In thermochemistry, what happens when a system exhibits an exothermic reaction?

When a system undergoes an exothermic reaction, it releases heat to the environment. This process involves the transformation of reactants into products, during which energy is released in the form of heat. As a result, the total internal energy of the system decreases, and heat flows out of the system, causing the surroundings to gain energy and, often, experience a temperature rise.

This release of heat is a defining characteristic of exothermic reactions, and it distinguishes them from endothermic reactions, where a system absorbs heat from the environment. In an exothermic reaction, because the system is losing heat, the temperature of the system may decrease, but this is a consequence of the heat flow rather than a defining feature of the reaction itself. Constant temperature can also occur under specific conditions, such as during phase changes, but it is not a general characteristic of exothermic reactions. Thus, the key takeaway is that an exothermic reaction is characterized primarily by the release of heat to the surroundings, which aligns with the correct answer.