Excess $\mathrm{KCN}$ is added to a mixture containing $\mathrm{Cu}{}^{2+}$ and $\mathrm{Cd}{}^{2+}$ and $\mathrm{H}{}_2\mathrm{S}$ is then passed. The precipitate obtained is

Copper is converted to a very stable cyanocuprate(I) complex, so $[\mathrm{Cu}{}^{+}]$ is too small for any sulphide to form. The cadmium cyanide complex is less stable, so enough $\mathrm{Cd}{}^{2+}$ is present to precipitate yellow $\mathrm{CdS}$ with $\mathrm{H}{}_2\mathrm{S}$.