Nuclear and subatomic particle reactions emit large quantities of energy, but contrary to popular belief they do not convert mass into energy. Any mass lost by the reacting system is acquired by the surroundings along with the energy acquired. Chemical reactions are much less energetic. Nevertheless, when a chemical reaction emits energy to its surroundings, its reactants lose an equivalent quantity of mass in the process. The change in mass is so extremely small, however, that it cannot be directly detected. For all practical purposes it can be ignored.

Mass–energy conservation is discussed in various reactions, such as matter–antimatter annihilation, nuclear fusion and fission, chemical reactions, and changes of physical state. The concept of mass–energy equivalence is reinforced by the use of everyday analogies and by asking what mathematical equations actually mean.