In the course of chemical reactions the concentration of reactants typically decreases whereas the concentration of products increases. In addition, more complex reactions are also known, in which in time both the concentrations of reactants and products periodically once increase and then again decrease or oscillate in time.
Such reactions are also known as Beloussov–Zhabotinsky reactions or clock reactions. One of the best known clock reactions was demonstrated in this experiment. Equal volumes of three different solutions were poured into the flask:
1) Solution 1: 10% hydrogen peroxide (H2O2)
2) Solution 2: 5% potassium iodate (KIO3), 0.9% sulfuric acid (H2SO4)
3) Solution 3: 0.03% starch (C6H10O5)n, 0.57% manganese sulfate (MnSO4), 1.6% malonic acid CH2(COOH)2
During the reaction, the concentrations of reactants as well as products change periodically. Iodine and starch combine and give a blue complex compound, but for low concentrations of iodine, the solution is yellowish.
Mechanisms of oscillating reactions are complicated and there is no simple chemical equation.
The clock reaction in this experiment is known as Briggs-Rauscher reaction.