Ice

[carriesmum]

What temperature does water freeze at?

[pinus]

Assuming normal atmospheric pressure -- Zero on the celsius (or centigrade scale) Zero on the Reaumur scale 32 degrees on the Fahrenheit scale Before you ask the boiling point of water on these scales is 100 celsius 80 reaumur 212 fahrenheit

[pinus]

I should have added. It is possible in certain conditions to produce supercooled water, which is still liquid below freezing point. Supercooled water features in the creation of hail and snow

[Waltonator]

It depends a lot on where you are (there is liquid oxygen at the centre of jupiter because of the immense pressure). It can also depend on what you have put into the water (things like salt and methanol can change the freezing point)

[Waltonator]

Forgot to mention - the supercooled water that was mentioned before was probably produced somewhere with very high pressure

[pinus]

Educational sites will give classroom experiments in producing supercooled water at notmal temperature. http://www.exploratorium.edu/cooking/candy/activity-drops.ht ml When it is produced in clouds, the pressure is lower

[pinus]

That should have been normal pressure :-)

[ianess]

Interesting question. I was always taught that ice MELTED at zero centigrade, and by assumption freezing started only if temp fell below zero. Will follow this debate with interest.

[Drewhound]

It does both at zero centigrade is8748. When the temperature is dropping the water starts to freeze at zero and stays at that temperature until it has frozen When the temperature is rising the opposite happens. In each case the temperature can only continue to rise or fall, as applicable, after the thawing or freezing has taken place

[ianess]

That, Drewhound, I`m sorry to say, was as clear as mud. Can`t accept that both things can happen at the same temperature. Let the debate continue!

[Gef]

Sorry is8748 but Drewhound is absolutely correct. The melting point of pure water (ice) is the same as the freezing point of pure water, ie zero degrees Celcius.

[Drewhound]

Thanks Gef, I confess it wasn't the clearest explanation I've ever given

[thekraut]

Thermodynamically speaking, the difference between solid and liquid water at 0 degree is the amount of energy in the system. if energy is added to ice its not used to increase the temperature but goes into the melting process which, in itself consumes energy as the disorder (or entropy) of the system increases. Its similar to the phase equilibrium of the water/vapour system. Evaporation consumes energy, condensation gives off energy, but both happens at the same temperature (not necessarily 100�C, though)