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Difficulty: HardTypes of Solutions: True Solutions, Suspensions, and Colloidal Systems

An aqueous mixture passes completely through standard filter paper without leaving any residue, but its particles are retained when passed through a parchment (semi-permeable) membrane. When illuminated by a narrow light beam in a dark room, the mixture scatters light, and under an ultramicroscope, its particles exhibit continuous zig-zag movement without settling under gravity. Which of the following correctly classifies this mixture and describes the characteristic behavior of its dispersed phase?

  1. It is a colloidal system, and its particles undergo electrophoresis toward an oppositely charged electrode when an electric field is applied.Answer
  2. B
    It is a true solution, and its particles will rapidly precipitate if a small amount of an electrolyte is added.
  3. C
    It is a suspension, and its particles undergo rapid gravitational sedimentation upon standing for a few minutes.
  4. D
    It is a colloidal system, but its particles lack any net surface charge and cannot be coagulated by adding ionic compounds.

Answer

The mixture is a colloidal system whose dispersed particles carry electric charges and migrate toward an oppositely charged electrode during electrophoresis.
The mixture exhibits all hallmark physical and optical characteristics of a colloidal system: particle size between 1 nm1\text{ nm} and 100 nm100\text{ nm} (retained by parchment membrane but passing filter paper), Tyndall effect (light scattering), Brownian motion, and surface electrical charge that allows directional movement in an electric field (electrophoresis).

Step-by-Step Solution

1
Analyze permeability and filtration behavior
Passing through standard filter paper rules out suspensions (>100 nm>100\text{ nm}). Retention by parchment (semi-permeable) membrane indicates a particle size between 1 nm1\text{ nm} and 100 nm100\text{ nm}, characteristic of colloidal systems.
True solution particles (<1 nm<1\text{ nm}) pass through both filter paper and semi-permeable membranes.
2
Analyze optical and kinetic behavior
Scattering light (Tyndall effect) and random continuous motion (Brownian motion) without settling confirm colloidal properties.
True solutions do not scatter light (no Tyndall effect), while suspensions settle out under gravity.
3
Determine electrical properties of the colloidal system
Colloidal particles preferentially adsorb ions from the medium, acquiring a net positive or negative surface charge. When subjected to an electric field, they migrate toward the electrode of opposite charge (electrophoresis).
Surface charges prevent colloidal particles from aggregating under normal conditions and make them responsive to electric potential.

Key Concept

Physical distinction and electrical properties of true solutions, colloidal systems, and suspensions
Estimated Time:1m 30s
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Topics

Types of Solutions: True Solutions, Suspensions, and Colloidal Systems