5.
When a non-volatile solute is dissolved in a pure solvent, the difference between the
freezing points of the solvent alone and the solution is
a. positive
b. negative
c. zero
d. none of the above

The correct answer is a. positive. When a non-volatile solute is dissolved in a pure solvent, the freezing point of the solution decreases compared to the pure solvent, making the difference (freezing point of solvent minus freezing point of solution) positive.​

Option Analysis

• a. positive: This holds true due to freezing point depression, a colligative property. The non-volatile solute lowers the solvent’s vapor pressure, requiring a lower temperature for the solid-liquid equilibrium where vapor pressures match.​

• b. negative: Incorrect, as the solution’s freezing point is always lower than the pure solvent’s, so ΔT_f = T_f(solvent) – T_f(solution) > 0.​

• c. zero: Incorrect, since solute addition disrupts solvent crystallization, causing depression proportional to molality (ΔT_f = K_f × m).​

• d. none of the above: Incorrect, as option a accurately describes the phenomenon.​

When a non-volatile solute dissolves in a pure solvent, freezing point depression occurs, making the difference between the freezing points of the solvent alone and the solution positive. This colligative property, key for exams like CSIR NET Life Sciences, follows Raoult’s law: the solute reduces vapor pressure, shifting equilibrium to lower temperatures.​

Why Positive Difference?

The formula ΔT_f = K_f × m × i shows depression (ΔT_f) is positive, where K_f is the cryoscopic constant, m is molality, and i is the van’t Hoff factor. Pure solvent freezes higher (e.g., water at 0°C), but solution freezes lower (e.g., -1.86°C for 1m NaCl). Fewer solvent molecules escape to solid phase due to dilution.​

Real-World Applications

• Antifreeze in cars prevents freezing.​

• Ice cream making uses solutes for creamier texture.

• Biological cryopreservation protects cells.​

This freezing point depression non-volatile solute concept clarifies why option a is correct in MCQs.​