Question

Give one example of each of the following double displacement reaction in which

(ii) a black precipitate is formed.

Answer 1

1. Double displacement reaction
The reactions in which there occurs a replacement of the anions and cations among two compounds (reactants). This gives rise to another two new compounds (products) in which ions are exchanged.

2. Example of a reaction where a black precipitate is formed

An example of a reaction producing black precipitate is shown below.

$$\begin{gathered} {\mathrm{H}}_2\mathrm{S}\left(\mathrm{g}\right)+{\mathrm{CuSO}}_4\left(\mathrm{aq}\right)\to \mathrm{CuS}\left(\mathrm{s}\right)+{\mathrm{H}}_2{\mathrm{SO}}_4\left(\mathrm{aq}\right) \\ \mathrm{Hydrogen}\mathrm{Copper}\mathrm{Copper}\mathrm{Sulphuric}\mathrm{acid} \\ \mathrm{sulphide}\mathrm{sulphate}\mathrm{sulphide} \end{gathered}$$
When a reaction occurs between Hydrogen sulphide $\left({\mathrm{H}}_2\mathrm{S}\right)$ and Copper sulphate $\left({\mathrm{CuSO}}_4\right)$, a black precipitate of Copper sulphide $\left(\mathrm{CuS}\right)$ is formed.