Definitions

zone melting

Any of a group of techniques used to purify an element or a compound or to control its composition by melting a short region (called a zone) and causing this liquid zone to travel slowly through a relatively long ingot, or charge, of the solid. In zone refining, the most important of the zone-melting techniques, a solid is refined by multiple molten zones being passed through it in one direction. Each zone carries a fraction of the impurities to the end of the solid charge, thereby purifying the remainder. Zone refining is particularly important as a method of purifying crystals, especially for use in semiconductor devices.

Temperature at which the solid and liquid states of a pure substance can exist in equilibrium. As heat is applied to a solid, its temperature increases until it reaches the melting point. At this temperature, additional heat converts the solid into a liquid without a change in temperature. The melting point of solid water (ice) is 32°F (0°C). Though the melting point of a solid is generally considered to be the same as the freezing point of the corresponding liquid, they may differ because a liquid may freeze into different crystal systems and impurities can lower the freezing point.

Melting is a 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) to a specific temperature (called the melting point) at which it changes to the liquid phase. An object that has melted completely is molten.

The melting point of a substance is a characteristic property. The melting point may not be equal to the freezing point. This is evident in the phenomenon known as supercooling. In the case of water, ice crystals typically require a seed on which to begin formation. 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 the nucleation of ice taking place. For this reason, melting point is a characteristic property of a substance while freezing point is not.

Molecular vibrations

When the internal energy of a solid is increased by the application of an external energy source, the molecular vibrations of the substance increases. As these vibrations increase, the substance becomes less and less ordered. Fusion is also another term used for this.

Constant temperature

Substances melt at a constant temperature, the melting point. Further increases in temperature (even with continued application of energy) do not occur until the substance is molten.

The thermodynamics of melting

From a thermodynamics point of view, at the melting point the change in Gibbs free energy ($Delta G$) of the Material is zero, because the enthalpy ($H$) and the entropy ($S$) of the material are increasing ($Delta H, Delta S > 0$). Melting phenomenon happens when the Gibbs free energy of the liquid becomes lower than the solid for that material. At various pressures this happens at a specific temperature. It can also be shown that:

$Delta S = frac \left\{Delta H\right\} \left\{T\right\}$

The "$T$","$Delta S$", and "$Delta H$" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting.

Other meanings

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