Melting

In physics and chemistry, melting is the process of heating a solid substance to a temperature called the melting point, when it turns to the liquid form. An object that has melted is said to be "molten." The reverse process of turning a liquid to a solid is called freezing. The melting point and freezing point are usually the same temperature.

These ice cubes, having been placed in a glass above their melting point, are beginning to melt.

Freezing

In physics and chemistry, freezing is the process of cooling a liquid to the temperature (called freezing point) where it turns solid. Melting, the process of turning a solid to a liquid, is the opposite of freezing. For most substances, melting and freezing temperatures are equal. For example, the melting point and freezing point of the element mercury are the same. Rapid cooling by exposure to cryogenic temperatures can cause a substance to freeze below its melting point, a process known as flash freezing.

For some pure substances, such as pure water, the freezing temperature is lower than the melting temperature. The freezing point for water is only the same temperature as the melting point when nucleators are present to prevent supercooling. The melting point of water is 0°C (32°F, 273 K). In the absence of nucleators water will supercool to −42°C (−43.6°F, 231 K) before freezing. But in the presence of nucleating substances the freezing point of water is the same as the melting point. Nucleating agents, such as dust, are commonly present in the environment, which is why rain water and tap water will normally freeze at the melting point of water.

Freezing is a common method of food preservation which slows both food decay and the growth of micro-organisms and, by turning water to ice, makes it unavailable for bacterial growth and chemical reactions.

In biology, freezing is the reaction of an animal to a fear-eliciting situation, enabling it to remain undetected by a predator and prepare a fight-or-flight-reaction.

Melting point

The melting point of a crystalline solid is the temperature at which it changes state from solid to liquid. When considered as the temperature of the reverse change from liquid to solid, it is referred to as the freezing point.

For most substances, melting and freezing points are equal. For example, the melting point and freezing point of the element mercury is 234.32 kelvins (−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 32 °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.

The melting point of water at 1 atmosphere of pressure is 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 characterise organic compounds and to ascertain the purity. The melting point of a pure substance is always higher than the melting point of that substance when a small amount of an impurity is present. The more impurity is present, the lower the melting point. 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.

Some examples

The chemical element with the highest melting point is tungsten, at 3695 K (3422 °C, 6192 °F). 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). 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.

Thermodynamics of melting

To melt a solid, its temperature must first be raised to its melting point, and then additional heat must be supplied for the melting to occur. The amount of heat absorbed by 1 gram of a substance at its melting point when changing from the solid state to the liquid state is called the latent heat of fusion or the enthalpy of fusion (symbol: $\Delta {}_{fus}H$). For the reverse process, when 1 gram of the same substance in the liquid state is frozen to its solid state, it releases the same amount of heat.

From a thermodynamics viewpoint, at the melting point the change in Gibbs free energy ($\Delta G$) of the material is zero, because the enthalpy ($H$) and the entropy ($S$) of the material are increasing ($\Delta H,\Delta S>0$). 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 "$T$","$\Delta S$", and "$\Delta H$" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting.

Other meanings

• In genetics, melting DNA means to separate the double-stranded DNA into two single strands by heating or the action of chemicals.

Reference Values

The heat of fusion of water is 79.72 calories per gram, or 334.5 joules per gram. The heat of fusion of some other common substances are given in the table below.

Substance	Heat of fusion (cal/g)	Heat of fusion (kJ/kg)
methane:	13.96	58.41
ethane:	22.73	95.10
propane:	19.11	79.96
methanol:	23.70	99.16
ethanol:	26.05	108.99
glycerol:	47.95	200.62
formic acid:	66.05	276.35
acetic acid:	45.91	192.09
acetone:	23.42	97.99
benzene:	30.45	127.40
myristic acid:	47.49	198.70
palmitic acid:	39.18	163.93
stearic acid:	47.54	198.91

Source: CRC Handbook of Chemistry and Physics, 62nd edition.

See also

• Phase (matter)
• Boiling

References

ISBN links support NWE through referral fees

• Kleinert, Hagen, Gauge Fields in Condensed Matter, Vol. II, "STRESSES AND DEFECTS; Differential Geometry, Crystal Melting", pp. 743-1456, World Scientific (Singapore, 1989); Paperback ISBN 9971-50-210-0 (readable online here)

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