Chemical clock

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. 
Types.
Clock reactions may be classified into three or four types:
One reaction may fall into more than one classification above depending on the circumstance. For example, iodate−arsenous acid reaction can be substrate-depletive clock reaction, autocatalysis-driven clock reaction and crazy clock reaction.
Examples.
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.