Daniel Thompson, of Ilam School, asks :-

When water is boiled, where do the bubbles come from?

John Boereboom, a physics teacher at Burnside High School, responded.

There are two different types of bubbles which form when water is heated.

Water has air dissolved in it, much like a bottle of fizzy drink has lots of carbon dioxide dissolved in it. The amount of air dissolved in water depends on the temperature of the water, the higher the temperature the less air is dissolved. When you start heating water its temperature gradually increases so some of the air is expelled from the water, bubbles of air form and when these are large enough they rise to the surface of the water because air is less dense than water.

When water reaches 100 degrees Celsius at normal atmospheric pressure the water boils, that is to say it changes from the liquid state to the gaseous state. The bubbles that form when water is boiling consist of water molecules in the gaseous state ie water vapour.

Molecules cannot be much closer together than they are in the liquid state because liquids cannot be compressed into smaller volumes very easily. Gases can be easily compressed as you know if, for example, you have used a bike pump. In the gaseous state molecules are about ten times further apart than in the liquid state, the gas is much less dense than the liquid and so the bubbles of water vapour rise to the surface of the liquid where the bubbles explode releasing water vapour which, like air, is invisible.