Melting point
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The '''melting point''' of a solid is the temperature range at which it changes [[states of matter|state]] from solid to [[liquid]]. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds actually melt over a range of a few degrees or less. At the melting point the solid and liquid phase exist in equilibrium. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the '''freezing point'''. Because of the ability of some substances to [[supercool]], the freezing point is not considered to be a characteristic property of a substance.
==Fundamentals==
[[Image:Carboxylic.Acids.Melting.&.Boiling.Points.jpg|thumb|right|Melting points (in blue) and boiling points (in pink) of the first eight [[carboxylic acids]] (°C)]]For most substances, [[melting]] and [[freezing]] points are essentially equal. For example, the melting point ''and'' freezing point of the [[chemical element|element]] [[mercury (element)|mercury]] is 234.32 [[kelvin]] (−38.83 [[Celsius|°C]] or −37.89 [[Fahrenheit|°F]]). However, certain substances possess differing solid-liquid transition temperatures. For example, [[agar]] melts at 85 °C (185 °F) and solidifies from 31 °C to 40 °C (89.6 °F to 104 °F); this process is known as [[hysteresis]].
Certain materials, such as [[glass]], may harden without crystallizing; these are called [[amorphous solid]]s. Amorphous materials as well as some polymers do not have a true melting point as there is no abrupt phase change at any specific temperature. Instead, there is a gradual change in their [[Viscoelasticity|viscoelastic]] properties over a range of temperatures. Such materials are characterized by a [[glass transition temperature]] which may be roughly defined as the "knee" point of the material's density vs. temperature graph.
The melting point of [[ice]] at 1 atmosphere of pressure is very close <ref>The melting point of purified water has been measured to be 0.002519 +/- 0.000002 degrees Celsius - see {{cite journal | author = R. Feistel and W. Wagner | year = 2006
| title = A New Equation of State for H<sub>2</sub>O Ice Ih
| journal = J. Phys. Chem. Ref. Data | volume = 35
| pages = 1021–1047
| doi = 10.1063/1.2183324 }}
</ref> to 0 °C (32 °F, 273.15 K), this is also known as the '''ice point'''. In the presence of [[Nucleation|nucleating substances]] the freezing point of water is the same as the melting point, but in the absence of nucleators water can [[Supercooling|supercool]] to −42 °C (−43.6 °F, 231 K) before freezing.
Unlike the [[boiling point]], the melting point is relatively insensitive to [[pressure]] 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 |accessdate= 2008-02-19 }}</ref>
Melting points are often used to characterize organic compounds and to ascertain the [[purity]]. The melting point of a pure substance is always higher and has a smaller range than the melting point of an impure substance. The more impurity is present, the lower the melting point and the broader the range. Eventually, a minimum melting point will be reached. The mixing ratio that results in the lowest possible melting point is known as the [[eutectic point]].
The [[chemical element]] with the highest melting point is [[tungsten]], at 3695 K (3422 °C, 6192 °F) making it excellent for use as filaments in light bulbs. The often-cited [[carbon]] does not melt at ambient pressure but [[sublimation (physics)|sublimates]] at about 4000 K; a liquid phase only exists above pressures of 10 [[Pascal (unit)|MPa]] and estimated 4300–4700 K. [[Tantalum hafnium carbide]] (Ta<sub>4</sub>HfC<sub>5</sub>) is a [[refractory]] compound with a very high melting point of 4488 K (4215 °C, 7619 °F).<ref>[http://www.britannica.com/nobel/micro/254_6.html hafnium entry at Britannica.com]</ref> At the other end of the scale, [[helium]] does not freeze at all at normal pressure, even at temperatures infinitesimally close to [[absolute zero]]; pressures over 20 times normal atmospheric pressure are necessary.
==Melting point measurements==
Many [[laboratory techniques]] exist for the determination of melting points.
A '''Kofler bench''' is a metal strip with a temperature gradient (range room temperature to 300°C). Any substance can be placed on a section of the strip revealing its thermal behaviour at the temperature at that point. [[Differential scanning calorimetry]] gives information on melting point together with its [[enthalpy of fusion]].
A basic melting point apparatus for the analysis of crystalline solids consists of a [[oil bath]] with a transparent window (most basic design: a [[Thiele tube]]) and a simple magnifier. The several grains of a solid are placed in a thin glass tube and partially immersed in the oil bath. The oil bath is heated (and stirred) and with the aid of the magnifier (and external light source) melting of the individual crystals at a certain temperature can be observed. In contemporary devices, the sample is placed in a heating block, and optical detection is automated.
==Thermodynamics==
[[Image:Melting curve of water.jpg|thumb|right|Pressure dependence of water melting point (MPa/K)]]Not only is heat required to raise the temperature of the solid to the melting point, but the melting itself requires heat called the [[heat of fusion]].
From a thermodynamics point of view, at the melting point the change in [[Gibbs free energy]] (<math>\Delta G</math>) of the material is zero, because the [[enthalpy]] (<math>H</math>) and the [[entropy]] (<math>S</math>) of the material are increasing (<math>\Delta H, \Delta S > 0</math>). 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>
The "<math>T</math>","<math>\Delta S</math>", and "<math>\Delta H</math>" in the above are respectively the [[temperature]] at the melting point, change of [[entropy]] of melting, and the change of [[enthalpy]] of melting.
==Carnelley’s Rule==
In [[organic chemistry]] '''Carnelley’s Rule''' established in 1882 by Thomas Carnelley, states that ''high [[molecular symmetry]] is associated with high melting point'' <ref>''Melting Point and Molecular Symmetry'' R. J. C. Brown, R. F. C. Brown [[Journal of Chemical Education]] 724 Vol. 77 No. 6 June '''2000'''</ref>. Carnelley based his rule on examination of 15,000 chemical compounds. For example for three [[structural isomer]]s with [[molecular formula]] C<sub>5</sub>H<sub>12</sub> the melting point increases in the series [[isopentane]] −160 °C (113 K) [[n-pentane]] −129.8 °C (143 K) and [[neopentane]] −18 °C (255 K). Likewise in [[xylene]]s and also [[dichlorobenzene]]s the melting point increases in the order [[arene substitution patterns|meta, ortho and then para]]. [[Pyridine]] has a lower symmetry than [[benzene]] hence its lower melting point but the melting point again increases with [[diazine]] and [[triazine]]s. Many cage-like compounds like [[adamantane]] and [[cubane]] with high symmetry have very high melting points.
A high melting point results from a high [[heat of fusion]] or a low [[entropy of fusion]] or a combination of both. In highly symmetrical molecules the crystal phase is densely packed with many efficient intermolecular interactions resulting in a higher enthalpy change on melting.
==See also==
* [[Boiling point]]
* [[Freezing-point depression]]
* [[List of elements by melting point]]
* [[Melting]]
* [[Melting points of the elements (data page)|Melting Points for various elements]]
* [[Phase (matter)|Phases of matter]]
* [[Triple point]]
* [[Vicat softening point]] - the determination of the softening point for materials that have no definite melting point.
==References==
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==External links==
* [http://www.archive.org/details/meltingboilingpo01carnuoft Melting and boiling point tables vol. 1] by Thomas Carnelley (Harrison, London, 1885-1887)
* [http://www.archive.org/details/meltingboilingpo02carnuoft Melting and boiling point tables vol. 2] by Thomas Carnelley (Harrison, London, 1885-1887)
{{State of matter}}
[[Category:Fundamental physics concepts]]
[[Category:atmospheric thermodynamics]]
[[Category:Phase changes]]
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