Iodine clock reaction is a combination of hydrogen peroxide and sulfuric acid. When these are poured together, there is no reaction at first, but after time, a colored reaction will happen to the liquid and may change back to the original color. This idea was discovered by Hans Landolt in the late 1800s.
It represents the <em>number of atoms</em> of that particular element present in the compound. In C₂H₄O₂ there are 2 Carbon atoms, 4 Hydrogen atoms and 2 Oxygen atoms.
