$\begin{array}{ccc}\underset{\mathrm{Rotten} \mathrm{fish} \mathrm{like} \mathrm{smell}}{\mathrm{Gas} \mathrm{X}}+{\mathrm{CuSO}}_4& \to & \underset{\mathrm{Black} \mathrm{ppt}}{\mathrm{Compound} \mathrm{Y}}+\mathrm{Solution} \mathrm{Z}\\ \mathrm{Compound} \mathrm{Y}& \stackrel{\mathrm{Hydrolysis}}{\to }& \mathrm{Gas} \mathrm{X}\end{array}$

Black precipitate, Y is

Tricopper phosphide is a black precipitate (ppt) when phosphine gas reacts with copper sulfate.

$\underset{\mathrm{Rotten} \mathrm{fish} \mathrm{smell}}{2{\mathrm{PH}}_3 \left(\mathrm{X}\right)}+3{\mathrm{CuSO}}_4\to \underset{\mathrm{black} \mathrm{ppt}.\left(\mathrm{Y}\right)}{{\mathrm{Cu}}_3{\mathrm{P}}_2} +\underset{\left(\mathrm{Z}\right)}{ 3{\mathrm{H}}_2{\mathrm{SO}}_4}$

Hydrolysis of tricopper phosphide regains phosphine gas.

${\mathrm{Cu}}_3{\mathrm{P}}_2+6{\mathrm{H}}_2\mathrm{O}\to 3\mathrm{Cu}{\left(\mathrm{OH}\right)}_2+2{\mathrm{PH}}_3$