Question: Assertion (A) : When NaCl is added to water, a depression in freezing point is observed. Reason (R) : The lowering of vapour pressure of a solution causes depression in the freezing point.

Step 1 (Governing Principle): [0.5 Mark]

According to the colligative properties of solutions, the addition of a non-volatile solute (like NaCl) to a solvent (water) results in a decrease in the vapour pressure of the solution. The freezing point of a substance is defined as the temperature at which the vapour pressure of the substance in its liquid phase is equal to its vapour pressure in the solid phase.

Step 2 (Logical Deduction): [0.25 Mark]

When the vapour pressure of the liquid solution is lowered due to the presence of the solute, it intersects the vapour pressure curve of the solid solvent at a lower temperature. This relationship is mathematically expressed by the equation for depression in freezing point:

Where:

• $\Delta T_f$ = Depression in freezing point
• $K_f$ = Molal freezing point depression constant
• $m$ = Molality of the solution
• $i$ = van't Hoff factor (for NaCl, $i \approx 2$)

Step 3 (Final Concluded Answer): [0.25 Mark]

Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation for Assertion (A). The lowering of vapour pressure directly shifts the freezing point to a lower value.

💡 EL10 Concept Insight: Think of it like this: adding salt makes it harder for water molecules to "lock" into a solid ice crystal structure because the salt particles get in the way, so you have to make the water much colder than usual to force it to freeze.