How does the (a) average kinetic energy (b) average potential energy of molecules of a substance change during its change in phase at a constant temperature?
(a) Average kinetic energy does not change. (b) Average potential energy increases.
Explanation: When a substance is heated at constant temperature (i.e. during its phase change state), the heat supplied makes the vibrating molecules gain potential energy to overcome the intermolecular force of attraction and move about freely. This means that the substance changes its form. However, this heat does not increase the kinetic energy of the molecules, and hence, no rise in temperature takes place during the change in phase of a substance. This heat supplied to the substance is known as latent heat and is utilized in changing the state of matter without any rise in temperature.