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Q & A: air pressure and water freezing

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Q:
Does air pressure effect the rate at which water freezes? If water is placed in a vacuum, will it freeze at a slower or quicker rate that water placed in a container with normal air pressure and placed into the freezer?
- Michelle
Lenart, Chicago,ILL
A:
Air pressure certainly affects the freezing temperature. The higher the pressure, the lower the freezing temperature. Since it will take water longer to reach the lower temperature, I'd expect that it would freeze more slowly.

There are two ways that the higher pressure lowers the freezing temperature. One is direct. Ice occupies more volume than liquid water, so squeezing harder favors the liquid over the ice state. The second is less direct. At higher pressure, more air (nitrogen and oxygen molecules) goes into solution. The more molecules dissolved in the water, the lower the freezing point. That's discussed more in other answers here on the freezing of saltwater.

There are two other points to worry about here. One is that at higher pressures, the air will have a higher heat capacity per unit volume than at lower pressures, and so flowing high-pressure cold air past the water can freeze the water faster. If the high-pressure air does not flow (that is, if you pressurize a closed container) then this should have almost no effect.

You asked about placing water in a vacuum. Liquid water will boil if its vapor pressure is greater than the ambient air pressure. In a vacuum, liquid water will start to boil regardless of what the temperature is. Turning water from a liquid to a vapor takes 540 calories per gram, and this heat is taken from the liquid water, cooling it off. In a vacuum, the water will continue to boil until so much heat has been removed that the remaining water will freeze. This is a very quick way to freeze water.

If you put the water in a sealed container with a vacuum, then the water will only boil for a very short time until the container is full of water vapor (no vacuum any more). If you want to freeze the water by boiling it, you have to constantly pump away the water vapor with a vacuum pump.

Mike W. and Tom J.

(republished on 07/24/06)

Follow-Up #1: sealed vapor

Q:
You say "If you put the water in a sealed container with a vacuum, then the water will only boil for a very short time until the container is full of water vapor " How can water in a sealed container create water vapor, if there is not enough space for the water vapor to exist. Pretend the container is full of water nearly, or full of water entirely. How can that water expand and take up space as vapor - if there is no room for it! Water vapor takes up a tremendous amount of space compared to liquid water.
- Lars Allen Olson (age 25)
Canada
A:
If the liquid initially fills the whole container, then just as you say nothing will boil. However, we were dealing with the case in which there was some leftover space, initially empty.

Mike W.

(published on 06/09/09)

Follow-up on this answer.