Decomposition Reaction: Definition, Types, Uses and Examples

A decompositon reaction is a reaction that takes place when one reactant breaks down into two or more simpler substances. The decomposition reaction can be represented by the general equation: AB → A + B. Most of the decomposition reactions require an input of energy in the form of heat, light or electricity. 

Read Also: Class 10 Chemical Reactions

The topic Decomposition Reaction is covered in the unit 1 chapter 1 of NCERT Class 10 Science. As per the latest updates, in the revised syllabus of CBSE, no topics have been excluded from the above mentioned chapter. The entire unit 1 i.e., Chemical Substances- Nature and Behaviour will carry a weightage of 26 marks in CBSE Class 10 Board Examination. 

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Define Decomposition Reaction

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The reaction in which a single substance decomposes to produce more than one substance is known as the decomposition reaction. The key characteristic of a decomposition reaction is that one substance acts as a reactant forming more than one product.

For example, in the decomposition of sodium hydrogen carbonate three products are formed - sodium carbonate, carbon dioxide, and water.

2 NaHCO₃(s) → Na₂CO₃(s) + CO₂(g) + H₂O(?)

Explanation

Heat

2FeSO₄(s) → Fe₂O₃(s) + SO₂(g) + SO₃(g)

In this reaction, a single reactant breaks down into simpler products. This is a decomposition reaction. During heating of Ferrous Sulphate, crystals (FeSO₄, 7H₂O) lose water, and the color of the crystals changes. Then it decomposes to produce ferric oxide (Fe2O₃), sulfur dioxide (SO₂), and sulfur trioxide (SO₃). Ferric oxide is in solid-state, while SO₂, and SO₃ are in the gaseous state. Few examples for decomposition reactions are as follows:

• Decomposition of calcium carbonate on heating produces calcium oxide and carbon dioxide. This decomposition reaction is used in various industries. Calcium oxide is known as lime or quicklime. One of its uses is to manufacture cement. The decomposition reaction, which is carried out by heating, is known as thermal decomposition.

Heat

CaCO₃(s) → CaO(s) + CO₂(g)

• On heating nitrate salt emission of brown fumes is observed. These brown fumes are of nitrogen dioxide (NO₂). 

Heat

2Pb(NO₃)₂(s) → 2PbO(s)+ 4NO₂(g) + O₂(g)

• White silver chloride turns grey in sunlight which is an example of a decomposition reaction. In the presence of light silver chloride breaks into silver and chlorine.

Sunlight

2AgCl(s) → 2Ag(s) + Cl₂(g)

From the examples above, we can see that for a decomposition reaction to take place, some form of energy is required as an external source of the reaction. As energy is required in the case of the decomposition reaction, it may be exothermic or endothermic but the former type is more common in day-to-day life.

Other examples of decomposition reaction are:

1. Electrolysis of water which breaks it into oxygen and hydrogen:

2H₂O(I) → 2H₂ + O₂

2. Carbonates are decomposed when heated:

H₂CO₃ → H₂O + CO₂

3. Decomposition of Metal chlorates in presence of heat:

2MClO₃ → 2MCl+ 3O₂

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Types of Decomposition Reaction

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There are three types of decomposition reactions, which are discussed as under:

i) Thermal decomposition reaction

A thermal decomposition reaction is considered as a decomposition reaction which is activated by thermal energy. In this type of decomposition reaction, energy is supplied to the reactants in the form of heat energy. As energy is required to break the chemical bond of the reactants, these are endothermic. A common example of a thermal decomposition reaction is the decomposition of carbonate salt.

CaCO₃ → CaO + CO₂

On heating, calcium carbonate decomposes to produce calcium oxide and carbon dioxide. Quicklime is produced by this process which is an important substance for many industries.

Read more: Balanced Chemical Equations

ii) Electrolytic decomposition reaction

When the reactant is broken down into multiple products by receiving energy from electricity, it is known as an Electrolytic decomposition reaction. Electrolysis of water is a simple example for the same:

2H₂O → 2H₂ + O₂

iii) Photochemical decomposition

When the reactant is broken down into several products using the energy from the light source is termed photochemical decomposition. Breaking down of silver chloride in presence of sunlight is an example of photochemical decomposition:

 Sunlight

2AgCl(s) → 2Ag(s) + Cl₂(g)

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Uses of Decomposition Reactions

• Manufacture of cement from calcium carbonate.
• Extraction of metals from their ores using electrolytic decomposition.
• Breaking up of starch and other sugars in the body for proper digestion.
• Thermite welding process.

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Double Decomposition reactions

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A double decomposition reaction is a type of decomposition reaction involving 2 reactants that exchange their cations and anions to form 2 new compounds.

General form of double decomposition reaction is:

AB + CD→CB + AD

Example:

1. i) AgNO₃(aq)+NaCl(aq)→AgCl(s)+NaNO₃(aq)
2. ii) HCl(aq)+NaOH(aq)→NaCl(aq)+HOH(l)

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