What characterizes an exothermic reaction?

An exothermic reaction is characterized by the release of heat to the surroundings. When a chemical reaction occurs and is classified as exothermic, it means that the energy stored in the reactants is transformed and released, typically in the form of thermal energy. This release of heat can result in an increase in the temperature of the immediate surroundings, which is often how exothermic reactions are identified in practice.

This property is critical in various applications, such as combustion reactions, where fuels release heat and light when they react with oxygen. Additionally, many biological processes, including cellular respiration, are exothermic as they release energy for the organism's use.

In contrast, a reaction that absorbs heat would be endothermic, and while reactions may require specific conditions like high pressure or occur in specific states of matter (gases, liquids, or solids), these characteristics do not define exothermic reactions. Thus, the essence of an exothermic reaction lies in its ability to release heat, making the correct choice clear.