THE BUBBLES IN A BEAKER OF WATER WHICH IS BOILING STEADILY ARE AIR BUBBLES
WRONG!!
It surprised me, when I once marked an exam question on this, how many people thought that the bubbles in the beaker of
boiling water were air bubbles. Many other people think that they are bubbles of hydrogen and bubbles of oxygen, or bubbles of a mixture of hydrogen or oxygen (because water
molecules are made of hydrogen and oxygen atoms).
The bubbles are made of water vapour: ie water in its gas state. We can also call this steam. The water (at its boiling temperature) is given so much energy that the water
molecules push each other so far apart that the gas state is achieved. This water vapour is less dense than the liquid water so it rises.
Water cannot be split up into hydrogen and oxygen just by heating (ie by physical means) it needs a different process to do this - electrolysis.
Water from a tap does contain some dissolved air, but most of this is driven out before the water gets to boiling point.