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Q & A: to freeze or not to freeze

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Most recent answer: 10/22/2007
Q:
I put a liter of water into the freezer accidentally for a couple of hours. I thought owe no itís going to blow up. I ran to the freezer and it was fine. I then pulled it out of the freezer and within about 5 secs the hole thing turned to ice. I just sat there and watched the ice start forming from top to bottom. What caused this? It didnít make sense to me that inside the freezer it was a liguid and then outside at room temperature it started forming ice.
- Philip Washington (age 18)
Richardson,TX,US
A:
Philip- That's an interesting question, but I'm sure that things didn't happen quite the way you say. I'll explain why, then give a guess as to what you might have seen, then try to slip in a little philosophy.

It's true that water can be cooled below its freezing point, and that then a small shake or a speck of dust can 'nucleate', i.e. trigger, the formation of some ice. The question is: how much ice? When water freezes, it releases 80 calories of heat per gram. That's called the 'latent heat'. 80 cal is enough to heat up 80 gm by a degree (Celsius). Now there is no reasonable way that you could have kept that water supercooled more than a few degrees below the freezing point (say less than 4 degrees below). [Note added much later. I was wrong. Water can often be cooled down to -20įC or even more below freezing, in principle down even to -42įC.] At that rate the first 5% [this should read more like 25%.] or less of the water to freeze would heat up the rest all the way to the melting/freezing point, and the freezing would stop. Although a little heat could escape the container, as long as the container is colder than the room, the net heat flow is in, in addition to the latent heat.

So what might account for what you describe? One possibility is that you saw the sides frost up, and confused that with the contents freezing. Or perhaps a sheet of ice formed around the entire inner surface of the container, which should have been the coldest part. To the eye that might look like the whole thing was turning to ice. I guess that's my favorite explanation. Or maybe what you saw was 'slush'- some needles of ice forming throughout the container, but most of the water staying liquid.

It may seem unscientific for me to say that you didn't see what you say you saw. After all, I wasn't there. Nevertheless, science only works because there are some very general, reliable facts about the universe. The laws of thermodynamics are among them. So sometimes 'theory' is much more reliable than eyewitness testimony. Actually, eyewitness testimony is notoriously unreliable. I personally just failed to see a gorilla parade across a basketball court, but that's another story.

Anyway, why not try this again and see if it fits any of these descriptions? Please write back.

Mike W.
[I left in the original errors, with corrections, to remind readers of the sorts of mistakes we make.]


(published on 10/22/2007)

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