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.
Which of the following is not a cause of the charge on sol particles?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → Scattering of light by colloidal particles.
The charge on colloidal particles arises in three ways: electron capture during electro-dispersion of metals, preferential adsorption of ions from the solution (the most accepted reason), and the formation of an electrical double layer. Scattering of light is the Tyndall effect, an optical property that comes from the size of the particles; it produces no charge.
Option (1) Electron capture by sol particles. — A cause of the charge on sol particles. Metal particles formed in Bredig’s arc capture electrons and become charged.
Option (2) Scattering of light by colloidal particles. — Not a cause of the charge on sol particles. It is the Tyndall effect, an optical effect of particle size.
Option (3) Preferential adsorption of ion from solution. — A cause of the charge on sol particles. For example, AgI particles in excess KI adsorb \( \ce{I-} \) and become negative.
Option (4) Due to formation of electrical double layer. — A cause of the charge on sol particles. The fixed layer of adsorbed ions and the diffused layer of counter ions together make up the double layer.
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):
Charge on Colloidal Particles – “… The charge on the sol particles is due to one or more reasons, viz., due to electron capture by sol particles during electrodispersion of metals, due to preferential adsorption of ions from solution and/or due to formulation of electrical double layer.”
Tyndall Effect – “… The Tyndall effect is due to the fact that colloidal particles scatter light in all directions in space. This scattering of light illuminates the path of beam in the colloidal dispersion.”