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For the biological process, see Decomposition. For chemical decomposition in general, see Chemical decomposition.
'Thermal decomposition' is a chemical reaction whereby a chemical substance breaks up into at least two chemical substances when heated. It is an endothermic reaction as heat is required to break chemical bonds in the compound undergoing decomposition. ''Thermolysis'' (from ''thermo-'' meaning heat and -lysis meaning break down) is a chemical process by which a substance is decomposed into other substances by use of heat. The ''decomposition temperature'' of a substance is the temperature at which the substance decomposes into smaller substances or into its constituent atoms.
For example, calcium carbonate decomposes into calcium oxide and carbon dioxide. Some compounds, on the other hand simply decompose into their constituent elements. Water, when heated to well over 2000 degrees Celsius, breaks up into hydrogen and oxygen.
A common example is the decomposition of copper carbonate into copper oxide and carbon dioxide, seen here:
:CuCO₃ → CuO + CO₂
The copper carbonate turns from a green powder into a black copper oxide, and carbon dioxide is released in a gaseous state.
Decomposition may be aided by the presence of a catalyst. For example, hydrogen peroxide decomposes more quickly with the use of manganese(IV) oxide:
:2H₂O_2(aq) → 2H₂O_(l) + O_2(g)
High temperatures can also induce polymerization, which produces larger molecules, possibly also causing thermal decomposition and evaporation of smaller molecules in the process. Such reactions called pyrolysis. This is not only decomposition, although may be inaccurately referred to as such. A common example is coking, formation of an amorphous carbon structure along with the evaporation of hydrogen and other pyrolysis gases.