Question

Which of the following is not a colligative property?

• A. Osmotic pressure
• B. Depression of freezing point
• C. Elevation of boiling point
• D. Refractive index

Hint:

Colligative properties are independent of solute type and depend on solute particle concentration.

Answer 1

The Correct Option is D

Step 1: Define colligative properties
Colligative properties include:
- Osmotic pressure: Pressure required to prevent osmosis, proportional to solute concentration.
- Depression of freezing point: Lowering of freezing point, proportional to molality.
- Elevation of boiling point: Increase in boiling point, proportional to molality.
- Vapor pressure lowering: Decrease in vapor pressure due to solute particles.

Step 2: Analyze each option
- Osmotic pressure: Depends on the number of solute particles (van’t Hoff factor times concentration). Colligative.
- Depression of freezing point: Depends on molality and the number of solute particles. Colligative.
- Elevation of boiling point: Depends on molality and solute particle concentration. Colligative.
- Refractive index: Measures how light bends in a medium. It depends on the chemical nature of both solute and solvent, not just the number of particles. Not colligative.

Step 3: Conclusion
Refractive index is not a colligative property.