Decomposition Reaction

Our Objective

To study decomposition reactions.

The Theory

What is a decomposition reaction?

Decomposition is a type of chemical reaction.  It is defined as the reaction in which a single compound splits into two or more simple substances under suitable conditions. It is just the opposite of the combination reaction.

In a combination reaction, a substance is formed as a result of chemical combination, while in a decomposition reaction, the substance breaks into new substances.

For example:  The digestion of food in our body is accompanied by a number of decomposition reactions. The major constituents of our food such as carbohydrates, fats proteins, etc., decompose to form a number of simpler substances.  These substances further react, releasing large amounts of energy, which keeps our body working.

The general equation that describes a decomposition reaction is:


Types of Decomposition Reactions

Decomposition reactions can be classified into three types:

  • Thermal decomposition reaction
  • Electrolytic decomposition reaction
  • Photo decomposition reaction

Thermal decomposition is a chemical reaction where a single substance breaks into two or more simple substances when heated. The reaction is usually endothermic because heat is required to break the bonds present in the substance.

Photo decomposition is a chemical reaction in which a substance is broken down into simple substances by exposure to light (photons).

  • Thermal decomposition reaction (Thermolysis)
    • Decomposition of calcium carbonate: Calcium carbonate (lime stone) decomposes into calcium oxide (quick lime) and carbon dioxide when heated. Quick lime is the major constituent of cement.


    • Decomposition of potassium chlorate: When heated strongly, potassium chlorate decomposes into potassium chloride and oxygen. This reaction is used for the preparation of oxygen.


      If the decomposition is carried out in the presence of manganese dioxide (MnO2), it takes place at a lower temperature. In this case, MnO2 is used as a catalyst.

    • Decomposition of ferric hydroxide: Ferric hydroxide undergoes decomposition in the presence of heat, giving ferric oxide and water molecules.


    • Decomposition of hydrated oxalic acid:

      Hydrated oxalic acid (H2C2O4.2H2O) decomposes into oxalic acid and water when heated.

  • Electrolytic decomposition reaction (Electrolysis)

    Electrolytic decomposition may result when electric current is passed through an aqueous solution of a compound. A good example is the electrolysis of water.


    • Electrolysis of water: Electrolysis of water is the decomposition of water into hydrogen and oxygen due to the passage of electric current through it.
    • Decomposition of sodium chloride: On passing electricity through molten sodium chloride, it decomposes into sodium and chlorine.
  • Photo decomposition reaction (Photolysis)


    • Decomposition of silver chloride: Place a small quantity of silver chloride (AgCl) taken in a watch glass under sunlight for some time. The crystals slowly acquire a grey colour. On analysis, it is found that the sunlight has caused decomposition of silver chloride into silver and chlorine.


      Silver bromide also decomposes in the same way.

    • Decomposition of hydrogen peroxide: In the presence of light, hydrogen peroxide decomposes into water and oxygen


Why are decomposition reactions mostly endothermic in nature?

Most decomposition reactions require energy either in the form of heat, light or electricity. Absorption of energy causes the breaking of the bonds present in the reacting substance which decomposes to give the product.

Learning Outcomes

  1. Students understand the characteristics of a decomposition reaction & different types of such reactions.
  2. Students identify the compounds that may give a decomposition reaction.
  3. Students acquire skills to perform a decomposition reaction in the lab.
  4. Students will be able to distinguish a decomposition reaction from a given set of chemical reactions.

Let’s discuss the decomposition reaction of ferrous sulphate crystals by the action of heat.