Skip to content

Boiling point and pressure

Heat and matter · level 6, lesson 3 of 6

A question to start

On top of Mount Everest, water boils at about 71 °C, too cool to make a good cup of tea. On Mars it would boil at less than 1 °C. Why does the boiling point change?

The idea

At any temperature, some particles at the surface of a liquid escape as vapour. The pressure that vapour can push with is the vapour pressure, and it rises steeply as the liquid warms.

A liquid boils when its vapour pressure equals the pressure of the air around it: then bubbles of vapour can form inside the liquid without being crushed. Lower air pressure means a lower boiling point.

Water at Vapour pressure
0 °C 611 Pa
5 °C 872 Pa
10 °C 1,228 Pa
20 °C 2,339 Pa
50 °C 12.3 kPa
71 °C 33 kPa
100 °C 101.3 kPa
120 °C 199 kPa
sea level: boils at 100 °CEverest: about 71 °CMars: just above 0 °Ctemperature
Boiling point of water Mars Earth
Average surface pressure 636 Pa 101,400 Pa
Boiling point there about 0.5 °C 100 °C

So on Mars, water a little above freezing boils. Water in an open cup would freeze and boil at almost the same time.

Worked examples

1. Everest. The air pressure is about 33 kPa. From the table, water's vapour pressure reaches 33 kPa at about 71 °C.

2. A pressure cooker. It holds the pressure at about 200 kPa, so the water boils at about 120 °C and cooks food faster.

3. A deep Mars basin. Suppose the pressure there is 1,200 Pa. From the table, water boils at just under 10 °C.

Try it

  1. When does a liquid boil?
  2. Why does water boil at a lower temperature on a mountain?
  3. At about what temperature does water boil where the pressure is 12.3 kPa?
  4. Mars's pressure rises to about 870 Pa in some seasons and places. About what is the boiling point then?
Answers
  1. When its vapour pressure equals the pressure around it.
  2. The air pressure is lower, so the vapour pressure reaches it at a lower temperature.
  3. About 50 °C.
  4. About 5 °C.

Common mistake

"Boiling means hot." Boiling means the vapour pressure has reached the outside pressure. On Mars, water near 0 °C boils; in a pressure cooker, water at 100 °C does not.

Where it leads