Alum helps in purifying water by:
A. Coagulation of mud particles
B. Sedimentation of mud particles
C. Dialysis
D. forming true solution with water
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → A & B only
Mud particles in water are negatively charged colloidal particles. Alum (\( \ce{K2SO4} \)·\( \ce{Al2(SO4)3} \)·\( \ce{24H2O} \)) supplies \( \ce{Al^{3+}} \) ions, which have a high coagulating power (Hardy-Schulze rule); they neutralise the charge on the particles.
A. Coagulation of mud particles — True. The neutralised particles come together into larger aggregates.
B. Sedimentation of mud particles — True. The aggregates become heavy and settle down, leaving clear water.
C. Dialysis — False. Dialysis removes dissolved electrolytes from a sol through a membrane; alum plays no part in it.
D. forming true solution with water — False. Alum does not dissolve the mud; it makes the mud settle out.
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):
Applications of Colloids – …
