Question

When hydrogen sulfide gas is passed through a blue solution of copper sulfate, a black precipitate of copper sulfide is obtained and the sulphuric acid so formed remains in the solution. The reaction is an example of a:
(A) Combination reaction.
(B) Displacement reaction.
(C) Decomposition reaction.
(D) Double displacement reaction.

Answer 1

Here, two compounds react to form two new compounds by the exchange of their elements. The compounds hydrogen sulfide and copper sulfate break into their respective cations and anions and then exchange their ions to form two new compounds as copper sulfide and sulphuric acid.

Complete step by step answer:
Let us first write the given changes in the form of a chemical equation. Since hydrogen sulfide is passed through the copper sulfate solution, here we have hydrogen sulfide and copper sulfate as the reactants. The chemical formula for hydrogen sulfide is $H_2S$, and that of copper sulfate is $CuSO_4$. The compounds formed after the reaction are a black precipitate of copper sulfide and sulphuric acid. So, we have copper sulfide and sulphuric acid as the products. The chemical formula for copper sulfide is $CuS$, and that of sulphuric acid is $H_2SO_4$. The overall reaction can be represented as under:

$H_2S + CuSO_4 \rightarrow CuS + H_2SO_4$

In the above reaction, we find that the reactants exchange their ions while forming the products. The sulfide anion in $H_2S$ reacts with the copper cation in $CuSO_4$, while the sulfate anion in $CuSO_4$ reacts with the hydrogen cation in $H_S$. Therefore, it can be stated that an exchange of ions takes place in the reaction. The anion of one reactant displaces the anion of the other reactant and takes its place. Or, it can also be said that the cation of one reactant displaces the cation of the other reactant and takes its place. Since the displacement is taking place in both the reactants, it is an example of a double displacement reaction.
Therefore, the passing of hydrogen sulfide gas through a blue solution of copper sulfate is an example of a double displacement reaction.

So, the correct answer is Option D.

Note: 1.Combination reaction – A reaction that takes place by the addition or combination of two reactants to form a single product is called a combination reaction. It is generally of the form $A + B \rightarrow AB$.
For example - $2H_2 + O_2 \rightarrow 2H_2O$

2.Displacement reaction – A displacement reaction takes place between an element and a compound, in which the element displaces an element from the compound and takes its place. It can be generally represented as $A + BC \rightarrow AC + B$.
For example - $2Na + 2HCl \rightarrow 2NaCl + H_2$

3.Decomposition reaction – A reaction in which a compound breaks into its constituent elements or simpler compounds is called a decomposition reaction. Its general chemical representation is as follows:
$AB \rightarrow A + B$
For example - $CaCO_3 \rightarrow CaO + CO_2$

4.Double displacement reaction – A reaction in which two compounds react and exchange their elements to form two new compounds is called a double displacement reaction. In a simple form, it can be represented as:
$AB + CD \rightarrow AD + BC$
For example - $H_2S + CuSO_4 \rightarrow CuS + H_2SO_4$