When a substance melts or boils the temperature remains constant. why?

When a substance melts or boils, the temperature remains constant because the energy added during these phase changes is used to overcome intermolecular forces rather than increase the temperature. For melting, energy breaks the bonds holding the solid structure together, while for boiling, it allows molecules to escape from the liquid phase into the gas phase. This constant temperature reflects the balance between energy input and the energy required for the phase transition. Thus, the temperature only changes once the entire substance has transitioned to the new phase.