Why does the temperature of a substance remain constant during its melting point or boiling point ?

Explanation

As long as a substance has not started either melting or boiling, the heat energy supplies increases the kinetic energy of the particles. Therefore, the temperature of the substance increases. Once it starts melting or boiling , the further heat energy is absorbed either as latent heat of fusion or as latent heat of vaporisation. This does not cause any increase in kinetic energy because all the particles of a substance have sufficient kinetic energy to change into liquid or vapour state. However, interparticle forces do not allow them to do so. The energy supplied simply overcomes these interparticle forces and they undergo change in state.