What characterizes exothermic reactions?

Exothermic reactions are characterized by the release of heat energy into their surroundings. This means that as the reaction occurs, the temperature of the environment increases due to the excess energy being released. This is a fundamental feature of exothermic processes, distinguishing them from endothermic reactions, which absorb heat energy to proceed.

When a chemical reaction releases energy, it typically results in the formation of products that are lower in energy compared to the reactants. This energy difference is often manifested as heat, making these reactions spontaneous in nature under standard conditions. Common examples of exothermic reactions include combustion, where fuels react with oxygen, leading to the release of heat and light.

The other options presented do not accurately define exothermic reactions. Reactions that absorb heat energy are classified as endothermic. While some reactions may involve work being done or occur at constant pressure, these characteristics do not specifically define an exothermic reaction. Instead, the hallmark of exothermicity is the liberation of heat, making the correct choice clear.