Freezing point of the solution will increase. Mercuric Iodide will react with Potassium Iodide to form dipotassium tetraiodomercury. The reaction is given below : 2KI + HgI2 = K2[HgI4] This association will decrease the number of ions in the solution as a result of which Van't Hoff Factor (i) will decrease as ndecreases from 4 to 3. Now, Freezing point depression when a solute is added to a solvent is given by ΔT = i Kf m Clearly now i has decreased from when it was an aqueous KI solution to when HgI is added so ΔT is less now, hence less depression in freezing point which means a higher freezing point. Hence the freezing point of solution will increase. Note: Van't Hoff factor (i) =1+alpha(n-1) where alpha=dissociation degree and n is the no of ions dissociated Hope you understand the reasoning behind it now.