Question

why does the temperature of a substance remain constant during the change of state?​

Answer 1

It is because when a substance changes from solid state into liquid state, the heat energy supplied is used up in overcoming the interparticle forces of attraction operating between the particles of the solid. As the kinetic energy remains unchanged, the temperature also remains unchanged or constant. After melting is over or when the changing of state (from solid to liquid) is over, the heat supplied increases the Kinetic energy of molecules of the liquid and therefore, temperature starts rising.