Final answer:
During the melting point and boiling point, the average molecular energy remains constant as energy is used for the phase change, but during the initial heating of a solid or a liquid to reach these points, the average molecular energy is changing as the temperature increases.
Step-by-step explanation:
When a solid is heated in a container and a phase change occurs, the behavior of the energy of the molecules can be observed through a heating curve. For example, when a solid is heated to its melting point, all the heat energy added to the system is used to break intermolecular bonds rather than increase the temperature, so the average molecular energy remains constant during this phase change. Similarly, when a liquid is heated to its boiling point, the temperature and therefore the average molecular energy again remain constant since the energy is used for the phase change from liquid to gas. However, when a solid is initially heated to reach its melting point or a liquid is initially heated to reach its boiling point, the temperature rises because the average molecular energy is changing; the molecules are moving faster as they absorb more thermal energy.