Modifications are presented for the iodine clock reaction to introduce the concept of activity and to help students better appreciate molecular aspects of chemical equilibrium. The addition of an unreactive salt affects the activity of the reactants in the iodine clock reaction. The difference in activity affects how long the iodine clock reaction takes to show the color change, even though the concentration of reactants has not changed. Resources and suggestions are provided to use the modified iodine clock reaction in both introductory/general chemistry courses and more advanced analytical and physical chemistry courses to expose students to the concept of activity and its role in chemical reactions and equilibrium.