How does the (a) average kinetic energy (b) average potential energy of molecules of a substance change during it's change in phase at a constant temperature, on heating?

(a) The average kinetic energy of molecules does not change during it's change in phase at a constant temperature, on heating.

(b) Average potential energy of molecules increases during it's change in phase at a constant temperature, on heating.

Explanation: During the change in phase of the substance at a constant temperature on heating, the heat supplied is utilized in increasing the separation against the attractive forces between the molecules. This increases the potential energy of the molecules.
As the temperature of the substance remains constant, the average kinetic energy of the molecules does not change. The heat energy supplied during melting is utilised only in increasing the potential energy of the molecules and is called the latent heat of melting.