When a liquid is heated, it's molecules absorb heat energy and thus their kinetic energy starts increasing. As the average kinetic energy of the molecules continues increasing, therefore, the temperature of the liquid rises. It is because the average kinetic energy of the molecules is the measure of the temperature of a body. Furthermore, with the increase in kinetic energy, molecules vibrate with greater amplitude about their mean positions. This in turn increases the intermolecular spaces and weakens the intermolecular forces. This continues until the kinetic energy of the molecules becomes more than the intermolecular forces. At this stage, the energetic molecules rapidly leave the liquid and change into gaseous state.
All heat energy supplied during boiling does not increase the kinetic energy of the molecules, so the temperature remains constant. The heat energy supplied is hidden in the molecules of the gas. This hidden energy is known as latent heat of vaporisation.
