When we heat a liquid, what happens is that the molecules of the liquid will absorb heat and thus develop kinetic energy that will make them move faster.
Now, as the liquid begins to boil, bubbles of will be formed inside the liquid and then rises to the surface. Now, when the temperature of the reaches 100°C which is the boiling point of a liquid, the molecules at the top of the liquid begin to change to gaseous state and escape in form of vapour.
