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) a displacement reaction. C) a decomposition reaction. D) a double displacement reaction.

The reaction described, in which hydrogen sulfide gas is passed through a blue solution of copper sulfate resulting in the formation of a black precipitate of copper sulfide while sulfuric acid remains in the solution, is an example of a double displacement reaction.

In a double displacement reaction, the ions in two compounds switch places to form two new compounds. In this case:

CuSO4 (copper sulfate) + H2S (hydrogen sulfide) -> CuS (copper sulfide) + H2SO4 (sulfuric acid)

The copper ions (Cu²⁺) in copper sulfate combine with the sulfide ions (S²⁻) in hydrogen sulfide to form copper sulfide (CuS), and the sulfate ions (SO4²⁻) in copper sulfate combine with the hydrogen ions (H⁺) in hydrogen sulfide to form sulfuric acid (H2SO4). This exchange of ions is characteristic of a double displacement reaction.