A simple way of classifying chemical reactions is to group them in one of four basic types:
single displacement - an element replaces another element in a compound
A + BC AC + B
A + BC BA + C
Mg + 2 HNO3 Mg(NO3)2 + H2
Cl2 + 2 NaBr 2 NaCl + Br2
double displacement - two compounds react and exchange positive and negative ions
AB + CD AD + CB
Ca(NO3)2 + Na2SO4 2 NaNO3 + CaSO4
synthesis - compounds or elements react to form more complex compounds
A + B AB
4 Fe + 3 O2 2 Fe2O3
decomposition - a compound reacts to form elements or less complex molecules
AB A + B
2 HgO 2 Hg + O2
On occasion a reaction does not clearly fall into one of these types or appears to be a combination of these types. For example, the reaction below could be thought of as a double displacement reaction followed by the decomposition of carbonic acid.