When chemical bonds between atoms are formed or broken, chemical reactions occur. The substances that are introduced into a chemical reaction are known as reactants, and the substances that are produced at the end of the reaction are known as products. To indicate the direction of the chemical reaction, an arrow is drawn between the reactants and products. The reactants are on the left-hand side whereas the products formed are on the right-hand side. The reactants and products are connected by a one-headed or two-headed arrows. For example, a reaction A + B → C + D Here, A and B are the reactants, which react to form the products C and D.