Melting point

The melting point of a 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.

Fundamentals

For most substances, hysteresis.

Certain materials, such as glass, may harden without crystallizing; these are called 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 supercool to −42 °C (−43.6 °F, 231 K) before freezing.

Unlike the pressure. Melting points are often used to characterize organic compounds and to ascertain the eutectic point.

The 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. Enthalpy of fusion.

A basic melting point apparatus for the analysis of crystalline solids consists of 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 this optical detection is automated.

Thermodynamics

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 enthalpy ( $H$ ) and the entropy ( $S$ ) of the material are increasing ( $Δ H,Δ S > 0$ ). 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:

$\Delta S = \frac {\Delta H} {T}$

The " $T$ "," $Δ S$ ", and " $Δ H$ " in the above are respectively the enthalpy of melting.

Carnelley’s Rule

In 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.

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

v • d • e Phases of Matter (Cooling curve

Categories: Phase changes

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