Editing Freezing (section)

==Exothermicity==
{{main|Enthalpy of fusion}}
Freezing is almost always an [[exothermic]] process, meaning that as liquid changes into solid, heat and pressure are released. This is often seen as counter-intuitive, since the temperature of the material does not rise during freezing, except if the liquid were [[supercooled]]. But this can be understood since heat must be continually removed from the freezing liquid or the freezing process will stop. The energy released upon freezing is a [[latent heat]], and is known as the [[enthalpy of fusion]] and is exactly the same as the energy required to [[melting|melt]] the same amount of the solid.

Low-temperature [[helium]] is the only known exception to the general rule.<ref>{{Citation |last1=Atkins |first1=Peter |last2=Jones |first2=Loretta | name-list-style = vanc |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 ''added'' to these substances in order to freeze them.<ref>{{cite book |last1=Ott |first1=J. Bevan |last2=Boerio-Goates |first2=Juliana | name-list-style = vanc  |year=2000 |title=Chemical Thermodynamics: Advanced Applications |publisher=Academic Press |isbn=0-12-530985-6 |pages=92–93}}</ref>