In a synthesis reaction, two or more reactants combine to form a product. The reactants are simpler than the product and can be either elements or compounds. The product is always a more complex compound than the reactants were.
Synthesis reactions are simple to identify because the number of products will always be less than the number of reactants. The general formula for a synthesis reaction is A + B ---> AB. A synthesis reaction can be a step in a larger chain of reactions. Examples of synthesis reactions include hydrogen and oxygen gas coming together to form water and carbon monoxide and oxygen gas coming together to form carbon dioxide. The reactants in a synthesis reaction can be metals or nonmetals.
Synthesis reactions are one of the main four categories of chemical reactions. The other categories of reactions are decomposition, single-displacement and double-displacement. Decomposition reactions are the direct opposite of synthesis reactions and involve breaking down a complex reactant into simpler products. Single-displacement and double-displacement reactions involve elements or ions that switch place with the elements or ions in another reactant. Among these categories there are more specific types of reactions, such as acid-base, redox, combustion and hydrolysis reactions.