Select the correct option based on statements below:
| 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. |
| 1. | Both (A) and (R) are true and (R) is the correct explanation of (A). |
| 2. | Both (A) and (R) are true but (R) is not the correct explanation of (A). |
| 3. | (A) is true but (R) is false. |
| 4. | (A) is false but (R) is true. |
Hint: When a non-volatile solute like NaCl is added to water, then depression in freezing point occur.
When a non-volatile solute like NaCl is added to water, then depression in freezing point occur. The non-volatile solute decreases the number of water molecules at the surface of a liquid because NaCl molecules that might occupy the surface.
This means that the solution now exerts a lower vapour pressure as compared to that of pure water.
The solution will freeze when its vapour pressure equals the vapour pressure of the pure solvent.
According to Raoult’s law, when a non-volatile solid is added to the solvent its vapour pressure decreases and now it would become equal to that of solid solvent at a lower temperature.
Thus, the freezing point of the solvent decreases.
Hence, Assertion and reason both are correct statements and reason is correct explanation for assertion.
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