Final answer:-
A cooling curve represents how the temperature of a substance decreases over time as it loses heat. On this curve, different segments correspond to either a change in temperature during a single phase or a phase change at a constant temperature.
Explanation:-
During the heat of vaporization (which is the amount of energy needed to convert a substance from a liquid to a gas without changing its temperature), if we are observing a cooling curve instead, the process is reversed—meaning we are looking at the phase change from gas to liquid, or condensation.
Here is a step-by-step explanation of the phase change that occurs during the heat of vaporization on a cooling curve:
1. **Starting in the Gas Phase**: The substance begins as a gas, and as it cools, it loses energy. At this point in the curve, the slope would be downward as the temperature decreases.
2. **Reaching the Condensation Point**: As the gas cools down to the temperature at which condensation occurs (the boiling point at a given pressure), it reaches a phase change. At this point, the temperature of the substance stops dropping despite the continued removal of heat. This happens because the energy is being used to change the phase of the substance, not to lower its temperature. The cooling curve levels off and becomes horizontal.
3. **Undergoing Condensation**: During condensation, the gas molecules lose enough energy that they start to move closer together, forming liquid. This phase change occurs at a constant temperature, and the heat removed from the system during this process is the heat of vaporization. Despite the removal of heat, the temperature remains constant until the phase change is complete.
4. **Completion of Condensation**: Once all the gas has turned into a liquid, the phase change is complete. If heat continues to be removed, the temperature of the now liquid substance will begin to decrease again. The cooling curve will start sloping downward once more as the substance continues to lose thermal energy.
Therefore, the phase change that can occur on a cooling curve during the heat of vaporization is condensation, the transition from gas to liquid. This takes place at a constant temperature while the substance is releasing the heat of vaporization, and is visually identified on the cooling curve as a flat, horizontal segment where the temperature does not change.