Atoms or molecules that make up a solid or a liquid are very close together. If you lower the pressure (in other words, make a vacuum) this means that there will be very few gas atoms or molecules near the surface of the solid (or liquid). This, in turn, makes it very unlikely for any molecule that leaves the surface to be replaced. So as you make the pressure lower, the ’stuff’ you’re dealing with will have a harder time coming together to make a solid (or a liquid).
Actually, below a certain pressure, you can’t get a liquid at all. Below that pressure, if you cool a gas down, it will do something called ’crystal lattice formation’ and go straight from a gas to a solid. (The opposite of this, going from a solid to a gas, is called ’sublimation.’)
The boiling point for a liquid will also drop at lower pressures. In fact, you can actually get a liquid to boil at room temperature if you have a vacuum. If you want to see this for yourself, here’s an experiment you can try: go to a doctor or veterinarian’s office and ask if you can borrow a syringe (you don’t need a needle). Suck a bit of water into the syringe. Now cover the hole on the syringe with your finger (or a plastic cap if you have one) and pull back on the plunger as hard and as fast as you can. This will create a partial vacuum inside the syringe, and with some luck you should be able to see the water in it boiling.
To see the relationship between the freezing/melting points and pressure, you can look at a phase diagram:
In this picture, the line between the solid and liquid phases is the graph of the freezing point, and the line between the liquid and vapor phases is the graph of the boiling point. Between the solid and vapor phases is the graph of the sublimation point, like I was describing before. (’nbp’ and ’nfp’ stand for ’normal boiling point’ and ’normal freezing point,’ respectively. ’tp’ stands for ’triple point,’ and ’cp’ stands for ’critical point,’ above which the liquid and gas phases sort of blur together.)
This graph is a general one, and the exact location of each line is different for every compound. Water is one well-known exception, where the freezing point actually goes up as pressure decreases. This is because for water, the solid form (ice) actually takes up more space than the liquid (water). But once you get below the point where you can have a liquid, the sublimation point does get lower as the pressure drops (since the solid takes up less space than the gas). Here’s a picture of that graph:
(published on 10/22/2007)