A chemical clock (or clock reaction) is a complex mixture of reacting chemical compounds in which the onset of an observable property occurs after a predictable induction time. In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change after a reproducible time lapse.
One class of example is the iodine clock reactions, in which an iodine species is mixed with redox reagents in the presence of starch. After a delay, a dark blue color suddenly appears due to the formation of a triiodide-starch complex.
Additional reagents can be added to some chemical clocks to build a chemical oscillator. For example, the Briggs-Rauscher reaction is derived from an iodine clock reaction by adding perchloric acid, malonic acid and manganese sulfate..