A combination reaction is a reaction in which two substances combine to form a third. A general equation that describes a combination reaction is A + B --> AB. Again A and B can be either elements or compounds. Decomposition and combination reactions can be considered to be the reverse of each other. Under some conditions it is possible to change conditions and cause a decomposition reaction to become a combination reaction or vice versa. The reaction of calcium oxide with sulfur dioxide to form calcium sulfite is an example of a combination reaction.
CaO(s) + SO2(g) ---> CaSO3(s)