Iodine clock reaction, an example of a redox reaction which produces a delayed colour change. Two solutions are mixed at room temperature. The first contains iodide ions, thiosulfate and starch, and the second contains hydrogen peroxide (H2O2) and sulphuric acid. A redox reaction occurs in which the iodide ions are oxidised to iodine by the peroxide. As soon as the iodine is formed, it reacts with thiosulphate ions (S2O32-) to reform iodide ions. The second reaction is far faster than the first. As soon as all the thiosulphate is used up, free iodine remains, which reacts with the starch to form the a blue-black complex.