Question

$HgC{l}_2+ 2I^{-}\underset{}{\overset{}{\to }}–Hg{I}_2+2C{\mathcal{l}}^{-}$
                                      $$\begin{gathered} \overline{)+ 2{\mathrm{I}}^{+}} \\ [{\mathrm{HgI}}_4{]}^{-2} \end{gathered}$$
Soluble complex
$I_{2 +} I^{-}\stackrel{}{\to }{I}_3^{-}$
      water soluble