An example of a spontaneous (without addition of an external energy source) decomposition is that of hydrogen peroxide which slowly decomposes into water and oxygen ):
: 2 HO → 2 HO + O
This reaction is one of the exceptions to the endothermic nature of decomposition reactions.
Other reactions involving decomposition do require the input of external energy. This energy can be in the form of heat, radiation, electricity, or light. The latter being the reason some chemical compounds, such as many prescription medicines, are kept and stored in dark bottles which reduce or eliminate the possibility of light reaching them and initiating decomposition.
When heated, carbonates will decompose. A notable exception is carbonic acid, (HCO). Commonly seen as the "fizz" in carbonated beverages, carbonic acid will spontaneously decompose over time into carbon dioxide and water. The reaction is written as:
: HCO → HO + CO
Other carbonates will decompose when heated to produce their corresponding metal oxide and carbon dioxide. The following equation is an example, where M represents the given metal:
: MCO → MO + CO
A specific example is that involving calcium carbonate:
: CaCO → CaO + CO
Metal chlorates also decompose when heated. In this type of decomposition reaction, a metal chloride and oxygen gas are the products. Here, again, M represents the metal:
: 2 MClO → 2 MCl+ 3 O
A common decomposition of a chlorate is in the reaction of potassium chlorate where oxygen is the product. This can be written as:
: 2 KClO → 2 KCl + 3 O