Answer the question on the basis of passage given below:
A colloid is a heterogeneous system in which a substance is dispersed (dispersed phase) as fine particles in another substance called dispersion medium. Their particle size is intermediate between that of pure solutions (<1 nm) and suspension (>1000 nm).
They are known for interesting properties such as enormous surface area, Tyndall effect, Brownian Motion etc. They are widely used in industries and day to day life. Some prominent uses of colloids are those in smoke precipitators, purification of water, medicines, cleansers etc.
Stability of a colloid is intermediate between that of a solution and suspension. This stability can be attributed to ____.
A. Presence of charge on the particles of dispersed phase.
B. Prolonged mechanical stirring.
C. Solvation by the dispersion medium.
D. Brownian Motion.
E. Dialysis
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → A, C and D only
A sol is stable when its particles are kept from coming together and settling. Like charges, a layer of solvent round each particle and constant random motion all do this.
A. Presence of charge on the particles of dispersed phase — Correct. All particles of a sol carry the same charge, so they repel one another and cannot come together to settle.
B. Prolonged mechanical stirring — Incorrect. Stirring is not a property of the sol; a sol stays stable on standing without any stirring.
C. Solvation by the dispersion medium — Correct. A layer of the medium held round each particle (as in lyophilic sols) keeps the particles apart; charge and solvation are the two factors that stabilise lyophilic sols.
D. Brownian motion — Correct. The constant zig-zag motion of the particles has a stirring effect that does not let them settle.
E. Dialysis — Incorrect. Dialysis is a method of purifying a sol; prolonged dialysis removes the stabilising electrolyte and makes the sol coagulate.
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):
Brownian Movement – “… The Brownian movement has been explained to be due to the unbalanced bombardment of the particles by the molecules of the dispersion medium. The Brownian movement has a stirring effect which does not permit the particles to settle and thus, is responsible for the stability of sols.”
Coagulation of Lyophilic Sols – “There are two factors which are responsible for the stability of lyophilic sols. These factors are the charge and solvation of the colloidal particles.”