Write the balanced chemical equations for the following reactions.
Copper sulphate on treatment with potassium iodide precipitates cuprous iodide $\left( {CuI} \right)$, liberates iodide gas and also forms potassium sulphate.
A: $2CuS{O_4} + 4KI\xrightarrow[{}]{}C{u_2}{I_2} + 2{K_2}S{O_4} + 2{I_2}$
B: $2CuS{O_4} + 2KI\xrightarrow[{}]{}2CuI + {I_2} + 2{K_2}S{O_4}$
C: $CuS{O_4} + 4KI\xrightarrow[{}]{}CuI + 2{I_2} + 2{K_2}S{O_4}$
D: $2CuS{O_4} + 4KI\xrightarrow[{}]{}2CuI + {I_2} + 2{K_2}S{O_4}$

Hint: A chemical reaction is said to be balanced when it has equal number of atoms of each element on both reactant side and product side. If a chemical reaction does not have an equal number of atoms in the reactant and product then the chemical reaction is not balanced.

Complete step by step answer:
In this question we have to write a balanced chemical equation when Copper sulphate reacts with potassium iodide and precipitates cuprous iodide $\left( {CuI} \right)$, liberates iodide gas and potassium sulphate. Chemical formula of copper sulphate is $CuS{O_4}$, potassium iodide is $KI$, cuptous iodide is $CuI$, iodide gas is ${I_2}$ and potassium sulphate is ${K_2}S{O_4}$. According to the given information reaction can be written as:
$CuS{O_4} + KI\xrightarrow[{}]{}CuI + {I_2} + {K_2}S{O_4}$
But this reaction is not balanced as the number of atoms of each element in the reactant is not equal to the number of atoms in the product. Number of atoms of iodine on the reactant side is one while the number of atoms of iodine in the product is three. To balance the number of atoms of iodine, there should be two molecules of $CuI$ and four molecules of $KI$. Now the reaction is:
$CuS{O_4} + 4KI\xrightarrow[{}]{}2CuI + {I_2} + {K_2}S{O_4}$
In this reaction the number of atoms of iodine is balanced but atoms of copper and potassium are not balanced. To balance copper and potassium atoms there should be two molecules of $CuS{O_4}$ and two molecules of ${K_2}S{O_4}$. Now, the reaction is:
$2CuS{O_4} + 4KI\xrightarrow[{}]{}2CuI + {I_2} + 2{K_2}S{O_4}$
Now, this reaction is completely balanced.
Hence, the correct answer is option D.

Note:
According to law of conservation of mass can neither be created nor be destroyed. If these atoms will not be balanced then mass will not be balanced because mass of reactant will not be equal to mass of product in this case and law of conservation of mass will be violated. So, the number of atoms of each element must be balanced on the reactant and product side.