Question

Name a gas which gives a black ppt when passed through copper sulphate solution.

Answer 1

Hydrogen chloride (H₂S) gas gives black precipitate when passed through Copper sulphate (CuSO₄) solution.

The chemistry of the test is as follows:

$$\begin{gathered} {\mathrm{CuSO}}_4\left(\mathrm{aq}\right)+{\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)\to \mathrm{CuS}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \\ (\mathrm{Copper}\mathrm{sulphate})(\mathrm{Hydrogen}\mathrm{sulphide})(\mathrm{Copper}\mathrm{sulphide})(\mathrm{Sulphuric}\mathrm{acid}) \\ \left(\mathrm{black}\right) \end{gathered}$$

Blue colored Copper sulphate solution turns black due to the formation of black precipitate of Copper sulphide (CuS).

This reaction occurs as a result of double displacement reaction where Hydrogen (H) , being more reactive than Copper(Cu), displaces Copper to form Sulphuric acid and Copper sulphide.