When mercuric iodide is added to an aqueous solution of potassium iodide, the 

In a reaction with $\mathrm{KI}$, $\mathrm{HgI}$ forms ${\mathrm{K}}_2\left[{\mathrm{HgI}}_4\right]$. This causes the amount of ions to decline, which lowers the van't Hoff factor.
${\mathrm{\Delta T}}_{\mathrm{f}}=\mathrm{i}\times {\mathrm{k}}_{\mathrm{f}}\times \mathrm{m}$
As ′ i ′ falls, the freezing point experiences less depression, which causes the freezing point to rise.