What characterizes an exothermic reaction?

An exothermic reaction is characterized by the release of heat energy into the surroundings. During such a reaction, the total energy of the products is lower than that of the reactants, resulting in the excess energy being emitted, often in the form of heat. This release of heat can lead to a rise in temperature of the immediate environment, making exothermic reactions important in various processes, including combustion and respiration.

In contrast, a reaction that absorbs heat energy would be considered endothermic, and reactions occurring in the presence of water or those that produce gas do not necessarily define an exothermic reaction. While some exothermic reactions may indeed produce gas or occur in aqueous solutions, these traits are not exclusive to exothermic reactions. The key defining characteristic is the release of heat.