Iodine clock reactions are a class of reactions which are used to demonstrate reaction kinetics. In the setup, two reactions, one fast and one slow, are set against each other. The slow reaction converts a form of iodine to triiodide ions, while the fast one converts the triiodide ions into a transparent form of iodine. Once the reactants involved in the fast reaction are depleted, triiodide ions become visible in solution. The addition of starch makes the appearance of triiodide far more visible and dramatic.