Question

Which one of the following is an example of a decomposition reaction?
(A) $AgCl \to 2Ag + C{l_2}$
(B) $C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O$
(C) $CaC{O_3} + heat \to CaO + C{O_2}$
(D) $Fe + CuS{O_4} \to FeS{O_4} + Cu$

Answer 1

To identify the decomposition reactions, carefully observe the reactants and products involved in all the given reactions. Whenever a single molecule is getting cleaved into its fragments (i.e. bonds are being broken), then it can be called a decomposition reaction.

Complete answer:
A chemical reaction is an irreversible process in which the reacting species get transformed into completely new substances with different chemical and physical properties.
Chemical reactions can be classified on the basis of the type of reactants or the type of products obtained.
A decomposition reaction is said to occur when a single chemical species gets broken down into fragments (giving more than one product) by the action of heat or light on it.
(A) - $AgCl \to 2Ag + C{l_2}$
This is a decomposition reaction because the ionic solid i.e. silver chloride is breaking down into solid silver metal and chlorine gas. A single reactant is giving two products.
(B) - $C{H_4} + 2{O_2} \to C{O_2} + 2{H_2}O$
This reaction is a combustion reaction as a hydrocarbon (methane in this gas) is reacting with sufficient amounts of oxygen to produce carbon dioxide and water.
(C) - $CaC{O_3} + heat \to CaO + C{O_2}$
This is a decomposition reaction as it involves the breakdown of an alkaline earth metal (calcium in this case) carbonate into its oxide and carbon dioxide gas. A single reactant is giving two products.
(D) - $Fe + CuS{O_4} \to FeS{O_4} + Cu$
This is a displacement reaction as the more reactive metal (iron) is displacing copper (a less reactive metal) from its salt solution. The sulphate ions get segregated from copper ions and forma new ionic bond with ferrous ions.
Hence, the correct options are (a) and (c) as both of them are examples of decomposition reaction.

Note:
A chemical reaction may satisfy more than one condition of classification and can be identified as two different types of reaction. But the dissociation of any ionic compound into its constituent ions in an aqueous medium cannot be identified as a decomposition reaction.