Hi Clint, On question number 1
The one at lower pressure will probably freeze first because the evaporation rate will be higher at lower pressures, assuming the same fraction of water molecules in the air at both pressures. If the partial pressure of water molecules is the same at both air pressures, then this effect will not help, although the diffusion rate of water molecules away from the water surface will be higher at lower pressures.
If it is cold air which is making the water freeze, then the fact that there is less of it at lower pressures might make the freezing rate go down. Evaporation might be more important however. Question 2
Water with lots of impurities freezes at a lower temperature than cleaner water. If it takes more time to get to the lower temperatures, then the impure water will take more time to freeze. Question 3
For most materials (not water though), you can raise the freezing temperature by increasing the pressure. Most materials are more dense in the solid state than the liquid state and squeezing on them makes them want to be solids.
Water expands when it freezes, and you actually reduce its freezing temperature by squeezing on it.
However, if you squeeze really *really* **REALLY** hard, you can pack the water molecules more tightly than they are in ice, and more tightly than they are in the liquid, and you can even get water to solidify at room temperature, but the ice you get won’t be the same as the stuff in your freezer -- it will have a different crystal structure.
Have a look at http://van.physics.illinois.edu/qa/listing.php?id=1723
to making water freeze at room temperature. It takes about 10,000 atmospheres of pressure!
(republished on 07/25/06)