Freezing vs. Melting Points

I have read that water can be found in clouds at up to minus 40 F and in labs at minus 42. Is it true, then, that the "freezing point of water" could more accurately be called "the melting of ice"? Or are both terms slightly untrue? (Which is a wonderful term when you think about it, the idea that things can be "slightly untrue" meaning of course that they're also slightly true).
- Colum McCann (age 53)
New York

In true thermal equilibrium, there's only one freezing/melting temperature, only one temperature where the liquid and solid states have the same Gibbs fre energy and are thus equally stable. Nevertheless, for very pure water at temperatures below that true freezing point, it can be hard for the molecules to find their way drom the liquid state over to the special ordered solid state. These superooled liquids can persist for very long times, even though they aren't thermodynamically stable.

 

See https://van.physics.illinois.edu/qa/listing.php?id=1618 
https://van.physics.illinois.edu/qa/listing.php?id=1618 
for more on this phenomenon.

Mike W.

(published on 01/08/2019)