Science Puzzle
Why Does Water Boil Below 100 Degrees on Everest?
At sea level, water boils at 100 °C. On the summit of Everest, climbers melting snow for a drink find their pan bubbling furiously at only about 70 °C. The water is boiling, yet it's far cooler than boiling water at home, and a boiled egg up there takes an age to set.
Why does water boil at a lower temperature high on a mountain?
The Answer
Because there's less air pressing down on it. Water boils when its vapour can push back as hard as the air pushes in; high up, the air pushes much more gently.
Water is always evaporating a little from its surface, and the hotter it gets, the harder its vapour pushes outwards. This push is called vapour pressure. Boiling is different from gentle evaporation: bubbles of vapour form deep inside the liquid and grow. A bubble can only survive if the vapour inside it pushes outwards as hard as the surroundings push in. Those surroundings are pressed on by the weight of all the air above.
At sea level that air pressure is about 101 kilopascals, and water's vapour pressure only reaches that at 100 °C. On top of Everest the air pressure is about a third of that, around 34 kilopascals. Water's vapour pressure reaches that level at about 70 °C, so that's where bubbles can form and the water boils.
Cooking depends on how hot the water is, not on how hard it bubbles, so food cooks slowly up there. A pressure cooker uses the same idea backwards: it traps steam to raise the pressure inside, so the water doesn't boil until about 120 °C and food cooks much faster.
The principle: Boiling point depends on pressure. A liquid boils when its vapour pressure equals the pressure around it, so lower air pressure lowers the boiling point and higher pressure raises it.