Answer:
Boiling
Explanation:
As. you heat a liquid, its molecules gain kinetic energy and move faster.
Some of the molecules at the surface have enough kinetic energy to escape into the space above the surface. In a closed container, these molecules exert a pressure (called the <em>vapour pressure</em>) on the walls of the container.
In an open container, the molecules simply escape into the atmosphere.
If you add enough heat, the liquid will reach a temperature at which the <em>vapour pressure equals the atmospheric pressure</em>.
The surface molecules then have enough energy to escape freely into the atmosphere. The water has reached its boiling point.
If you keep on heating, you will see bubbles of vapour will form in the liquid and rise to the surface. When that happens, you know that the liquid is boiling.