Colligative effects: When a nonvolatile solute is dissolved in a solvent, which property (or properties) of the solution decreases compared with the pure solvent?

Difficulty: Easy

Correct Answer: both (b) and (c)

Explanation:


Introduction:
Adding a nonvolatile solute to a solvent changes several properties known as colligative properties. Understanding the direction of change for boiling point, freezing point, and vapor pressure is important in designing evaporation and crystallization operations and in interpreting laboratory measurements.


Given Data / Assumptions:

  • Solute is nonvolatile and does not dissociate unusually.
  • Ideal or near-ideal solution behavior is assumed for trends.
  • Comparison is with the pure solvent at the same pressure.


Concept / Approach:
Adding solute lowers the solvent’s escaping tendency, thereby reducing the solution’s vapor pressure (Raoult’s law). Lower vapor pressure causes boiling point elevation (so boiling point increases, not decreases) and freezing point depression (freezing point decreases). Thus, among the listed properties, freezing point and vapor pressure decrease upon solute addition.


Step-by-Step Solution:
Vapor pressure: decreases due to reduced mole fraction of solvent.Boiling point: increases because a higher temperature is needed for the solution’s vapor pressure to reach ambient pressure.Freezing point: decreases due to disruption of solid–liquid equilibrium.Select properties that decrease: freezing point and vapor pressure.


Verification / Alternative check:
Standard equations: ΔTb = Kb * m > 0 (elevation), ΔTf = Kf * m > 0 (depression magnitude), and p_solution = x_solvent * p^0_solvent < p^0_solvent.


Why Other Options Are Wrong:

  • Boiling point: It increases.
  • Density: May change either way depending on solute; not a colligative certainty.


Common Pitfalls:
Confusing the directions of change—remember: vapor pressure ↓, boiling point ↑, freezing point ↓ for nonvolatile solutes.


Final Answer:
both (b) and (c)

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