Question

Why does the temperature of a subtract region constant during its melting or boiling point?

Answer 1

The temperature of any substance is related to its average kinetic energy where energy is the ability to perform work. Since the average kinetic energy of the substance remains the same during phase change, the temperature will remain the same too.

Complete step-by-step answer:
Total energy can be explained as sum of kinetic energy and potential energy where kinetic energy
here refers to the temperature while potential energy is the force of attraction between molecules.
Initially, there is an increase in kinetic energy due to which the temperature also increases. This happens because molecules gain that kinetic energy, they get excited and starts to vibrate.
But as the phase changes gradually while melting or boiling any substance, the spacing between molecules start to increase and force of attraction decreases. This results in an increase in potential energy while kinetic energy remains constant. Thus, the temperature stays constant during the course of phase change since the added heat is used to increase only potential energy. Therefore, heat is not manifested outside which is why temperature stays constant.
Or we can say that during phase change the energy given or taken in is referred to as latent heat which is used only to form bonds between molecules or break the bonds between molecules. This heat does not affect the average kinetic energy of the substance and it remains constant during melting or boiling.
Hence, we can say that the temperature of a subtract region constant during its melting or boiling point.

Note: This can also be understood by considering the internal energy of a substance. If the next exchange of internal energy remains constant, the whole system is in thermal equilibrium, which is a condition that exists during boiling and melting point calculation.