Question: Why does temperature not change during a change of state?
The particles have reached the maximum vibrational-kinetic energy that they can have without breaking the bonds holding them in place. Any extra energy absorbed by the material breaks a bond, which is part of the process of changing state. Since the extra energy did not cause an increase in the particle's kinetic energy, there is no temperature increase.
Exam answer: The temperature does not change during a change of state because the heat energy is being used to overcome the intermolecular forces of attraction (break the bonds) between the particles; instead of increasing the kinetic energy of the particles.