• Those reactions in which a compound splits up into two or simpler substances are known as decomposition reactions.
  • The decomposition reactions are carried out by applying heat, light or electricity.
  • Heat, light or electricity provide energy which breaks a compound into two or more simpler compounds.
  • The decomposition reaction is just the opposite of a combination reaction.

Examples of Decomposition Reaction

  • When calcium carbonate is heated, it decomposes to give calcium oxide and carbon dioxide:    decomposition of lime stone into quick lime and carbon dioxideDecomposition of ferrous sulphate into ferric oxide, sulphur dioxide and sulphur trioxide

decomposition of ferrous sulphate into ferric oxide, sulphur dioxide and sulphur trioxide

When a decomposition reaction is carried out by heating, it is called thermal decomposition.


  • The emission of brown fumes of nitrogen dioxide(NO2) after decomposition of lead nitrate.

decomposition of lead nitrate into lead oxide, oxygen and brown fumes of nitrogen dioxide

  • White silver chloride turns grey in sunlight.
  • This is due to the decomposition of silver chloride into silver and chlorine by light.

thermal decomposition of silver chloride into silver and chloride in presence of sunlight

  • Silver bromide turns grey in sunlight.

thermal decomposition of silver chloride into silver and chloride in presence of sunlight

NOTE –The above reactions of SILVER CHLORIDE and  SILVER BROMIDE  are used in black and white photography. 

decomposition of ferrous sulphate into ferric oxide

ACTIVITY 1.5 Decomposition Reaction of Ferrous sulphate

  • Take about 2 g ferrous sulphate crystals in a dry boiling tube  
  •  Heat the boiling tube over the flame of a burner
  • Observe the colour of the crystals after heating. 
  • Green coloured ferrous sulphate crystals decompose to form a reddish-brown powder after heating.

ACTIVITY 1.6  Decomposition of lead nitrate, Thermal decomposition reaction

Decomposition of lead nitrate produces reddish-brown gas
  •  Take about 2 g lead nitrate powder in a boiling tube.
  • Hold the boiling tube with a pair of tongs and heat it over a flame

  • What do you observe?
  • Emission of brown fumes. These fumes are of nitrogen dioxide (NO2) 

ACTIVITY 1.7  Electrolysis of water 

  • step-1: Take a plastic mug. Drill two holes at its base and fit rubber stoppers in these holes. Insert carbon electrodes in these rubber stoppers as shown in Fig.
  • step-2: Connect these electrodes to a 6-volt battery.    
  • step-3: Fill the mug with water such that the electrodes are immersed. Add a few drops of dilute sulfuric acid to the water.
  • step-4: Take two test tubes filled with water and invert them over the two carbon electrodes. Switch on the current and leave the
    apparatus undisturbed for some time.
  • Observation: formation of bubbles at both the electrodes.
  • These bubbles displace water in the test tubes.
  • Once the test tubes are filled with the respective gases, remove them carefully. Test these gases one by one by bringing a burning candle close to the mouth of the test tubes.
  • CAUTION! This step must be performed carefully by the teacher
  • What happens in each case?
  • 1) one of the gas helps to burn the flame
  • 2) the second one burns off the flame
  • Which gas is present in each test tube?
  • 1) the gas is hydrogen at the cathode and
  • 2) oxygen at anode                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     ACTIVITY 1.8

Take about 2 g silver chloride in a china dish.

  •  Place this china dish in sunlight for some time
  • OBSERVATION:  white silver chloride turns grey in sunlight


  • This is due to the decomposition of silver chloride into silver and chlorine by light.
  • 2AgCl(s) Sunlight  → 2Ag(s) + Cl2(g)     


1. (a) What is the colour of ferrous sulphate crystals? How does this colour change after heating?
   (b) Name the products formed on strongly heating ferrous sulphate crystals. What type of chemical reaction occurs in this change?

ANS: a)Ferrous sulphate crystals (FeSO4.7H2O) are light green in colour. When it is heated, white colour solid is formed.
(b) When anhydrous ferrous sulphate is further heated strongly, it decomposes to give ferrous oxide (Fe2O3) and oxides of sulphur. this is type of chemical reaction is known as decomposition reaction. 

2. What type of chemical reactions take place when :
(a) a magnesium wire is burnt in air?
(b) lime-stone is heated?
(c) silver bromide is exposed to sunlight?
(d) electricity is passed through water ?
(e) ammonia and hydrogen chloride are mixed ? 

ANS: (a) Combination (b) Decomposition (c) Decomposition (d) Decomposition (e) Combination    

 3.Why are decomposition reactions called the opposite of combination reactions? Write equations for these reactions.

 Answer. In a decomposition reaction, a compound is broken down into simpler compounds or elements, e.g.
Combination reaction is a reaction in which two or more elements or compounds combine to form a new compound, e.g.
Thus, decomposition and combination reactions are opposite to each other.

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