Question

for any substance why does the temp remain constant during the change of state

Answer 1

Because during the change of state all energy
(heat energy) consume in breaking the force of
attraction of particles of object not in raising the temperature .

Answer 2

The temperature of any substance remain constant during the change of state because the heat energy supplied is used up in changing the state of matter  and also this heat is used for breaking of several bonds or attractive forces.This heat, which does not raise the temperature of the body, is called Laten heat.