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{{about||the EP by Hyuna|Melting (EP)|the album by Mamamoo|Melting (album)}}
{{Redirect|Molten}}
[[File:Melting icecubes.gif|thumb|Melting ice cubes illustrate the process of fusion.]]

'''Melting''', or [[Enthalpy of fusion|fusion]], is a physical process that results in the [[phase transition]] of a [[chemical substance|substance]] from a [[solid]] to a [[liquid]]. This occurs when the [[internal energy]] of the solid increases, typically by the application of [[heat]] or [[pressure]], which increases the substance's [[temperature]] to the [[melting point]]. At the melting point, the ordering of [[ion]]s or [[molecule]]s in the solid breaks down to a less ordered state, and the solid '''melts''' to become a liquid.

Substances in the molten state generally have reduced [[viscosity]] as  the temperature increases. An exception to this principle is the element [[sulfur]], whose viscosity increases in the range of 160 °C to 180 °C due to [[polymerization]].<ref name="Sofekun Evoy Lesage Chou 2018 pp. 469–476">{{cite journal | last=Sofekun | first=Gabriel O. | last2=Evoy | first2=Erin | last3=Lesage | first3=Kevin L. | last4=Chou | first4=Nancy | last5=Marriott | first5=Robert A. | title=The rheology of liquid elemental sulfur across the λ-transition | journal=Journal of Rheology | publisher=Society of Rheology | volume=62 | issue=2 | year=2018 | issn=0148-6055 | doi=10.1122/1.5001523 | pages=469–476}}</ref>

Some [[organic compound]]s melt through [[mesophase]]s, states of partial order between solid and liquid.

== As a 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 |last=Atkins |first=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 |last=Ott |first=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>

==Criteria==
Among the theoretical criteria for melting, the [[Frederick Lindemann, 1st Viscount Cherwell|Lindemann]]<ref>{{cite journal|first=F.A.|last=Lindemann|author-link=Frederick Lindemann, 1st Viscount Cherwell|journal=Physikalische Zeitschrift|volume=11|issue=14|year=1910|page=609–614|title=Über die Berechnung molekularer Eigenfrequenzen|url=https://babel.hathitrust.org/cgi/pt?id=njp.32101054770845&view=1up&seq=147|language=de}}</ref> and [[Max Born|Born]]<ref>{{cite journal | last=Born | first=Max |author-link=Max Born| title=Thermodynamics of Crystals and Melting | journal=The Journal of Chemical Physics | publisher=AIP Publishing | volume=7 | issue=8 | year=1939 | issn=0021-9606 | doi=10.1063/1.1750497 | pages=591–603}}</ref> criteria are those most frequently used as a basis to analyse the melting conditions . The Lindemann criterion states that melting occurs because of vibrational instability, e.g. [[crystal]]s melt when the average amplitude of thermal vibrations of atoms is relatively high compared with interatomic distances, e.g. <''δu<sup>2</sup>''><sup>1/2</sup> > ''δ<sub>L</sub>R<sub>s</sub>'', where ''δu'' is the atomic displacement, the Lindemann parameter ''δ<sub>L</sub>'' ≈ 0.20...0.25 and ''R<sub>s</sub>'' is one-half of the inter-atomic distance.<ref name="Rice2008">{{cite book|author=Stuart A. Rice|title=Advances in Chemical Physics|date=15 February 2008|publisher=John Wiley & Sons|isbn=978-0-470-23807-3}}</ref>{{rp|177}} The Lindemann melting criterion is supported by experimental data both for [[crystal]]line materials and for [[glass-liquid transition]]s in amorphous materials. The Born criterion is based on rigidity catastrophe caused by the vanishing elastic shear modulus, e.g. when the crystal no longer has sufficient rigidity to mechanically withstand load.<ref>[[Robert W. Cahn]] (2001) [http://www.nature.com/nature/journal/v413/n6856/full/413582a0.html Materials science: Melting from Within], [[Nature (journal)|Nature]] 413 (#6856)</ref>

==Supercooling==
{{main|Supercooling}}

Under a standard set of conditions, the melting point  of a substance is a characteristic property.  The melting point is often equal to the [[melting point|freezing point]]. However, under carefully created conditions, supercooling or superheating past the melting or freezing point can occur. [[properties of water|Water]] on a very clean glass surface will often supercool several degrees below the freezing point without freezing.  Fine emulsions of pure water have been cooled to −38 degrees Celsius without nucleation to form [[ice]].{{Citation needed|date=April 2008}} 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 liquid is in the metastable state with respect to the crystalline phase, and it is likely to crystallize suddenly.

== Amorphous solids (glasses)==
Glasses are [[amorphous solid]]s which are usually fabricated when the molten material cools very rapidly to below its glass transition temperature, without sufficient time for a regular crystal lattice to form. Solids are characterised by a high degree of connectivity between their molecules, and fluids have lower connectivity of their structural blocks. Melting of a solid material can also be considered as a percolation via broken connections between particles e.g. connecting bonds.<ref>{{cite journal | last=Park | first=Sung Yong | last2=Stroud | first2=D. | title=Theory of melting and the optical properties of gold/DNA nanocomposites | journal=Physical Review B | publisher=American Physical Society (APS) | volume=67 | issue=21 | date=11 June 2003 | issn=0163-1829 | doi=10.1103/physrevb.67.212202 | page=212202| arxiv=cond-mat/0305230 }}</ref> In this approach melting of an amorphous material occurs when the broken bonds form a percolation cluster with ''T<sub>g</sub>'' dependent on quasi-equilibrium thermodynamic parameters of bonds e.g. on enthalpy (''H<sub>d</sub>'') and entropy (''S<sub>d</sub>'') of formation of bonds in a given system at given conditions:<ref>{{cite journal | last=Ojovan | first=Michael I | last2=Lee | first2=William (Bill) E | title=Connectivity and glass transition in disordered oxide systems | journal=Journal of Non-Crystalline Solids | publisher=Elsevier BV | volume=356 | issue=44-49 | year=2010 | issn=0022-3093 | doi=10.1016/j.jnoncrysol.2010.05.012 | pages=2534–2540}}</ref>

:<math>T_g = \frac{H_d}{S_d+ R \ln(\frac{1-f_c}{f_c})},</math>

where ''f<sub>c</sub>'' is the percolation threshold and ''R'' is the universal gas constant. Although ''H<sub>d</sub>'' and ''S<sub>d</sub>'' are not true equilibrium thermodynamic parameters and can depend on the cooling rate of a melt they can be found from available experimental data on [[viscosity of amorphous materials]].

Even below its melting point, quasi-liquid films can be observed on crystalline surfaces. The thickness of the film is temperature dependent. This effect is common for all crystalline materials. Pre-melting shows its effects in e.g. frost heave, the growth of snowflakes and, taking grain boundary interfaces into account, maybe even in the movement of [[glacier]]s.

==Related concepts==
In [[genetics]], melting [[DNA]] means to separate the double-stranded DNA into two single strands by heating or the use of chemical agents, cf. [[polymerase chain reaction]].

==See also==
* [[List of chemical elements]] providing melting points
* [[Phase diagram]]
* [[Zone melting]]

==References==
{{reflist}}

{{Table of phase transitions}}

==External links==
{{Wiktionary|melting}}
{{State of matter}}

[[Category:Phase transitions]]
[[Category:Materials science]]
[[Category:Thermodynamics]]