Have you ever sat around a campfire or watched flames flicker in a fireplace? The burning of wood is a chemical reaction. A chemical reaction always involves changes in chemical bonds that join atoms in compounds. The elements or compounds that enter into a chemical reaction are called reactants. The elements or molecules produced by a chemical reaction are called products. As wood burns, molecules of cellulose are broken down and combine with oxygen to form carbon dioxide and water vapor, and energy is released. What are the reactants when wood burns? What are the products when wood burns?