A chemical reaction that shows a periodic colour change between magenta and blue with a period of about one minute. It occurs with a mixture of sulphuric acid, potassium bromate(V), cerium sulphate, and propanedioic acid. The colour change is caused by alternating oxidation-reductions in which cerium changes its oxidation state (Ce3+ gives a magenta solution while Ce4+ gives a blue solution). The B-Z reaction is an example of a chemical oscillating reaction - a reaction in which there is a regular periodic change in the concentration of one or more reactants. The mechanism is highly complicated, involving a large number of steps.