Decomposition reactions involve the breakdown of a compound into its elements or into simpler. In other words, decomposition reactions result in the breakdown of a complex substance into two or more simpler substances. A decomposition reaction is the opposite of a combination reaction.
Many decomposition reactions occur when electricity or heat is applied to a substance. For example, the decomposition of water occurs when an electric current is passed through the water. The products are hydrogen and oxygen. This decomposition reaction is the opposite of the synthesis reaction in which water is formed.
Thermal decomposition is heating the substance with a Bunsen flame and is a simple method used in a lab. Not all the substance will decompose under these conditions.