Water/Freezing point
Not measurable
Note: The freezing point of "pure" water is not measurable,[1][2] 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.
- ↑ For more information on why the freezing point of pure water is not measurable see:Revised Release on the Equation of State 2006 for H2O Ice Ih The International Association for the Properties of Water and Steam, The Netherlands, September 2009
- ↑ For more information on the colligative property of freezing point depression of water by adding of a solvent (such as a salt) see:Freezing Point Depression in Solutions Rod Nave, Department of Physics and Astronomy, Georgia State University
- ↑ http://www.newton.dep.anl.gov/askasci/gen01/gen01672.htm
- ↑ http://polymer.bu.edu/hes/articles/ms98.pdf
- ↑ http://polymer.bu.edu/hes/articles/ds03.pdf
- ↑ http://www.bipm.org/en/si/si_brochure/chapter2/2-1/kelvin.html
- ↑ http://physics.nist.gov/cuu/Units/kelvin.html
- ↑ http://physics.nist.gov/cuu/Units/kelvin.html
- ↑ http://www.energyquest.ca.gov/scientists/celsius.html