In combination reactions, two or more substances combine to form a new compound. Only one product is obtained in such reactions. The reactants in such reactions can be elements as well as compounds. The general equation used to represent a combination reaction is: A + Z AZ For example, calcium oxide reacts vigorously with water to produce calcium hydroxide. A large amount of heat is also evolved during this process, which increases the temperature of the system. Hence, the combination of calcium oxide and water is exothermic in nature.