A solution of a metal ion when treated with KI gives a red...

When a solution of a $Hg^{2 +}$ is treated with KI a red precipitate of $( Hg{I}_{2} )$ is obtained which dissolves in excess KI to give a colourless solution $( Hg{I}_{4}{)}^{2 -}$. When the solution of $Hg^{2 +}$ is treated with a solution of cobalt (II) thiocyanate a deep blue crystalline precipitate of $Hg ( SCN{)}_{2}$ is formed. The reactions are as follows
$Hg^{2 +} + 2I^{–} \to Hg{I}_{2}$
$\mathop{Hg{I}_{2}}_{( excess )} + 2I^{–} \to \mathop{[ Hg{I}_{4}{]}^{2 –}}_{Soluble}$
{\text{Hg}^{2 +} + \text{Co}{( \text{SCN} )}_{2} \to \mathop{\text{Hg}{( \text{SCN} )}_{2}}_{\text{Blue}\text{crystal}\text{line}\text{ppt}.}

Here other ion $Pb^{2 +}$ , $Hg^{2 +}$ and $Co^{2 +}$ do not give precipitates formed in these reactions.