A solution contains Cu^2+ and Cd^2+. Excess KCN is added, and then H2S is passed. The result is

yellow CdS precipitates; copper stays in solution as a cyano complex

no precipitate forms, as both cyano complexes are inert to H2S

black CuS precipitates; cadmium stays in solution

yellow CdS precipitates; copper stays in solution as a cyano complex

JEE Main levelSingle correct optionDifficulty 2 of 3qc024

A solution contains $Cu X^{2 +}$ and $Cd X^{2 +}$ . Excess $KCN$ is added, and then $H X_{2} S$ is passed. The result is

Cu X^{2 +}

is reduced by

CN X^{-}

to copper(I), which forms the very stable

[Cu (CN) X_{4}] X^{3 -}

with negligible free

Cu X^{+}

. The

[Cd (CN) X_{4}] X^{2 -}

complex is less stable and releases enough

Cd X^{2 +}

for the very insoluble yellow

CdS

to form.