Final answer:
As you move down the cooling curve, the process is exothermic, with energy being released into the surroundings during phase changes such as gas to liquid to solid.
Step-by-step explanation:
As you move down the cooling curve, the process is exothermic. An exothermic reaction is one in which energy is released into the surroundings, often in the form of heat. This is evidenced when substances transition from gas to liquid to solid, releasing heat and causing temperatures to drop. For example, when water is placed in a freezer to make ice, it releases heat into the surrounding environment, which is an exothermic process. The shift in reaction due to cooling in exothermic reactions tends toward the product side, which often includes the formation of solids or liquids from gases.
Conversely, endothermic reactions absorb heat from their surroundings, leading to a temperature increase in the system but a temperature decrease in the surroundings. For instance, when ice melts in your hand, heat transfers from your hand to the ice, which is endothermic. In cooling curves, the temperature plateau during phase changes, such as freezing, indicates that energy is being used to change the substance's state rather than altering its temperature, emphasizing the exothermic nature of these changes.