What reaction occurs when a substance changes from one state to another and gives off heat?

The reaction that occurs when a substance changes from one state to another while giving off heat is termed exothermic. In an exothermic process, energy is released into the surroundings as the substance transitions from a higher energy state to a lower energy state. This is commonly observed in processes such as condensation, where gas turns into a liquid, or freezing, where a liquid turns into a solid. As heat is released during these transitions, the surrounding environment may experience an increase in temperature.

In contrast, endothermic reactions absorb heat from the environment, resulting in a temperature drop in the surroundings. This occurs during processes like melting or boiling, where heat is required for the substance to change to a higher energy state. Considering these definitions, the exothermic process is clearly the correct answer here, as it refers specifically to the release of heat during a state change.