Final answer:
In a diagonal section of a heating curve, added energy increases the substance's temperature and internal energy, but not during phase changes, where the temperature stays constant as intermolecular forces are overcome.
Step-by-step explanation:
On a diagonal section of a heating curve, the energy that is added creates a change in temperature of the substance. As energy in the form of heat is added to a system, it increases the system's internal energy and results in a rise in temperature, except during phase changes. During a phase change, the substance's temperature remains constant, even though energy is still being absorbed. This energy goes into breaking the intermolecular forces between particles, which is why no temperature change is observed during these plateaus on a heating curve.