A common classification of the chemical reactions is as combination (or synthesis), decomposition, single replacement and double replacement. Knowing the reactants and products, you can tell to which one of those four classifications a reaction pertains. When two reactants combine into a single product, it is a combination (synthesis) reaction. When a single reactant yields two or more products, it is a decomposition reaction. When the cation of a single element replaces the cation in a compound, it is a single replacement. When the cations of two different reactants exchange and end combined with the anions of the others compounds, it is a double replacement reaction. Knowing the reactants and products can help you classify a chemical reaction base on the atoms in the substances involved and the number of substances involved. For instance, when you are given two reactants and only one product is made, then you can classify it as a synthesis reaction. Another would be if an organic compound is rotated with oxygen, CO2 and water is produced, then you can classify it as a combustion reaction.
