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Q & A: Water freezing under a good vacuum

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Most recent answer: 07/23/2010
Q:
At what temperature does water freeze at 20" of vacuum?
- Dan W. (age 36)
Kenai, Ak. USA
A:
Ice may melt when the pressure is increased, as you can find out from other questions on the site. You may then expect that reducing the pressure will raise the freezing temperature, and you'd be right (water takes up more space when it freezes, and so if the pressure is lower, it is easier for the water to expand, and so freezing is easier and happens at a higher temperature). The only problem is that the effect is very tiny for only one atmosphere's worth of pressure, which is the most you can take away (there's no real limit to how much pressure you can add, though).

One atmosphere corresponds to 30 inches of mercury in these units. 20 inches of vacuum gets us to 10 inches of mercury's pressure. At one atmosphere, the freezing temp. of water is zero degrees Celsius. At 4.6 torr (at the triple point of water, which is at about 0.2 inches of mercury) the freezing temp is 0.01 degrees Celsius. We went about 2/3 of the way there in pressure, so I'd interpolate the answer to be T_water freezing(20 inches of vacuum) = 0.007 degrees Celsius, which is very close to its freezing temperature at atmospheric pressure, but ever so slightly higher.

Tom

(published on 10/22/2007)

Follow-Up #1: freezing via evaporation

Q:
we have a vacuum furnace which will pull a vacuum < 1 micron. An operator put parts in the furnace with water in them and we were unable to pull a hard vacuum, so we opened the furnace and the water had turned to ice, yet our thermocouples indicated temperature had never fallen below 75 deg. F. Why?
- Jim Huston (age 56)
ormond beach, FL.
A:
This is a well-known effect. As the water evaporates, the left-over water cools, the same way your skin cools as sweat evaporates. The reason is that it takes energy to pull a molecule out of the liquid, an that energy comes from the liquid itself. When the process goes rapidly, as it does in your furnace, the remaining water can cool enough to freeze. If you pump long enough the ice itself will all evaporate ("sublime") and you should be able to pull a harder vacuum. (Think of  freeze-dried coffee.) I don't know what other constraints you have, but the process can of course be sped up if you keep the chamber a bit warmer until the water is gone.

Mike W.

(published on 07/23/2010)

Follow-up on this answer.