Pressure and Phase Changes
When the pressure is increased on a substance, the particles are pushed closer together. The result is that increasing pressure tends to drive substances into their densest phase. This can be seen and understood most easily with respect to a gas.
When a gas is placed under higher pressure, the molecules are forced closer together and as a result they spend more time near each other and collide more often. Thus the attractions between the particles become more effective regardless of the temperature. If the pressure becomes high enough, and the attractions become effective enough the particles will stick together and become a liquid even at a temperature at which they would normally be a gas.
We can also think about how pressure will affect boiling. In order to evaporate, particles must break free from the attractions that holds them together. However, in order to boil, particles must not only break away from their neighbors, but must also push their neighbors aside to form a bubble. This requires substantially more energy than simple evaporation. How much additional energy is required depends on how how difficult it is to form a bubble. The formation of such a bubble requires that liquid on all sides, including above, be pushed aside. A particle at the bottom of a liquid must, therefore lift up all of the liquid above it. The more liquid is piled on top of the particle, the more energy it must have in order to boil. This is why water does not boil around thermal vents in the deep ocean even thought the temperature reaches over 300oC.
Although it is less obvious, the depth of the liquid is not the only factor that determines how difficult it is for bubbles to form. Above the liquid is the atmosphere. The harder the atmosphere pushes down on the surface, the harder it is for bubbles to form and the more energy is required for boiling. As a result, the temperature at which the liquid boils will be higher when the pressure is higher and lower at lower pressures.