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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." The reverse process of turning a liquid to a solid is called '''freezing'''. The melting point and freezing point are usually the same [[temperature]].
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{{Copyedited}}{{Paid}}{{Images OK}}{{Submitted}}{{Approved}}
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[[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.]]
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In [[physics]] and [[chemistry]], '''melting''' is the process of converting a [[solid]] substance to its [[liquid]] form, typically by [[heat|heating]] the substance to a [[temperature]] called its '''melting point'''. 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]].
  
== Freezing ==
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The melting or freezing of a substance is known as a '''phase change''' or '''phase transition'''. 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]].
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{{toc}}
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Our knowledge of melting and freezing can be applied in various ways. For instance, the purity of many solid compounds can be determined by checking their melting points. Also, freezing is a familiar method of preserving food. Melting a solid, such as a metal or plastic piece, is an extremely valuable means of molding it into a desired shape. The preparation of [[alloy]]s involves the combining of metals after they have been melted. Given a mixture of several substances in the solid state, melting is a way by which some of these substances can be separated from others (with higher melting points). Materials with high melting points are valuable for making products that need to resist high heat.
  
In [[physics]] and [[chemistry]], '''freezing''' is the process of cooling a [[liquid]] to the temperature (called [[Melting point|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 [[chemical element|element]] [[mercury (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]].
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== Comparing melting and freezing ==
  
For some pure substances, such as [[Impurity|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 [[Nucleation|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.  
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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]]).
  
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.
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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.
  
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.  
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The melting point of [[water]] at 1 atmosphere pressure is 0&nbsp;°C (32&nbsp;°F, 273.15&nbsp;K), also known as the '''ice point'''. In the presence of [[Nucleation|nucleating agents]]<ref>A nucleating agent for freezing or crystallization is something that provides tiny "nuclei" (such as dust particles or minute crystals) that promote the onset of the phase transition process.</ref> 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 &minus;42&nbsp;°C (&minus;43.6&nbsp;°F, 231&nbsp;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.
  
== Explanatory==
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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]]."
  
Not only is heat required to raise the temperature of the solid to the melting point, but the melting itself requires heat; see also [[latent heat]] and [[heat of fusion]].
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== Impurities affect the melting point ==
  
== Melting point ==
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If a substance contains a small amount of impurity, its melting point (and freezing point) becomes lower than that of the pure substance. The greater the amount of impurity present, the lower the melting point. Eventually, at a certain composition of the mixture, a minimum melting point will be reached. At that stage, the mixture is said to be at its "[[eutectic point]]," or it is called a "eutectic mixture."
  
The '''melting point''' of a [[crystal]]line [[solid]] is the [[temperature]] at which it changes [[states of matter|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'''.  
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== Some examples ==
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* The [[chemical element]] with the highest melting point is [[tungsten]], at 3,695 K (3,422 °C; 6,192 °F).
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* [[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&nbsp;K. A liquid phase exists only above pressures of 10&nbsp;megapascals (MPa) and an estimated temperature of 4,300&ndash;4,700&nbsp;K.
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* Tantalum hafnium carbide (Ta<sub>4</sub>HfC<sub>5</sub>) is a [[refractory]] compound with a very high melting point of 4,488&nbsp;K (4,215&nbsp;°C, 7,619&nbsp;°F).<ref>[http://www.britannica.com/nobel/micro/254_6.html Hafnium entry at Britannica.com]</ref>
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* 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.
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== Latent heat of fusion ==
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If a solid substance is heated, its temperature will rise until it reaches its melting point. If additional heat is supplied, the substance starts melting, and the temperature remains the same until the material is entirely melted.
  
For most substances, [[melting]] and [[freezing]] points are equal.  For example, the melting point ''and'' freezing point of the [[chemical element|element]] [[mercury (element)|mercury]] is 234.32 [[kelvin]]s (&minus;38.83&nbsp;[[Celsius|°C]] or &minus;37.89&nbsp;[[Fahrenheit|°F]]). However, certain substances possess differing solid-liquid transition temperatures.  For example, [[agar]] melts at 85&nbsp;°C (185&nbsp;°F) and solidifies from 32&nbsp;°C to 40&nbsp;°C (89.6&nbsp;°F to 104&nbsp;°F); this process is known as [[hysteresis]]. Certain materials, such as [[glass]], may harden without crystallizing; these are called [[amorphous solid]]s.
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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 example, the latent heat of fusion of ice is approximately 80 calories per gram. This means that 1 gram of ice at its melting temperature (0 &deg;C) will absorb 80 calories of heat before it melts completely.
  
The melting point of [[water]] at 1 atmosphere of pressure is 0&nbsp;°C (32&nbsp;°F, 273.15&nbsp;K), this is also known as the '''ice point'''. In the presence of [[Nucleation|nucleating substances]] the freezing point of water is the same as the melting point, but in the absence of nucleators water can [[Supercooling|supercool]] to &minus;42&nbsp;°C (&minus;43.6&nbsp;°F, 231&nbsp;K) before freezing.  
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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.
  
