Question

for any substances , why does temperature remains constant during the change of state ?

Answer 1

When there is a change of state there is no change in temperature because the heat given by you while changing the state is used up to change the state has the heat of fusion or heat of vaporization that is needed to overcome the intermolecular forces of the atoms inside hence it does not increasing the temperature just changes the state of the material by overcoming the intermolecular forces.

Answer 2

Because with the increase of temperature, more number of particles get enough kinetic energy to go into the vapour state. Hence temperature remains constant during the change of state.