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Q & A: freezing water at fixed volume

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Most recent answer: 10/22/2007
Q:
if you place water in an unbreakable container (completely fill the inner chamber), what will happen to the water as you drop the temperature slowly down to 0 degrees K?
- bill morrison
puyallup ,washington,usa
A:
Bill- Nice question. I'll assume you mean a very rigid container, since otherwise the answer is boring- the container just expands.

When the water reaches 0C, a little bit starts to freeze. (Here I'm assuming that there's some dirt or something to nucleate the freezing, so the water doesn't form a supercooled liquid.) That frozen part occupies a little extra volume, so the remaining water is compressed, under higher pressure. That lowers its freezing point. As you keep lowering the temperature, more and more ice forms. Finally, at some lower temperature the whole thing is compressed ice.
In other words, at fixed volume theres a range of temperatures at which ice and liquid coexist, instead of just one temperature. With a little effort to look things up, one could calculate how big the range is, using the volume difference between ice and water at atmospheric pressure, the compressibilities, and the latent heat of freezing.
Im assuming that ordinary ice forms under these pressures, although at high enough pressures different types of crystal ice become stable.

Mike W.

(published on 10/22/2007)

Follow-Up #1: freezing lakes

Q:
In regard to the freezing of a lake: What is the temp. of the water in a lake as it freezes over. Is the water colder on the bottom of a froze over lake than the water near the ice layer on the top?

Thank you.
- Arden 70
Glen Burnie MD
A:
No, water is most dense at about 4C, so water near that temperature will sink. As the temperature drops below that toward 0C, the density decreases, so the coldest water will float on top, just below the ice.

Mike W.

(published on 10/22/2007)

Follow-Up #2: cooking times

Q:
Is that the same reason why some food boxes say "cook at 425* F" where the * states "*for higher altitudes, change cooking time and temperature as needed"
- Jeffrey Twomey (age 15)
Kenosha, Wisconsin
A:
This is a little different. The boiling temperature of water depends on pressure, which depends on altitude. So the way things cook depends on altitude. You have to boil foods longer at high altitude, because boiling isn't quite as hot. Baking is also affected, because the rate at which water leaves the food is changed.

Mike W.

(published on 10/22/2007)

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