When a substance is heated or cooled it can go through a physical change. The substance may change its size, its shape, or its state, but remain the same substance. When atoms link together to form new substances, a chemical change takes place . The reactants are the original substances. New substances produced when a chemical reaction takes place are called the products. A common example of this kind of change is rust or iron oxide. One way to prevent it from forming on steel parts is to paint them.