Constant Temperature during Melting or Boiling Point
Understanding the phenomenon of why the temperature of a substance remains constant during its melting or boiling point requires a closer look at the process involved.

Phase Transition
When a substance is at its melting or boiling point, it undergoes a phase transition from a solid to a liquid or from a liquid to a gas, respectively. During this transition, the energy being supplied to the substance is utilized to break the intermolecular forces holding the molecules together rather than increasing the kinetic energy of the molecules.

Energy Absorption
As the substance absorbs energy to overcome the intermolecular forces, the temperature remains constant because the energy is being used for the phase transition rather than increasing the kinetic energy of the molecules. This energy is known as the latent heat of fusion for melting or latent heat of vaporization for boiling.

Equilibrium State
At the melting or boiling point, the substance exists in a state of equilibrium where the energy being absorbed is balanced by the energy required for the phase transition. This equilibrium results in a constant temperature until all the substance has completed the phase transition.

Conclusion
In conclusion, the temperature of a substance remains constant during its melting or boiling point because the energy being supplied is utilized for the phase transition rather than increasing the kinetic energy of the molecules. This phenomenon is crucial in understanding the behavior of substances under different conditions.