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A reaction in which a single reactant breaks down into more than one product is called Decomposition Reaction.


Example 1

Decomposition of Ferrous Sulphate

When we heat green coloured Ferrous Sulphate crystals in a boiling tube,

We observe that the green colour of Ferrous Sulphate changes.

We can also observe smell on burning of sulphur

This is because on heating,

Ferrous Sulphate breaks down into Ferric Oxide (solid) , Sulphur dioxide (gas) and Sulphur Trioxide(gas)

2FeSO 4 (s)               →               Fe2O 3 (s) + SO 2 (g) ​+ SO 3 (g)



Here, one reactant breaks down into more than one product . Hence it is a decomposition reaction.


Note - The chemical formula of ferrous sulphate crystals is FeSO 4 .7H 2 O .

           The water molecules are called water of crystallization


Example 2

Decomposition of Calcium Carbonate (CaCO 3

On heating of Calcium Carbonate (limestone),

It decomposes into Calcium Oxide (Quick Lime) + Carbon Dioxide


CaCO 3 (s)         →          CaO(s) + CO 2 (g) 

                                            (Limestone)    Heat       (Quick lime)



Example 3

Decomposition of Lead Nitrate 

On heating of Lead Nitrate (Pb(NO 3 ) 2 ) powder in boiling tube,

It decomposes to produce Lead Oxide (PbO), Nitrogen Dioxide (NO 2 ) and Oxygen (O 2 )

We observe brown fumes of Nitrogen dioxide (NO 2 )

2Pb(NO 3 ) 2 (s)     →      2PbO(s) + 4NO 2 (g) + O 2 (g)



Example 4

Silver Chloride turns grey in sunlight

It is because, the heat of sunlight decomposes Silver Chloride into Silver and Chlorine gas

The reaction is called Photolysis (Photo means light and lysis means breakdown)


2AgCl(s)      →      2Ag(s) + Cl 2 (g)



Silver Bromide also behaves the same way in the presence of sunlight,


2AgBr(s)      →      2Ag(s) + Br 2 (g)


This reaction is used for black and white photography .


Silver Chloride and Bromide are are light-sensitive compounds and thus stored in dark bottles

What is thermal decomposition reaction?

Decomposition carried out by heating is called thermal decomposition reaction.


Note: The 4 examples above are decomposition due to heating. Hence they are all examples of Thermal Decomposition Reaction .


Example 5

Electrolysis of water

The breaking down of H 2 O into H 2 and O 2 by passing electricity is known as electrolysis of water.



Endothermic Reaction

Reactions in which energy is absorbed/utilized are called endothermic reactions.


For example,


In decomposition of ferrous sulphate crystals, heat was provided to carry out the reaction.

Hence, it is an example of endothermic reaction


2FeSO 4 (s)               →               Fe2O 3 (s) + SO 2 (g) ​+ SO 3 (g)




Are decomposition reactions endothermic or exothermic?

Reactions in which energy is released is called exothermic reaction

Reactions in which energy is absorbed is called endothermic reaction

In case of decomposition reaction , heat energy is absorbed

Hence it is an example of endothermic reaction


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