Editing Melting point (section)

==Thermodynamics==
[[File:Melting curve of water.svg|thumb|upright=1.45|Pressure dependence of water melting point.]]
For a solid to melt, [[heat]] is required to raise its temperature to the melting point. However, further heat needs to be supplied for the melting to take place: this is called the [[heat of fusion]], and is an example of [[latent heat]].<ref>{{Cite web |date=2024-06-04 |title=Latent heat {{!}} Definition, Examples, & Facts {{!}} Britannica |url=https://www.britannica.com/science/latent-heat |access-date=2024-06-28 |website=www.britannica.com |language=en}}</ref>

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 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:

: <math>\Delta S = \frac {\Delta H} {T}</math>

Here ''T'', ''ΔS'' and ''ΔH'' are respectively the temperature at the melting point, change of entropy of melting and the change of enthalpy of melting.

The melting point is sensitive to extremely large changes in [[pressure]], but generally this sensitivity is orders of magnitude less than that for the [[boiling point]], because the solid-liquid transition represents only a small change in volume.<ref>The exact relationship is expressed in the [[Clausius–Clapeyron relation]].</ref><ref>{{cite web |url= http://mpec.sc.mahidol.ac.th/RADOK/physmath/PHYSICS/j10.htm |title= J10 Heat: Change of aggregate state of substances through change of heat content: Change of aggregate state of substances and the equation of Clapeyron-Clausius |access-date= 19 February 2008 |archive-date= 28 February 2008 |archive-url= https://archive.wikiwix.com/cache/20080228000000/http://mpec.sc.mahidol.ac.th/RADOK/physmath/PHYSICS/j10.htm |url-status= dead }}</ref> If, as observed in most cases, a substance is more dense in the solid than in the liquid state, the melting point will increase with increases in pressure. Otherwise the reverse behavior occurs. Notably, this is the case of water, as illustrated graphically to the right, but also of Si, Ge, Ga, Bi. With extremely large changes in pressure, substantial changes to the melting point are observed. For example, the melting point of silicon at ambient pressure (0.1 MPa) is 1415&nbsp;°C, but at pressures in excess of 10 GPa it decreases to 1000&nbsp;°C.<ref>Tonkov, E. Yu. and Ponyatovsky, E. G. (2005) ''Phase Transformations of Elements Under High Pressure'', CRC Press, Boca Raton, p. 98 {{ISBN|0-8493-3367-9}}</ref>

Melting points are often used to characterize organic and inorganic compounds and to ascertain their [[wiktionary:Purity|purity]]. The melting point of a pure substance is always higher and has a smaller range than the melting point of an impure substance or, more generally, of mixtures. The higher the quantity of other components, the lower the melting point and the broader will be the melting point range, often referred to as the "pasty range". The temperature at which melting begins for a mixture is known as the ''[[Solidus (chemistry)|solidus]]'' while the temperature where melting is complete is called the ''[[liquidus]]''. Eutectics are special types of mixtures that behave like single phases. They melt sharply at a constant temperature to form a liquid of the same composition. Alternatively, on cooling a liquid with the eutectic composition will solidify as uniformly dispersed, small (fine-grained) mixed crystals with the same composition.

In contrast to crystalline solids, [[glass]]es do not possess a melting point;
on heating they undergo a smooth [[glass transition]] into a [[viscous liquid]].
Upon further heating, they gradually soften, which can be characterized by certain [[softening point]]s.