Water/Freezing point: Difference between revisions

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imported>Johan Förberg
(Kelvin is not a 'degree', says SI)
imported>Johan Förberg
(Further de-degreeification)
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Note: The freezing point of "pure" water is not measurable, whereas the melting point is. This is because pure water does not freeze without help of a solid crystallization kernel.<ref>http://www.newton.dep.anl.gov/askasci/gen01/gen01672.htm</ref> Very cold (metastable) ''pure liquid water'' can be obtained by "[[supercooling]]" pure water.  Pure liquid water has been reported to be possible down to various extremely low temperatures: (-38°C to -45°C<ref>http://polymer.bu.edu/hes/articles/ms98.pdf</ref>) and (231 K=-43.9°C<ref>http://polymer.bu.edu/hes/articles/ds03.pdf</ref>).
Note: The freezing point of "pure" water is not measurable, whereas the melting point is. This is because pure water does not freeze without help of a solid crystallization kernel.<ref>http://www.newton.dep.anl.gov/askasci/gen01/gen01672.htm</ref> Very cold (metastable) ''pure liquid water'' can be obtained by "[[supercooling]]" pure water.  Pure liquid water has been reported to be possible down to various extremely low temperatures: (-38°C to -45°C<ref>http://polymer.bu.edu/hes/articles/ms98.pdf</ref>) and (231 K=-43.9°C<ref>http://polymer.bu.edu/hes/articles/ds03.pdf</ref>).


The standard unit of thermodynamic temperature, currently defined in the [[SI system]] as °K (degrees Kelvin), selects as the fundamental fixed point the [[triple point]] of water.  One degree Kelvin, and therefore 1°C ([[Celsius]]), is specified by multiple standards bodies<ref>http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html</ref><ref>http://physics.nist.gov/cuu/Units/kelvin.html</ref> as the fraction 1/273.16 of waters triple point.  Formerly (until 1954<ref>http://physics.nist.gov/cuu/Units/kelvin.html</ref>) the definition developed by [[Anders Celsius]] had fixed the 0°C point at the "freezing point" of water.<ref>http://www.energyquest.ca.gov/scientists/celsius.html</ref>  It is now generally accepted that while the [[phase transition]] from solid to liquid water occurs at a predictable temperature (namely 0°C), the transition from liquid to solid water does not.  This is because the actual "Freezing" is dependent upon the previously mentioned [[nucleation]] as well as the temperature.{{Reflist}}</noinclude>
The standard unit of thermodynamic temperature, currently defined in the [[SI system]] as K (Kelvin), selects as the fundamental fixed point the [[triple point]] of water.  One Kelvin, and therefore 1°C ([[Celsius]]), is specified by multiple standards bodies<ref>http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html</ref><ref>http://physics.nist.gov/cuu/Units/kelvin.html</ref> as the fraction 1/273.16 of waters triple point.  Formerly (until 1954<ref>http://physics.nist.gov/cuu/Units/kelvin.html</ref>) the definition developed by [[Anders Celsius]] had fixed the 0°C point at the "freezing point" of water.<ref>http://www.energyquest.ca.gov/scientists/celsius.html</ref>  It is now generally accepted that while the [[phase transition]] from solid to liquid water occurs at a predictable temperature (namely 0°C), the transition from liquid to solid water does not.  This is because the actual "Freezing" is dependent upon the previously mentioned [[nucleation]] as well as the temperature.{{Reflist}}</noinclude>

Revision as of 15:34, 28 July 2010

Not measurable [1] [2]


Note: The freezing point of "pure" water is not measurable, whereas the melting point is. This is because pure water does not freeze without help of a solid crystallization kernel.[3] Very cold (metastable) pure liquid water can be obtained by "supercooling" pure water. Pure liquid water has been reported to be possible down to various extremely low temperatures: (-38°C to -45°C[4]) and (231 K=-43.9°C[5]).

The standard unit of thermodynamic temperature, currently defined in the SI system as K (Kelvin), selects as the fundamental fixed point the triple point of water. One Kelvin, and therefore 1°C (Celsius), is specified by multiple standards bodies[6][7] as the fraction 1/273.16 of waters triple point. Formerly (until 1954[8]) the definition developed by Anders Celsius had fixed the 0°C point at the "freezing point" of water.[9] It is now generally accepted that while the phase transition from solid to liquid water occurs at a predictable temperature (namely 0°C), the transition from liquid to solid water does not. This is because the actual "Freezing" is dependent upon the previously mentioned nucleation as well as the temperature.

  1. ↑ For more info on why the freezing point of pure water is not measurable see:
    http://www.iapws.org/relguide/Ice-Rev2009.pdf (http://www.iapws.org)
  2. ↑ For more info on the colligative property of freezing point depression of water by adding of a solvent (such as a salt) see:
    http://hyperphysics.phy-astr.gsu.edu/hbase/chemical/meltpt.html
  3. ↑ http://www.newton.dep.anl.gov/askasci/gen01/gen01672.htm
  4. ↑ http://polymer.bu.edu/hes/articles/ms98.pdf
  5. ↑ http://polymer.bu.edu/hes/articles/ds03.pdf
  6. ↑ http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html
  7. ↑ http://physics.nist.gov/cuu/Units/kelvin.html
  8. ↑ http://physics.nist.gov/cuu/Units/kelvin.html
  9. ↑ http://www.energyquest.ca.gov/scientists/celsius.html
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