Melting point

Melting point
From Wikipedia, the free encyclopedia
Jump to: navigation, search
It has been suggested that melting be merged into this article or section. (Discuss)
The melting point of a crystalline solid is the temperature range at which it changes 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.

Contents [hide]
1 Fundamentals
2 Melting point measurements
3 Thermodynamics
4 Carnelley’s Rule
5 See also
6 References



[edit] Fundamentals

Melting points (in blue) and boiling points (in pink) of the first eight Carboxylic acids (°C)For most substances, melting and freezing points are equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvin (−38.83 °C or −37.89 °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 solids. 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 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 water at 1 atmosphere of pressure is very close [1] to 0 °C (32 °F, 273.15 K), this is also known as the ice point. In the presence of nucleating substances the freezing point of water is the same as the melting point, but in the absence of nucleators water can supercool to −42 °C (−43.6 °F, 231 K) before freezing.

Unlike the boiling point, the melting point is relatively insensitive to pressure. 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 sublimates at about 4000 K; a liquid phase only exists above pressures of 10 MPa and estimated 4300–4700 K. Tantalum hafnium carbide (Ta4HfC5) is a refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F).[2] 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.





[edit] 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 place 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.


[edit] Thermodynamics

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 (ΔG) of the material is zero, because 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:



The "T","ΔS", and "ΔH" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting.


[edit] 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 [3]. Carnelley based his rule on examination of 15,000 chemical compounds. For example for three structural isomers with molecular formula C5H12 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 xylenes and also dichlorobenzenes the melting point increases in the order 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 triazines. 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. In highly symmetrical molecules the crystal phase is densely packed with many efficient intermolecular interactions resulting in a higher enthalpy change on melting.





[edit] See also
Melting
Phases of matter
Triple point
Freezing-point depression
Boiling point
Melting Points for various elements

[edit] References
^ The ice point of purified water has been measured to be 0.000089 +/- 0.00001 degrees Celsius - see Magnum, B.W. (June 1995). "Reproducibility of the Temperature of the Ice Point in Routine Measurements" (PDF). Nist Technical Note 1411. Retrieved on 2007-02-11.
^ hafnium entry at Britannica.com
^ Melting Point and Molecular Symmetry R. J. C. Brown, R. F. C. Brown Journal of Chemical Education 724 Vol. 77 No. 6 June 2000
Retrieved from "http://en.wikipedia.org/wiki/Melting_point"
Categories: Articles to be merged since July 2007 | Fundamental physics concepts | Chemical properties | Atmospheric thermodynamics | Phase changes

ViewsArticle Discussion Edit this page History Personal toolsSign in / create account Navigation
Main page
Contents
Featured content
Current events
Random article
interaction
About Wikipedia
Community portal
Recent changes
Contact Wikipedia
Make a donation
Help
Search
Toolbox
What links here
Related changes
Upload file
Special pages
Printable version
Permanent link
Cite this article
In other languages
Afrikaans
العربية
Asturianu
Bosanski
Български
Català
Česky
Dansk
Deutsch
Eesti
Ελληνικά
Español
Esperanto
فارسی
Français
한국어
Hrvatski
Bahasa Indonesia
Íslenska
Italiano
עברית
Latviešu
Lietuvių
Lojban
Lumbaart
Magyar
Македонски
Bahasa Melayu
Nederlands
日本語
‪Norsk (bokmål)‬
‪Norsk (nynorsk)‬
Polski
Português
Română
Русский
Simple English
Slovenščina
Српски / Srpski
Srpskohrvatski / Српскохрватски
Suomi
Svenska
ไทย
Türkçe
Українська
O'zbek
粵語
中文

This page was last modified 17:43, 4 August 2007. All text is available under the terms of the GNU Free Documentation License. (See Copyrights for details.)
Wikipedia® is a registered trademark of the Wikimedia Foundation, Inc., a US-registered 501(c)(3) tax-deductible nonprofit charity.
Privacy policy About Wikipedia Disclaimers