Question

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

Answer 1

It is so because when the substance reaches its melting or boiling point it uses the heat supplied during that time to break its bonds between the atoms. hence, after breaking the bonds it changes its state.

Answer 2

During the change of state, the temperature remains constant because the heat provided for the change of the state is utilised for breaking the attractive forces between the particles of the substances.

This happens at melting point(or boiling point) of the substance and the heat used is called the latent heat of fusion (or vapourisation).

During the condensation or solidification, the vise-versa happens.