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

When a substance changes from one state to another and releases heat during that process, it undergoes an exothermic reaction. In an exothermic reaction, the system releases energy in the form of heat to its surroundings. This typically occurs during processes such as condensation (gas to liquid), freezing (liquid to solid), or combustion.

For example, when water vapor condenses into liquid water, it releases heat into the environment, which can be felt as warmth. This release of heat indicates that the system is losing energy in the form of thermal energy. The nature of exothermic reactions is crucial in various applications, including heating systems and understanding phase transitions in materials.

In contrast, an endothermic reaction absorbs heat, leading to a decrease in temperature in the surroundings, while the term isothermal refers to processes occurring at a constant temperature. These distinctions clarify why exothermic is the appropriate term for reactions that release heat during state changes.