Water/Freezing point

Not measurable

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. 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) and (231 K=-43.9°C).

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 as the fraction 1/273.16 of waters triple point. Formerly (until 1954) the definition developed by Anders Celsius had fixed the 0°C point at the "freezing point" of water. 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.