\(C{{u}^{2+}}\,+\,{{H}_{2}}S\,\to \,\underset{\text{black}}{\mathop{CuS}}\,\,+\,2{{H}^{+}}\)
$\mathrm{CrO}_2 \mathrm{Cl}_2+4 \mathrm{NaOH} \rightarrow \mathrm{A}+2 \mathrm{NaCl}+2 \mathrm{H}_2 \mathrm{O}$
$\mathrm{A}+2 \mathrm{HCl}+2 \mathrm{H}_2 \mathrm{O}_2 \rightarrow \mathrm{B}+3 \mathrm{H}_2 \mathrm{O}$