The chemical reaction where two substances join to form one is called a synthesis reaction or combination reaction. In this type of reaction, two or more reactants combine to create a more complex product. A common example of a synthesis reaction is the formation of water from hydrogen and oxygen gases: \(2H_2 + O_2 → 2H_2O\). These reactions are foundational in chemistry, playing pivotal roles in various biological and industrial processes, including the production of compounds and materials.