Why does water boil at low pressure?

1a contest

Does water always boil at 100 degrees? No. On Everest it boils at about 72. Boiling is a contest: vapour bubbles only survive if they push out harder than the air pushes in.

2less air

Up a mountain there's less air above, so the push is weaker. Bubbles win sooner: the water boils at a lower temperature, even though it's cooler.

3vapour pressure

Vapour pressure rises steeply with temperature, from about two kilopascals at 20 degrees to 101 kilopascals at 100. Water boils where this curve meets the pressure around it.

4three places

At sea level, 101 kilopascals, that's 100 degrees. Atop Everest the air gives only about 34, so water boils near 72. In a vacuum chamber, 2.3 kilopascals, it boils at about 20.

5the whole idea

So the rule: water boils when its vapour pressure equals the pressure around it. Less air above, lower boiling point. A pressure cooker does the reverse, and a vacuum boils water cold.