**Effect of Adding HgI2 to KI Solution on Freezing Point**

When HgI2 (mercury(II) iodide) is added to a solution of KI (potassium iodide), it has a significant effect on the freezing point of the solution. This effect can be explained by the concept of colligative properties, specifically the phenomenon of freezing point depression.

**Colligative Properties:**
Colligative properties are physical properties of a solution that depend on the number of solute particles present, rather than the nature of the solute itself. These properties include boiling point elevation, freezing point depression, vapor pressure lowering, and osmotic pressure.

**Freezing Point Depression:**
Freezing point depression is a colligative property that occurs when a solute is added to a solvent, causing the freezing point of the solvent to decrease. This is due to the fact that the presence of solute particles disrupts the crystal lattice formation of the solvent, making it more difficult for the solvent molecules to arrange themselves in an ordered manner required for freezing.

**Explanation:**
When HgI2 is added to a KI solution, it dissociates into Hg2+ and 2I- ions. The KI also dissociates into K+ and I- ions. The presence of these additional ions from the HgI2 and KI solutes increases the number of solute particles in the solution, leading to a greater freezing point depression.

The freezing point depression is directly proportional to the concentration of the solute particles in the solution. Therefore, as the concentration of HgI2 increases, the freezing point depression becomes more pronounced.

**Summary:**
In summary, when HgI2 is added to a KI solution, the freezing point of the solution decreases due to the phenomenon of freezing point depression. The additional solute particles from both HgI2 and KI contribute to a greater freezing point depression. This effect is a result of the disruption of the crystal lattice formation of the solvent by the solute particles.

Van't hoff factor decrease hence freezing point increase