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- For the biological process, see Decomposition. For chemical decomposition in general, see Chemical decomposition.
Thermal decomposition, also called thermolysis, is defined as a chemical reaction whereby a chemical substance breaks up into at least two chemical substances when heated. The reaction is usually endothermic as heat is required to break chemical bonds in the compound undergoing decomposition. 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 its components - hydrogen and oxygen.
A common example is the decomposition of copper carbonate into copper oxide and carbon dioxide, seen here:
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:
- 2H2O2(aq) → 2H2O(l) + O2(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 are called pyrolysis reactions. A common example is coking, which is the formation of an amorphous carbon structure along with the evaporation of hydrogen and other pyrolysis gases.
If thermal decomposition of a substance is significantly exothermic, then the substance is thermodynamically unstable. If initiated, its decomposition forms a positive feedback loop and undergoes thermal runaway up to the point of causing an explosion.
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