Question

give reason the temperature remains constant at the melting point?

Answer 1

Because during melting the substance is gaining latent heat of melting which we can't notice on thermometer.

Answer 2

when a liquid such as water-is heated at a constant pressure,some of its molecules get enough KE to escape the liquid.... Thus,more heat energyis needed to cause other moleculesto escape.This keeps the liquid at a temperature slightly below that of the boiling point.