Say, for the purposes of conjecture, that you have the necessary handwavium to uniformly cool it to any temperature you want, and decide to cool the water down to a temperature infinitely close to $0°$C.
What happens as $\lim_{T \to 0°C}$?
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asked Feb 11, 2021 at 21:13
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1$\begingroup$ The water freezes to form solid water (ice). And if you decide to heat the water ice up to a temperature infinitely close to 0 degC it melts to form liquid water. $\endgroup$– ArmadilloFeb 11, 2021 at 21:19
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2 Answers
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Nothing would happen.
You could even cool to $-1 ^\circ$C without a phase change. Then, if nucleated maybe by a bump, a small fraction of the water would crystallise to a solid.
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2$\begingroup$ You can 'supercool' it to much below $-1^{\circ}$ I believe $\endgroup$– GertFeb 11, 2021 at 22:03
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$\begingroup$ @Gert Absolutely, it is not even that difficult, just a plastic bottle in the freezer can do this. I was reacting to the infinitesimals in the question, and to that comment (which gives the wrong answer). $\endgroup$– user137289Feb 11, 2021 at 22:08
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$\begingroup$ Does only a small fraction freeze and not the entire body of water since freezing releases heat? $\endgroup$ Feb 11, 2021 at 22:18
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1$\begingroup$ @AccidentalTaylorExpansion Supercooled water may look like a solid after nucleation but almost all of it is still liquid water. The temperature then rises to $0\ ^\circ$C. $\endgroup$– user137289Feb 11, 2021 at 22:20
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1$\begingroup$ Reminds me of the time I left a 2 qt plastic bottle of apple juice in the freezer and forgot about it until months later. I took it out and it was still 100% liquid. I opened it... still liquid. I put the lid back on, and shook it. As you know, all those bubbles I introduced by shaking it were nucleation sites - instant apple juice ice slush. $\endgroup$– jpfFeb 12, 2021 at 1:18
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The molecular mechanics of particles are happening all the time as you go from one phase to another. Nothing about the processes changes as you lower the temperature, just the rate they occur.
Assuming you have some appropriate nucleation sites, then even at temperatures above freezing, some of the water molecules will bond to each other and join up. There's a chance that some other water molecule nearby will do the same. But at temperatures above freezing, the rate this happens is slower than the rate that one of the already-joined molecules leaves.
As you approach the freezing temperature, these two rates become similar. If you cool below freezing, then the only change is that the formation rate is now greater than the breakdown rate.
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2$\begingroup$ This seems to describe what is happening in a molecular dynamics simulation which has problems in dealing with phase transitions. $\endgroup$– user137289Feb 11, 2021 at 22:28