When two or more reactants combine in a reaction to form a single product, it is called a combination reaction.
Example:: When HCl vapours started coming out from the test tube, and NH3
out from the solution on the glass rod. The ammonia gas and hydrogen chloride gas reacted
to form the salt ammonium chloride .
NH3(g)+ HCl(g) ----> NH4Cl(s)
In this reaction ammonium chloride formed as the single product by the combination of Ammonia and Hydrogen Chloride .