JEE Advanced 2021 Paper 1 Q13 Chemistry Physical Chemistry Solutions & Colligative Properties Medium

JEE Advanced 2021 Paper 1 · Q13 · Solutions & Colligative Properties

The correct statement(s) related to colloids is(are):

  1. A. The process of precipitating colloidal sol by an electrolyte is called peptization.
  2. B. Colloidal solution freezes at higher temperature than the true solution at the same concentration.
  3. C. Surfactants form micelle above critical micelle concentration (CMC). CMC depends on temperature.
  4. D. Micelles are macromolecular colloids.
JEE Advanced multi-correct — pick every correct option, then check.
Reveal answer + step-by-step solution

Correct answer:B, C

Solution

(A) Wrong. Precipitating a colloid by adding an electrolyte is called coagulation (or flocculation). Peptization is the opposite process — converting a freshly precipitated solid into a colloidal sol using a small quantity of electrolyte.

(B) Correct. Colloidal particles have very large effective molar mass, so for the same mass concentration the mole fraction of colloidal solute is much smaller than that of a true solute. Hence the depression in freezing point ($\Delta T_f \propto$ molality) is smaller for the sol, and its freezing point is higher (closer to that of the pure solvent) than that of the true solution.

(C) Correct. Surfactant molecules begin to aggregate into micelles only above a certain concentration, the critical micelle concentration (CMC); CMC is temperature-dependent (and it also depends on the surfactant and on additives such as electrolytes).

(D) Wrong. Micelles are associated (associated/colloidal) colloids, not macromolecular colloids. Macromolecular colloids are large single molecules such as proteins, polymers and starches.

Hence (B) and (C) are correct.

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