1. The elements or compounds that enter into a chemical reaction are called reactants. The elements or compounds 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. Products are the species formed from chemical reactions. 2. Wood is made of fiber (cellulose) and minerals (metals). When wood is burned, oxygen and other elements in the air (mainly carbon, hydrogen and oxygen) react to form carbon dioxide that is released into the atmosphere, while the minerals turn into ashes. When an apple is cut (or bruised), oxygen is introduced into the injured plant tissue. When oxygen is present in cells, polyphenol oxidase (PPO) enzymes in the chloroplasts rapidly oxidize phenolic compounds naturally present in the apple tissues to o-quinones, colorless precursors to brown-colored secondary products
