B → A

B → A B Example: Hydrogen gas combines with chlorine gas to form hydrogen chloride gas. H (g) + Cl (g) → 2HCl (g) Depending on the chemical nature of the reactants, there are three classes of combination reactions:  Element + Element → Compound In this type of combination reaction, two elements react with one other to form a compound. The reaction may take place between a metal and a non-metal or two non-metals. Example : When solid sulphur reacts with oxygen, it produces sulphur dioxide. Here both the reactants are non-metals.