What is characterized by the release of heat?

The correct choice highlights that an exothermic reaction is characterized by the release of heat. In an exothermic reaction, energy is released into the surroundings, causing an increase in temperature. This release of energy often manifests as heat, which is a key factor in identifying the nature of these reactions. Such processes include combustion reactions, like burning fuel, where energy is released in the form of heat and light.

On the other hand, an endothermic reaction involves the absorption of heat, leading to a decrease in temperature in the surroundings. Irreversible reactions denote a process that cannot easily return to its original state but do not specifically relate to heat transfer. Physical changes might involve some energy exchanges but do not exclusively characterize heat release.

Understanding these distinctions is vital in grasping how different reactions function thermodynamically in chemistry.