Editing Melting (section)

== First order phase transition ==
From a [[thermodynamics]] point of view, at the melting point the change in [[Gibbs free energy]] ''∆G'' of the substances is zero, but there are non-zero changes in the [[enthalpy]] (''H'') and the [[entropy]] (''S''), known respectively as the [[enthalpy of fusion]] (or [[latent heat]] of fusion) and the [[entropy of fusion]]. Melting is therefore classified as a [[Phase transition#Modern classifications|first-order phase transition]]. 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.

Low-temperature [[helium]] is the only known exception to the general rule.<ref>{{Citation |last1=Atkins |first1=Peter |last2=Jones |first2=Loretta |year=2008 |title=Chemical Principles: The Quest for Insight |edition=4th |publisher=W. H. Freeman and Company |isbn=978-0-7167-7355-9 |page=236}}</ref> [[Helium-3]] has a negative enthalpy of fusion at temperatures below 0.3 K. [[Helium-4]] also has a very slightly negative enthalpy of fusion below 0.8 K. This means that, at appropriate constant pressures, heat must be ''removed'' from these substances in order to melt them.<ref>{{Citation |last1=Ott |first1=J. Bevan |last2=Boerio-Goates |first2=Juliana |year=2000 |title=Chemical Thermodynamics: Advanced Applications |publisher=Academic Press |isbn=978-0-12-530985-1 |pages=92–93}}</ref>