Difference between revisions of "Melting" - New World Encyclopedia
From New World Encyclopedia
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In [[physics]] and [[chemistry]], '''melting''' is the process of [[Heat|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." In the reverse process, called '''freezing''', the liquid substance is cooled to the temperature called the '''freezing point''', when it changes to the solid form. The melting point and freezing point are usually the same [[temperature]]. | In [[physics]] and [[chemistry]], '''melting''' is the process of [[Heat|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." In the reverse process, called '''freezing''', the liquid substance is cooled to the temperature called the '''freezing point''', when it changes to the solid form. The melting point and freezing point are usually the same [[temperature]]. | ||
| − | The melting or freezing of a substance is classified as a "phase change." Chemically, the substance remains the same, but its physical state (or "phase") changes. | + | The melting or freezing of a substance is classified as a "phase change." Chemically, the substance remains the same, but its physical state (or "phase") changes. Unlike the [[boiling point]] of a substance, its melting point is relatively insensitive to [[pressure]]. |
[[Image:Ice cubes in glass.jpg|thumb|right|200px|These ice cubes, having been placed in a glass above their melting point, are beginning to melt.]] | [[Image:Ice cubes in glass.jpg|thumb|right|200px|These ice cubes, having been placed in a glass above their melting point, are beginning to melt.]] | ||
| − | == | + | == Comparing melting and freezing == |
| − | In most cases, the [[melting]] point of a substance is equal to its freezing point. For example, the melting point ''and'' freezing point of the [[chemical element|element]] [[mercury (element)|mercury]] is 234.32 [[Kelvin]]* (K) (−38.83 [[Celsius|°C]]* or −37.89 [[Fahrenheit|°F]]*) | + | In most cases, the [[melting]] point of a substance is equal to its freezing point. For example, the melting point ''and'' freezing point of the [[chemical element|element]] [[mercury (element)|mercury]] is 234.32 [[Kelvin]]* (K) (−38.83 [[Celsius|°C]]* or −37.89 [[Fahrenheit|°F]]*). |
| + | Some substances, by contrast, 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 pressure is 0 °C (32 °F, 273.15 K), also known as the '''ice point'''. In the presence of [[Nucleation|nucleating agents]]* such as dust particles, the freezing point of water is the same as the melting point. On the other hand, in the absence of such agents, water can be [[Supercooling|supercooled]]* to −42 °C (−43.6 °F, 231 K) before it freezes. As nucleating agents are commonly present in the environment, [[rain]] water and [[tap water]]* normally freeze at (or close to) the melting point of pure water. | |
| + | Also, if a substance is rapidly cooled by sudden exposure to [[cryogenic]]* (extremely low) temperatures, it will freeze below its melting point. This process is known as "[[flash freezing]]*." | ||
| − | + | == Applications == | |
| − | + | * Melting points are often used to characterize organic compounds and ascertain their [[purity]]*. The melting point of a pure substance is always higher than the value obtained if that substance contained a small amount impurity. The greater the amount of impurity 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]]. | |
| − | Melting points are often used to | ||
| + | * Freezing is a common method of [[food preservation]] which slows both food decay and the growth of [[micro-organism]]s and, by turning [[water]] to [[ice]], makes it unavailable for [[bacterium|bacterial]] growth and [[chemical reaction]]s. | ||
| − | + | == Some examples == | |
| − | |||
| − | |||
| − | + | * The [[chemical element]] with the highest melting point is [[tungsten]], at 3695 K (3422 °C, 6192 °F). | |
| + | * [[Carbon]] does not melt at ambient pressure, but it [[sublimation (physics)|sublimes]]* (goes from the solid state directly to the gaseous state) at about 4,000 K. A liquid phase exists only above pressures of 10 megapascals (MPa) and an estimated temperature of 4,300–4700 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 extreme, [[helium]] does not freeze at all at normal pressure, even at temperatures infinitesimally close to [[absolute zero]]* (0 K). Pressures that are over 20 times the normal atmospheric pressure are necessary for it to freeze. | |
| − | |||
| − | == | + | == Latent heat of fusion == |
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: <math>\Delta{}_{fus}H</math>). 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. | 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: <math>\Delta{}_{fus}H</math>). 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. | ||
| − | + | == Other meanings == | |
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| − | |||
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| − | + | * In [[genetics]], the '''melting''' of [[DNA]] means the separation of double-stranded DNA into two single strands by heating or the action of chemicals. | |
| − | * In [[ | + | * 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. |
== Reference Values == | == Reference Values == | ||
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* [[Phase (matter)]] | * [[Phase (matter)]] | ||
* [[Boiling]] | * [[Boiling]] | ||
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| + | == Footnotes == | ||
| + | <references /> | ||
== References == | == References == | ||
Revision as of 05:58, 19 November 2006
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." In the reverse process, called freezing, the liquid substance is cooled to the temperature called the freezing point, when it changes to the solid form. The melting point and freezing point are usually the same temperature.
The melting or freezing of a substance is classified as a "phase change." Chemically, the substance remains the same, but its physical state (or "phase") changes. Unlike the boiling point of a substance, its melting point is relatively insensitive to pressure.
Comparing melting and freezing
In most cases, the melting point of a substance is equal to its freezing point. For example, the melting point and freezing point of the element mercury is 234.32 Kelvin (K) (−38.83 °C or −37.89 °F).
Some substances, by contrast, 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 pressure is 0 °C (32 °F, 273.15 K), also known as the ice point. In the presence of nucleating agents such as dust particles, the freezing point of water is the same as the melting point. On the other hand, in the absence of such agents, water can be supercooled to −42 °C (−43.6 °F, 231 K) before it freezes. As nucleating agents are commonly present in the environment, rain water and tap water normally freeze at (or close to) the melting point of pure water.
Also, if a substance is rapidly cooled by sudden exposure to cryogenic (extremely low) temperatures, it will freeze below its melting point. This process is known as "flash freezing."
Applications
- Melting points are often used to characterize organic compounds and ascertain their purity. The melting point of a pure substance is always higher than the value obtained if that substance contained a small amount impurity. The greater the amount of impurity 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.
- 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.
Some examples
- The chemical element with the highest melting point is tungsten, at 3695 K (3422 °C, 6192 °F).
- Carbon does not melt at ambient pressure, but it sublimes (goes from the solid state directly to the gaseous state) at about 4,000 K. A liquid phase exists only above pressures of 10 megapascals (MPa) and an estimated temperature of 4,300–4700 K.
- Tantalum hafnium carbide (Ta4HfC5) is a refractory compound with a very high melting point of 4488 K (4215 °C, 7619 °F).[1]
- At the other extreme, helium does not freeze at all at normal pressure, even at temperatures infinitesimally close to absolute zero (0 K). Pressures that are over 20 times the normal atmospheric pressure are necessary for it to freeze.
Latent heat of fusion
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: ). 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.
Other meanings
- In genetics, the melting of DNA means the separation of double-stranded DNA into two single strands by heating or the action of chemicals.
- 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.
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
Footnotes
ReferencesISBN 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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