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I left a water bottle in the freezer for it to cool since it was warm, so I could drink it sooner. After a couple hours I forgot I put it in there, so I opened the freezer, and to my surprise it was not frozen. But when I took it out, after 2 or 3 seconds, it froze from the bottom up over about 1 second.

Why did it not just freeze in the freezer? Why did I have to take it out of the freezer first?

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  • $\begingroup$ I put a mug into a bucket of water when I came to my country house in winter. Rays of ice streamed from the mug trough the bucket and the whole thing was a solid ice in an instant with only the mug handle sticking out. $\endgroup$
    – safesphere
    Sep 18, 2017 at 3:07

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You have discovered the effect of supercooling. There are also fascinating videos about this on YouTube.

It is basically that water needs some impurities in it to crystallize. If it is pure enough, the temperature can drop below the freezing point without becoming solid. What you witnessed was not based on temperature change, but rather that you introduced a shock wave into the supercooled liquid, which then acts as an impurity from which it can start to crystallize.

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The freezing process has to start somewhere.

If it was cooled slowly it's quite common for things to stay liquid just below freezing. When you opened it you created a starting point for the freezing, perhaps a bit of dust, or some water was squeezed in a crack in the wall.

The effect is called supercooling

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  • $\begingroup$ These answers don't seem to tell the full story. Yes, it's a known effect, but what is the physics behind it? Putting the "supercooled" label on it does not actually explain anything. Below the freezing point the energy of the ice crystal is lower than the energy of the liquid. Clearly water is awaiting a spontaneous symmetry breakdown, but where does water get the energy to stay liquid? $\endgroup$
    – safesphere
    Sep 18, 2017 at 3:21
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    $\begingroup$ It already has the energy: it just hasn't gotten a chance to lose it yet. It's in a metastable state: a local (and very shallow) energy minimum. The slightest disturbance will make it drop to the deeper minimum and release the extra energy. $\endgroup$
    – NickD
    Sep 18, 2017 at 4:28

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