Question

When hydrogen sulphide gas is passed through a blue solution of copper sulphate, a black precipitate of copper sulphide 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

The correct option is D

Double displacement reaction

Explanation for correct option:

(d) Double displacement reaction

${\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to \mathrm{CuS}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{l}\right)$

• When hydrogen sulphide gas is passed through a blue copper sulphate solution, a black deposit of copper sulphide is generated, and the subsequent sulphuric acid stays in the solution.
• This is an illustration of a Double Displacement reaction.
• A double displacement reaction occurs when two compounds react and the positive ions (cation) and negative ions (anion) of the two reactants transfer positions, resulting in the formation of two new compounds or products.

Explanation for incorrect options

(a) combination reaction

• A combination reaction occurs when two reactants unite to generate one product.
• Here no two reactants unite to generate one product.
• So it is not a combination reaction

(b) displacement reaction

• A displacement reaction occurs when an atom or a group of atoms in a molecule is replaced by another atom.
• In single displacement, either cation displaces cation or anion displaces anion, but both cannot occur together.
• So it is not a displacement reaction

(c ) decomposition reaction

• A decomposition reaction is a chemical reaction that occurs when one reactant disintegrates into two or more products.
• Here no single reactant breaks down into two or more products
• So it is not a decomposition reaction.

Hence the correct option is (d) Double displacement reaction