The change of a liquid into a gas is boiling. This occurs when a liquid absorbs heat.
The liquid molecules gain kinetic energy as they absorb heat, making them move around more vigorously.
At the boiling point, the molecules gain enough energy to overcome the intermolecular attractive forces and begin to move freely in a gaseous state.
The temperature at which a liquid turns into a gas is the boiling point.
The boiling point of a substance (at a particular pressure) is constant because the molecules have to possess a fixed amount of energy to overcome the attractive forces.
During boiling, the temperature remains constant because the heat absorbed at the boiling point is used to overcome the attractive forces between particles. The heat does not raise the temperature.
Heat added during boiling is latent heat.