Which phenomenon occurs when an electric field is applied to a colloidal solution and electrophoresis is prevented?
(A) Reverse osmosis takes place
(B) Electroosmosis takes place
(C) Dispersion medium begins to move in an electric field
(D) Dispersion medium becomes stationary
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B), (C) only
In electrophoresis the charged colloidal particles move to the oppositely charged electrode. The dispersion medium carries an equal and opposite charge. If the particles are held back (for example by a suitable membrane), the dispersion medium itself begins to move in the electric field. This is electroosmosis.
(A) Reverse osmosis takes place — False. Reverse osmosis is solvent flow through a semipermeable membrane driven by applied pressure, not by an electric field.
(B) Electroosmosis takes place — True. The movement of the dispersion medium when the particles are held back is called electroosmosis.
(C) Dispersion medium begins to move in an electric field — True. This is what electroosmosis is.
(D) Dispersion medium becomes stationary — False. It is the particles that are kept stationary; the medium moves.
Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.
NCERT (Pre-rationalised edition, 2022-23):
Properties of Colloidal Solutions – “… When electrophoresis, i.e., movement of particles is prevented by some suitable means, it is observed that the dispersion medium begins to move in an electric field. This phenomenon is termed electroosmosis.”