Why does the boiling temperature of water increase when the water is under increased pressure?

Question

Why does the boiling temperature of water increase when the water is under increased pressure?

Answer 1

Check out vapor pressure on wikipedia for details.

I think this is a good short explanation.

The surface air pressure is the resistance that keeps the water from turning to a gas, so as the resistance increases the temperature has to increase to over come the resistance caused by the higher pressure.

Answer 2

Remember that water boils because some of its molecules start moving about fast enough to become gaseous. These fast moving molecules eventually group together and form a bubble. When the pressure of this bubble is high enough (i.e. when enough high-energy molecules have packed themselves within it) to overcome to downwards pressure of the air above the water, the bubble will rise, and release its molecules (steam).

Imagine, though, that you increase the air pressure. It will take much longer for the bubbles to acquire the needed pressure, perhaps they will never get there at all. You are inhibiting the boiling process, even preventing it from occurring at all, effectively increasing the boiling temperature.

Hope that helps!