks down to a less-ordered state and the solid liquefies. An object that has melted completely is molten.

Contents 1 Melting point 2 Molecular vibrations 3 Constant temperature 4 Thermodynamics of melting 5 Other meanings 6 See also 7 Further reading

[edit] Melting point

Under a standard set of conditions, the melting point of a substance is a characteristic property. The melting point is often equal to the freezing point. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur. Water on a very clean glass surface will often supercool several degrees below the melting point without freezing. Fine emulsions of pure water have been cooled to -38 degrees Celsius without nucleation to form ice.^{[citation needed]}. Nucleation occurs due to fluctuations in the properties of the material. If the material is kept still there is often nothing (such a physical vibration) to trigger this change, and supercooling (or superheating) may occur. Thermodynamically, the supercooled material is unstable with respect to the frozen phase, and it is likely to change phase suddenly. This phenomenon is similar to hysteresis in permanent magnets, as they are heated and cooled near the Curie point.

[edit] Molecular vibrations

When the internal energy of a solid is increased by the application of external energy, the molecular vibrations of the substance increase. As these vibrations increase, the substance becomes less and less ordered until the stored vibrational energy exceeds the stabilization energy of the solid arrangement and molecules may break free of the lattice.

[edit] Constant temperature

Substances melt at a constant temperature, the melting point. As long as the solid is in equilibrium with its liquid form it does not increase its temperature. The energy used to transform the solid into a liquid is a latent heat. This characterizes the process of melting as a first-order phase transition.

[edit] Thermodynamics of melting

From a thermodynamics point of view, at the melting point the change in Gibbs free energy (ΔG) of the material is zero, but the enthalpy (H) and the entropy (S) of the material are increasing (ΔH,ΔS > 0). Melting occurs when the Gibbs free energy of the liquid becomes lower than the solid for that material. The temperature at which this occurs is dependent on the ambient pressure. It can also be shown that:

The "T","ΔS", and "ΔH" in the above are the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting, respectively.

[edit] Other meanings

Look up melting in Wiktionary, the free dictionary.

In genetics, melting DNA means to separate the double-stranded DNA into two single strands by heating or the use of chemicals.

See also : Polymerase chain reaction

[edit] See also

• List of elements by melting point
• Phase diagram

[edit] Further reading

• Kleinert, Hagen, Gauge Fields in Condensed Matter, Vol. II, "STRESSES AND DEFECTS; Differential Geometry, Crystal Melting", pp. 743-1456, World Scientific (Singapore, 1989); Paperback ISBN 9971-5-0210-0 (readable online here)

	To
From	Solid	Liquid	Gas	Plasma
Solid	Solid-Solid Transformation	Melting/Fusion	Sublimation	N/A
Liquid	Freezing	N/A	Boiling/Evaporation	N/A
Gas	Deposition	Condensation	N/A	Ionization
Plasma	N/A	N/A	Recombination/Deionization	N/A