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
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
Mike W. and Tom J.
(published on 10/22/2007)