A vapour is created when a substance forms a gas at a temperature below its boiling point; the vapour pressure of a liquid is the pressure of this vapour. A liquid boils when the vapour pressure of the liquid is equal to the atmospheric pressure of the air above it.

You can therefore boil a liquid in two ways: by heating it so that the vapour pressure increases to match the atmospheric pressure; or by decreasing the atmospheric pressure until it matches the vapour pressure at whatever the ambient temperature is.

In this video you can see me boiling water at room temperature: a beaker full of water at room temperature is placed in a vacuum chamber and the pressure lowered until it boils – the pressure gauge is on the right of the picture.

Notice how I can put my finger in the water both before and after boiling without scalding myself. It’s said that it’s impossible to make a good cup of tea at the summit of Mount Everest because water boils at only 70.4°C and this isn’t hot enough to properly brew tea.