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.
The general equation that describes a decomposition reaction is:

Decomposition reactions can be classified into three types:

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

1. Thermal decomposition reaction (Thermolysis)
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.
Example: Lead nitrate undergoes decomposition in the presence of heat, giving lead oxide, nitrogen dioxide and oxygen.

2. Electrolytic decomposition reaction (Electrolysis)
Electrolytic decomposition may result when an electric current is passed through an aqueous solution of a compound. A good example is the electrolysis of water.
Example: Electrolysis of water is the decomposition of water into hydrogen and oxygen due to the passage of electric current through it.

3. Photo decomposition reaction (Photolysis)
Photo decomposition is a chemical reaction in which a substance is broken down into simple substances by exposure to light (photons).
Examples: Decomposition of silver chloride: The sunlight causes the decomposition of silver chloride into silver and chlorine.

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