Boiling water bubbles are water vapor, not air, forming when liquid turns into gas.

The longer version

A bubble in boiling water is not trapped air but water in its gaseous form. Heat gives molecules near the bottom of the pot enough energy to break away from the liquid, and they gather into pockets that rise and burst at the surface. Small bubbles of dissolved air appear earlier, long before boiling begins.

Once the whole liquid is churning, the temperature stops climbing, because added heat goes into turning liquid into gas rather than raising the reading. That change of state absorbs a great deal of energy. Anything dissolved in the water, such as salt, shifts the temperature at which all this happens.

Where this entry sits

Everyday physics and chemistry hide small wonders in plain sight, from kitchen reactions to the light in the sky. These short facts explain matter, energy, electricity, magnets, sound, and the simple units we use to measure the world. The full list sits on the Science and Physics page, where every entry is listed in order.