Unlike the [[boiling point]], the melting point is relatively insensitive to [[pressure]].
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== Applications ==
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 ==
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* Melting points are often used to characterize compounds and ascertain their [[purity]].
 +
 
 +
* Freezing is a common method of [[food preservation]], because it slows food decay and the growth of [[microorganism]]s. In addition, by turning [[water]] to [[ice]], it makes the water unavailable for [[bacteria|bacterial]] growth and [[chemical reaction]]s.
 +
 
 +
* Given that ice has a relatively high latent heat of fusion (noted above), it is useful for refrigeration&mdash;that is, for cooling items that are warm and maintaining them at a low temperature.
 +
 
 +
* Melting a solid is an extremely valuable means of reshaping the solid. This is often done when metals and plastics are molded into new shapes.
 +
 
 +
* If one has a mixture of several solid substances, melting is a way by which some substances (with lower melting points) can be separated from others (with higher melting points).
 +
 
 +
* The preparation of [[alloy]]s involves the melting of metals, combining them when they are in the molten state, and then allowing them to solidify.
  
The [[chemical element]] with the highest melting point is [[tungsten]], at 3695 K
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* Materials with high melting points are valuable for making products that need to resist high heat. For example, tungsten, with its extremely high melting point, is used in filaments for light bulbs.
(3422 °C, 6192 °F). The often-cited [[carbon]] does not melt at ambient pressure but [[sublimation (physics)|sublimates]] at about 4000&nbsp;K; a liquid phase only exists above pressures of 10&nbsp;[[Pascal (unit)|MPa]] and estimated 4300&ndash;4700&nbsp;K. Tantalum hafnium carbide (Ta<sub>4</sub>HfC<sub>5</sub>) is a [[refractory]] compound with a very high melting point of 4488&nbsp;K (4215&nbsp;°C, 7619&nbsp;°F).<ref>[http://www.britannica.com/nobel/micro/254_6.html hafnium entry at Britannica.com]</ref> 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 ==
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== Other meanings of melting and freezing ==
  
From a thermodynamics point of view, at the melting point the change in [[Gibbs free energy]] (<math>\Delta G</math>) of the Material is zero, because the [[enthalpy]] (<math>H</math>) and the [[entropy]] (<math>S</math>) of the material are increasing (<math>\Delta H, \Delta S > 0</math>). 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:
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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.
  
<math>\Delta S = \frac {\Delta H} {T}</math>
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* 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.
  
The "<math>T</math>","<math>\Delta S</math>", and "<math>\Delta H</math>" in the above are respectively the temperature at the melting point, change of entropy of melting, and the change of enthalpy of melting.
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== Reference Values ==
  
== Other meanings ==
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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.
  
* In [[genetics]], '''melting''' [[DNA]] means to separate the double-stranded DNA into two single strands by heating or the action of chemicals.
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<table border=1 cellspacing=0>
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<tr><th>Substance</th><th>Heat of fusion<br> (cal/g) </th><th>Heat of fusion<br> (kJ/kg) </th></tr>
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<tr><td>methane:</td><td> 13.96</td><td>58.41</td></tr>
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<tr><td>ethane:</td><td> 22.73</td><td>95.10</td></tr>
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<tr><td>propane:</td><td> 19.11</td><td>79.96</td></tr>
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<tr><td>methanol:</td><td> 23.70</td><td>99.16</td></tr>
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<tr><td>ethanol:</td><td> 26.05</td><td>108.99</td></tr>
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<tr><td>glycerol:</td><td> 47.95</td><td>200.62</td></tr>
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<tr><td>formic acid:</td><td> 66.05</td><td>276.35</td></tr>
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<tr><td>acetic acid:</td><td> 45.91</td><td>192.09</td></tr>
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<tr><td>acetone:</td><td> 23.42</td><td>97.99</td></tr>
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<tr><td>benzene:</td><td> 30.45</td><td>127.40</td></tr>
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<tr><td>myristic acid:</td><td> 47.49</td><td>198.70</td></tr>
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<tr><td>palmitic acid:</td><td> 39.18</td><td>163.93</td></tr>
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<tr><td>stearic acid:</td><td> 47.54</td><td>198.91</td></tr>
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</table>
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''Source: CRC '''Handbook of Chemistry and Physics''', 62nd edition.''<ref>Lide, David R. (ed.). ''CRC Handbook of Chemistry and Physics'', 62nd edition. Updated 85th edition, 2005. Boca Raton, FL: CRC Press. ISBN 0849304865</ref>
  
 
==See also==
 
==See also==
Line 49: Line 88:
 
* [[Phase (matter)]]
 
* [[Phase (matter)]]
 
* [[Boiling]]
 
* [[Boiling]]
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== Notes ==
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<references />
  
