Why: Why Does Water Freeze at 32 Deg F?

[5-StarPlayer]

Why it is stated that 32 deg F is the freezing point of water when we know that water at STP (standard temperature and pressure) reaches maximum density at 39 deg F??

Thanks in advance for taking the time to answer this...

5-

 

[Topher925]

Its because water is a strange substance. Due to the molecular behavior of H2O it actually reaches its greatest density slightly above freezing. Past that point the density decreases and decreases even further freezing.

 

[billiards]

This is true of water at typical pressures we find at surface conditions; but at extreme pressures (such as those found at depth on a planetary ice body) water ice does get a lot denser as temperature is reduced and pressure is increased, check out the phase diagram of H₂O: http://www.es.ucl.ac.uk/research/pig/images/ice_phase_diagram.jpg.

Ice VII gets harder to melt at increasing pressures (it has a positive Clapeyron slope) whereas ice I (the stuff we're used to on Earth) actually gets easier to melt at increasing pressure - they're exactly the same substance chemically, yet they have different mineralogical structural properties (they are "polymorphs"). Interestingly, the fact that ice I has a negative Clapeyron slope is the reason that ice floats on water, which turns out to be essential for life as we know it.