Question

for any substance, why does the temperature remain constant during the change of state? please answer...

Answer 1

$\textbf{ Hello Mate }$

During the change in state of matter, the heat supplied is used by particles if matter. In actual, particles of matter absorb the heat to overcome intermolecular force of attraction and change its states.

Thus, this hidden form of heat also called latent heat is used to change state from one to other.

Hope it helps

Answer 2

During the change of state, the heat energy in the surroundings is being used up to break the bonds/attractive forces between the substance's molecules. The heat energy used is called latent heat.

Example:

When ice is heated, the water molecules have more kinetic energy to break the bonds holding them together. As the temperature continues to rise, the ice reaches its melting point and energy is being used up at the moment so the temperature will remain constant.

Then when all the latent heat has been used to change the state of ice into liquid, the temperature continues to rise again until the boiling point, where energy will again be used to change the liquid into gas.