At what temperature would water freeze in zero-pressure? (if air were pumped out of a container)?

Question

Assume the water is ordinary tap water, rather than pure water.

Answer 1

The other answers suggest that this is not a simple question.

If you had water in a container and the temperature was at 0 degrees C then the water would be freezing or melting depending on whether there was heat being added to the container or taken away (you need heat to change phase, right?)

OK, so, let's say the air in the container of half-melted water was removed. The vapor pressure of the container would drop and water would evaporate. Evaporation would suck up some heat from the half-melted ice, wouldn't it? And then you would have the evaporated water and some solid ice in a container with lowering pressure. The solid ice would sublimate into the vacuum and as you continued to withdraw more air some of the water vapor would be lost as well. I think you would continuously sublimate the ice into water vapor - so it wouldn't exactly be a vacuum, would it? As long as there was more water to evaoprate, you would not have a vacuum. Vacuums insulate, right (as in a thermos bottle) so the conditions in the container would also depend on whether you were continuously drawing down to a lower vacuum, and whether there was light or infrared entering the container to add heat.
Interesting question!

Answer 2

The molecules in water are not all at the same kinetic energy. Temperature is related to sort of the average kinetic energy of the molecules. Under reduced pressure, the molecules with higher energy evaporate first. It can look like boiling. The molecules with least energy get left behind. Therefore ice may form. (The demonstration I saw of this used distilled water - I'm not sure what the effect would be of using tap water.)

As some have said, the ice can sublimate but that's a slower process. Eventually you would be left with just the impurities.

Answer 3

Water freezes at 32F because enough thermal energy has been removed to permit water molecules to bond and form chains or crystals. In a perfect vacuum (all air and water vapor removed) any heat entering the container would cause ice to sublimate changing directly from a solid to a vapor without the liquid phase until water vapor pressure had been restored. Thereafter there would be melted water, as in canned tomatoes under near-perfect vacuum?

Answer 4

Pressure inside the container cannot be reduced to zero, as long as there is water inside it. When pressure is reduced, water will evaporate and it will exert some pressure on the walls of the container.
Pressure = 1/3 d c^2, where d is the density and c is the r.m.s. speed of the gas.
If pressure has to be zero, than the r.m.s. speed will have to be zero, which inturn will make the temperature of the system 0 kelvin which is practically not possible.
So it is not possible to make pressure zero, while having some water inside the container.

Answer 5

Water will be vapourised. It will not freeze. All the dissolved substance will be deposited at the bottom.

Note that when pressure is decreased boiling point of water decreases.

Answer 6

0 degrees C - it wont all evaporate instantly

Answer 7

Can you speak franch please.