 
== References ==
 
== References ==
 
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* Kleinert, Hagen. ''Gauge Fields in Condensed Matter'', Vol. II, "Stresses and Defects; [[Differential Geometry]], Crystal Melting", pp. 743-1456. Singapore: World Scientific, 1989. ISBN 9971502100. [http://www.physik.fu-berlin.de/~kleinert/kleiner_reb1/contents2.html online]. Retrieved October 13, 2014.
* Kleinert, Hagen, ''Gauge Fields in Condensed Matter'', Vol. II, "[[Stress|STRESSES]]* AND [[Defect|DEFECTS]]*; [[Differential Geometry]]*, Crystal Melting", pp. 743-1456, World Scientific (Singapore, 1989);  Paperback ISBN 9971-50-210-0 ''(readable online [http://www.physik.fu-berlin.de/~kleinert/kleiner_reb1/contents2.html here])''
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* Lide, David R. (ed.). ''CRC Handbook of Chemistry and Physics''. Boca Raton, FL: CRC Press, 2005.  ISBN 0849304865
  
 
[[Category:Physical sciences]]
 
[[Category:Physical sciences]]
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[[Category:Materials science]]
 
[[Category:Materials science]]
  
{{credit3|Melting|86901186|Melting_point|87706429|Freezing|87829961}}
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{{credit4|Melting|86901186|Melting_point|87706429|Freezing|87829961|Standard_enthalpy_change_of_fusion|88185693}}

Latest revision as of 21:17, 13 October 2014

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

In physics and chemistry, melting is the process of converting a solid substance to its liquid form, typically by heating the substance to a temperature called its melting point. 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 known as a phase change or phase transition. 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.

Our knowledge of melting and freezing can be applied in various ways. For instance, the purity of many solid compounds can be determined by checking their melting points. Also, freezing is a familiar method of preserving food. Melting a solid, such as a metal or plastic piece, is an extremely valuable means of molding it into a desired shape. The preparation of alloys involves the combining of metals after they have been melted. Given a mixture of several substances in the solid state, melting is a way by which some of these substances can be separated from others (with higher melting points). Materials with high melting points are valuable for making products that need to resist high heat.

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[1] 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, rainwater 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."

Impurities affect the melting point

If a substance contains a small amount of impurity, its melting point (and freezing point) becomes lower than that of the pure substance. The greater the amount of impurity present, the lower the melting point. Eventually, at a certain composition of the mixture, a minimum melting point will be reached. At that stage, the mixture is said to be at its "eutectic point," or it is called a "eutectic mixture."

Some examples

  • 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 4,300–4,700 K.
  • Tantalum hafnium carbide (Ta4HfC5) is a refractory compound with a very high melting point of 4,488 K (4,215 °C, 7,619 °F).[2]
  • 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

If a solid substance is heated, its temperature will rise until it reaches its melting point. If additional heat is supplied, the substance starts melting, and the temperature remains the same until the material is entirely melted.

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 example, the latent heat of fusion of ice is approximately 80 calories per gram. This means that 1 gram of ice at its melting temperature (0 °C) will absorb 80 calories of heat before it melts completely.

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.

Applications

  • Melting points are often used to characterize compounds and ascertain their purity.
  • Given that ice has a relatively high latent heat of fusion (noted above), it is useful for refrigeration—that is, for cooling items that are warm and maintaining them at a low temperature.
  • Melting a solid is an extremely valuable means of reshaping the solid. This is often done when metals and plastics are molded into new shapes.
  • If one has a mixture of several solid substances, melting is a way by which some substances (with lower melting points) can be separated from others (with higher melting points).
  • The preparation of alloys involves the melting of metals, combining them when they are in the molten state, and then allowing them to solidify.
  • Materials with high melting points are valuable for making products that need to resist high heat. For example, tungsten, with its extremely high melting point, is used in filaments for light bulbs.

Other meanings of melting and freezing

  • 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.

SubstanceHeat of fusion
(cal/g)
Heat of fusion
(kJ/kg)
methane: 13.9658.41
ethane: 22.7395.10
propane: 19.1179.96
methanol: 23.7099.16
ethanol: 26.05108.99
glycerol: 47.95200.62
formic acid: 66.05276.35
acetic acid: 45.91192.09
acetone: 23.4297.99
benzene: 30.45127.40
myristic acid: 47.49198.70
palmitic acid: 39.18163.93
stearic acid: 47.54198.91

Source: CRC Handbook of Chemistry and Physics, 62nd edition.[3]

See also

Notes

  1. A nucleating agent for freezing or crystallization is something that provides tiny "nuclei" (such as dust particles or minute crystals) that promote the onset of the phase transition process.
  2. Hafnium entry at Britannica.com
  3. Lide, David R. (ed.). CRC Handbook of Chemistry and Physics, 62nd edition. Updated 85th edition, 2005. Boca Raton, FL: CRC Press. ISBN 0849304865

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. Singapore: World Scientific, 1989. ISBN 9971502100. online. Retrieved October 13, 2014.
  • Lide, David R. (ed.). CRC Handbook of Chemistry and Physics. Boca Raton, FL: CRC Press, 2005. ISBN 0849304865

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