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Chemical reactions

Combustion, oxidation and thermal decomposition

Combustion, oxidation and thermal decomposition

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Combustion, oxidation and thermal decomposition

In a nutshell

Combustion involves burning a substance in oxygen, oxidation is when a substance gains oxygen and thermal decomposition is when a substance breaks down due to heat. In this summary you will learn more about these reactions with examples.




hydrocarbon + oxygenhydrocarbon \ +\ oxygen​​
carbon dioxide + watercarbon\ dioxide\ +\ water​​
     metal + oxygenmetal\ +\ oxygen ​​
          metal oxide metal\ oxide​​


Combustion is a type of reaction which involves burning a substance in oxygen. Combustion reactions release a lot of energy, mostly in the form of heat energy but it can also be released as light energy.

Three things are needed for a combustion reaction to take place:

  1. Fuel
  2. Oxygen
  3. Source of heat


Burning fuels involves combustion reactions. Fuels contain hydrocarbons (which are molecules made up of hydrogen and carbon atoms only). With a good supply of oxygen and heat, hydrocarbons burn in oxygen to produce water and carbon dioxide. 

hydrocarbon +oxygenwater +carbon dioxidehydrocarbon \space + oxygen \rightarrow water \space + carbon \space dioxide


An oxidation reaction involves a substance reacting with oxygen and thereby gaining oxygen.

Combustion is a type of oxidation reaction. Several types of metals react with oxygen in the air when heat is applied to product metals oxides. Rust is formed via an oxidation reaction: iron reacts with oxygen to produce iron oxide (rust).


When heat is applied, magnesium can react with oxygen in the air to form magnesium oxide.

magnesium +oxygenmagnesium oxidemagnesium \space + oxygen \rightarrow magnesium \space oxide
2Mg(s) + O2(g)2MgO(s)2Mg(s) \space + \space O_2(g) \rightarrow 2MgO(s)​​

Thermal decomposition  

Thermal decomposition is a process whereby a substance breaks down when heat is applied. 

Not every substance can undergo thermal decomposition; most substances melt when heated.


Calcium carbonate undergoes thermal decomposition to produce calcium oxide and carbon dioxide.

calcium carbonatecalcium oxide + carbon dioxidecalcium \space carbonate \longrightarrow calcium \space oxide \space+ \space carbon \space dioxide

CaCO3(s)CaO(s) + CO2(g)CaCO_3(s) \rightarrow CaO(s) \space+ \space CO_2(g)

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