The reaction in which one compound is broken down into two simpler substances is known as a decomposition reaction. This reaction is exactly the opposite of the combination reactions. Most of the decomposition reactions are endothermic but exceptions are also there. It is important to note that these reactions require energy to proceed, in the form of heat or light.

The general form of decomposition reactions is the following:


  • AB is the parent molecule
  • A and B are the daughter molecules or products

Types of Decomposition Reactions

There are the following types of decomposition reactions:

  1. Thermal decomposition reaction
  2. Electrolytic decomposition reaction
  3. Photolytic decomposition reaction

Thermal decomposition reactions

The decomposition reaction in which compounds are broken down in the presence of heat is known as the thermal decomposition reaction. Note that, heat provides energy for breaking the bonds and these reactions are mostly endothermic.

Thermal decomposition of acetaldehyde

Acetaldehyde decomposed to give methane and carbon monoxide.

CH3CHO → CH4 + CO 

The proposed mechanism of this reaction is given below:




CH3• + CH3CHO → CH4 + CH3CO

CH3CO → CH3 + CO


CH3• + CH3 → C2H6

Electrolytic decomposition reactions

An electrolytic decomposition reaction is a reaction in which energy is provided in the form of electricity. For example, the decomposition of water. Here, the water molecule undergoes electrolysis to produce hydrogen and oxygen.

2H2O → 2H2 + O2

Photolytic decomposition reactions

The photolytic decomposition reaction is a reaction in which a compound is broken down in the presence of light.

Decomposition of ozone

The ozone molecule is decomposed by absorbing photons. Hence, it gives O2 and O.

Decomposition reactions: ozone

Proposed mechanism of decomposition of ozone 

O3 → O2 + O


O2 + O• → O3

O• + O3 → 2O2

Examples of decomposition reactions

The following are the important examples of decomposition reactions:

Decomposition of carbonic acid

The carbonic acid spontaneously decomposes into carbon dioxide and water.

H2CO3 → CO2 + H2O

Decomposition of calcium carbonate

The calcium carbonate is decomposed into calcium oxide and carbon dioxide. calcium oxide has very important in industries.

CadCO3 → CaO + CO2

Decomposition of metal chlorates

When metal chlorates are heated. They decompose into metal chlorides and oxygen with intense emission of light.

2MClO3 → 2MCl + 3O2

Related Resources

Concepts Berg

What is decomposition reaction with examples?

The decomposition reaction involves the breakdown of the reacting molecules. For example, the breakdown of water molecules into hydrogen and oxygen.

What are the 3 types of decomposition reactions?

There are the following types of decomposition reactions such as thermal decomposition, electrolytic decomposition, and photolytic decomposition

What are the necessary conditions for a decomposition reaction?

The necessary condition for the decomposition reactions is energy. However, this can be provided in the form of heat or light.

Can a decomposition reaction be a redox reaction?

All decomposition reactions are not redox reactions. However, some can be redox reactions. For example, the decomposition of water is a redox reaction.

Why is cracking a thermal decomposition reaction?

Thermal decomposition reaction or cracking is a reaction in which long chains of hydrocarbons are broken down into the smaller hydrocarbon in the presence of heat,

Why is the decomposition reaction also known as the endothermic reaction?

The decomposition reactions are endothermic. This is because they involve the absorption of energy.

Do decomposition reactions always result in two or more substances?

The decomposition reaction always involves one reactant. Which is broken down into two or more products.