chain reaction

Picture a line of dominoes where knocking one over topples the next, which topples the next. The first push is small, but it propagates itself, tile after tile, with no extra effort from you. A chemical chain reaction works on the same self-sustaining principle: one reactive species makes a product and, in doing so, creates a new reactive species that carries the reaction onward.

More precisely, a chain reaction is a reaction that proceeds through a repeating cycle of steps in which a reactive intermediate — usually a free radical — is consumed in one step but regenerated in another, so it keeps the cycle going round and round. Such reactions have three characteristic stages: initiation, where the first reactive species is created (often by light or heat); propagation, the self-repeating cycle that does the real work; and termination, where two reactive species combine and quench the chain.

Why it matters: chain reactions explain a huge range of processes — the burning of fuels, the making of plastics, the chemistry of smog and the ozone layer, and (in the nuclear sense) the energy of reactors and bombs. The honest caveat for chemistry is that 'chain reaction' here means a self-propagating cycle of chemical steps; it is a different idea from nuclear chain reactions, even though both share the image of one event triggering the next.