A Decomposition Reaction is a reaction in which two compounds exchange parts with each other and form two different compounds.
2HgO ---> 2Hg + O2
2H2O ---> 2H2 + O2
MgCl2 ---> Mg + Cl2
FeS ---> Fe + S
Decomposition Reactions also occur when one compound is split into two similar compounds.
CaCO3 ---> CaO + CO2
Na2CO3 ---> Na2O + CO2
2KClO3 ---> 2KCl + 3O2
Ba(ClO3)2 ---> BaCl2 + 3O2
If the first two substances are bases and the last two are acids then the acid or base breaks down into the oxide of the metal, the case of bases, of the oxide of the nonmetal, in the case of acids, plus water.
All binary compounds, like the four in the first examples set above, will break down into their elements.
All carbonates, like the first two in the second example set above, break down to the oxide and carbon dioxide.
Chlorates, like KCLO3 and Ba(CLO3)2 in the example, will break down to the binary salt and oxygen.
In every decomposition reaction their is only one substance on the left-hand reactant side.