Final answer:
On a cooling curve during the Heat of Fusion, the phase change that occurs is freezing, where the substance transitions from liquid to solid, releasing heat and keeping the temperature constant until the phase change is complete.
Step-by-step explanation:
During the Heat of Fusion on a cooling curve, the phase change that can occur is the transition from liquid to solid, which is also known as freezing or solidification. At the melting point of a substance, if heat is removed rather than added, freezing occurs, causing the liquid to gradually change into the solid phase. Cooling curves graphically represent the relationship between temperature and the energy released as a substance cools and changes phase.
The enthalpy of fusion, ΔHfus, is the energy involved in this phase transition. For example, water at 0°C will begin to freeze and release energy equal to the heat of fusion. Throughout this process, the temperature of the substance remains constant until the phase change is complete. Cooling curves demonstrate how the temperature stays constant during a phase change despite the ongoing energy exchange.