Тачка топљења

Тачка топљења леда је 0^оС

Тачка топљења је температура при којој супстанција из чврстог агрегатног стања прелази у течно. То је карактеристична константа за сваку супстанцу. Она је обично једнака тачки очвршћавања. Тачке топљења и очвршћавања добро су дефинисане само за кристалне материјале. Аморфни материјали, на пример стакло, не топе се на одређеној температури већ са порастом темепратуре омекшавају.

Под посебним околностима, могуће је чврсто тело загревати изнад тачке топљења без фазног прелаза у течно стање (суперзагревање), односно хладити течност испод тачке мржњења (суперхлађење). То је рецимо случај са водом на веома чистој површини од стакла. Фине емулзије чисте воде су експериментално хлађене на -38 °C, а да се нису заледиле. Згрушавање течности се иницира путем мале промене у особини материјала (рецимо вибрација). Ако се материја остави у потпуно мирном стању, могуће је постићи појаву суперхлађења или суперзагревања. Материје у оваквом стању су термодинамчки нестабилне и могу нагло променити агрегатно стање. Ове појаве су сличне хистерезису код сталних магнета.

When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point or crystallization point. Because of the ability of substances to supercool, the freezing point can easily appear to be below its actual value. When the "characteristic freezing point" of a substance is determined, in fact the actual methodology is almost always "the principle of observing the disappearance rather than the formation of ice, that is, the melting point."

Примери

Melting points (in blue) and boiling points (in pink) of the first eight carboxylic acids (°C)

For most substances, melting and freezing points are approximately equal. For example, the melting point and freezing point of mercury is 23.432 kelvins (23.159 °C; 41.718 °F). However, certain substances possess differing solid-liquid transition temperatures. For example, agar melts at 85 °C (185 °F; 358 K) and solidifies from 31 °C (88 °F; 304 K); such direction dependence is known as hysteresis. The melting point of ice at 1 atmosphere of pressure is very close to 0 °C (32 °F; 273 K); this is also known as the ice point. In the presence of nucleating substances, the freezing point of water is not always the same as the melting point. In the absence of nucleators water can exist as a supercooled liquid down to −483 °C (−837 °F; −210 K) before freezing.

The metal with the highest melting point is tungsten, at 3.414 °C (6.177 °F; 3.687 K); this property makes tungsten excellent for use as electrical filaments in incandescent lamps. The often-cited carbon does not melt at ambient pressure but sublimes at about 3.700 °C (6.700 °F; 4.000 K); a liquid phase only exists above pressures of 10 MPa (99 atm) and estimated 4.030—4.430 °C (7.290—8.010 °F; 4.300—4.700 K) (see carbon phase diagram). Tantalum hafnium carbide (Ta₄HfC₅) is a refractory compound with a very high melting point of 4.215 K (3.942 °C; 7.127 °F). Quantum mechanical computer simulations have predicted that the alloy HfN_0.38C_0.51 will have an even higher melting point (about 4400 K), which would make it the substance with the highest melting point at ambient pressure. This prediction was later confirmed by experiment. At the other end of the scale, helium does not freeze at all at normal pressure even at temperatures arbitrarily close to absolute zero; a pressure of more than twenty times normal atmospheric pressure is necessary.

List of common chemicals
Chemical	Density (g/cm3)	Melt (K)	Boil (K)
Water @STP	1	273	373
Solder (Pb60Sn40)		456
Cocoa butter		614	-
Paraffin wax	0.9	310	643
Hydrogen	0.00008988	14.01	20.28
Helium	0.0001785	—	4.22
Beryllium	1.85	1560	2742
Carbon	2.267	—	4000
Nitrogen	0.0012506	63.15	77.36
Oxygen	0.001429	54.36	90.20
Sodium	0.971	370.87	1156
Magnesium	1.738	923	1363
Aluminium	2.698	933.47	2792
Sulfur	2.067	388.36	717.87
Chlorine	0.003214	171.6	239.11
Potassium	0.862	336.53	1032
Titanium	4.54	1941	3560
Iron	7.874	1811	3134
Nickel	8.912	1728	3186
Copper	8.96	1357.77	2835
Zinc	7.134	692.88	1180
Gallium	5.907	302.9146	2673
Silver	10.501	1234.93	2435
Cadmium	8.69	594.22	1040
Indium	7.31	429.75	2345
Iodine	4.93	386.85	457.4
Tantalum	16.654	3290	5731
Tungsten	19.25	3695	5828
Platinum	21.46	2041.4	4098
Gold	19.282	1337.33	3129
Mercury	13.5336	234.43	629.88
Lead	11.342	600.61	2022
Bismuth	9.807	544.7	1837
Notes

Термодинамика

Pressure dependence of water melting point.

Like many high symmetry compounds, tetrakis(trimethylsilyl)silane has a very high melting point (m.p.) of 319-321 °C. It tends to sublime, so the m.p. determination requires that the sample be sealed in a tube.

For a solid to melt, heat is required to raise its temperature to the melting point. However, further heat needs to be supplied for the melting to take place: this is called the heat of fusion, and is an example of latent heat.

From a thermodynamics point of view, at the melting point the change in Gibbs free energy (ΔG) of the material is zero, but 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:

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

Here T, ΔS and ΔH are respectively the temperature at the melting point, change of entropy of melting and the change of enthalpy of melting.

The melting point is sensitive to extremely large changes in pressure, but generally this sensitivity is orders of magnitude less than that for the boiling point, because the solid-liquid transition represents only a small change in volume. If, as observed in most cases, a substance is more dense in the solid than in the liquid state, the melting point will increase with increases in pressure. Otherwise the reverse behavior occurs. Notably, this is the case of water, as illustrated graphically to the right, but also of Si, Ge, Ga, Bi. With extremely large changes in pressure, substantial changes to the melting point are observed. For example, the melting point of silicon at ambient pressure (0.1 MPa) is 1415 °C, but at pressures in excess of 10 GPa it decreases to 1000 °C.

Карнелијево правило

In organic chemistry, Carnelley's rule, established in 1882 by Thomas Carnelley, states that high molecular symmetry is associated with high melting point. 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 −16.4 °C (256.8 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 relatively high melting points.

A high melting point results from a high heat of fusion, 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.

Предвиђање тачке топљења

In February 2011, Alfa Aesar released over 10,000 melting points of compounds from their catalog as open data. This dataset has been used to create a random forest model for melting point prediction which is now freely available. Open melting point data are also available from Nature Precedings. High quality data mined from patents and also models developed with these data were published by Tetko et al.

Види још

• Тачка кључања
• Топљење

Литература

• Haynes, William M., ур. (2011). CRC Handbook of Chemistry and Physics (92nd изд.). CRC Press. ISBN 978-1439855119. 
• Hofmann, Philip (2008). Solid state physics: an introduction. Wiley-VCH. ISBN 978-3-527-40861-0. Архивирано из оригинала 21. 03. 2015. г. Приступљено 13. 3. 2011. 

Спољашње везе

• Melting and boiling point tables vol. 1 by Thomas Carnelley (Harrison, London, 1885–1887)
• Melting and boiling point tables vol. 2 by Thomas Carnelley (Harrison, London, 1885–1887)
• Patent mined data Over 250,000 freely downloadable melting point data. Also downloadable at figshare