Answer the question on the basis of passage given below:
Adsorption is the phenomenon of attracting and retaining the molecules of a substance on the surface of a solid resulting into a higher concentration on the surface than in the bulk. The variation in the amount of gas adsorbed by the adsorbent with pressure at constant temperature is expressed as isotherm. A catalyst is a substance, when adsorbed enhances the rate of a chemical reaction without itself getting used up in the reaction. Depending on the type of the particles of the dispersed phase, colloids are classified as multimolecular, macromolecular and associated colloids and soap is such an example of associated colloids. Colloids show various properties such as Tyndall effect, colour, Brownian movement and charge development. There are positively charged sols and negatively charged sols. Due to this, electrophoresis and electroosmosis are shown by colloids.
The charge on the sol particles cannot be produced by-
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → presence of excess quantity of the dispersion medium.
Colloidal particles carry an electric charge of the same sign, and the mutual repulsion of these charges keeps them from coming together. The charge arises in three ways: electron capture by the particles during electro-dispersion of metals (Bredig’s arc), preferential adsorption of ions from the solution (the most accepted reason), and the formation of an electrical double layer of opposite charges at the particle surface.
The amount of dispersion medium has nothing to do with the charge. Diluting a sol with more medium does not create or remove charge on the particles, so option (4) is the one that cannot produce it.
Option (1) electron capture by sol particles during electro-dispersion of metals. — A cause of charge on sol particles. In Bredig’s arc method the metal particles capture electrons.
Option (2) preferential adsorption of ions from solution. — A cause of charge on sol particles. \( \ce{Fe(OH)3} \) from \( \ce{FeCl3} \) adsorbs \( \ce{Fe^{3+}} \) ions and becomes positive.
Option (3) formation of electrical double layer. — A cause of charge on sol particles. The fixed and diffuse layers of ions around a particle give it its charge.
Option (4) presence of excess quantity of the dispersion medium. — Cannot produce charge. Diluting a sol neither creates nor removes charge on the particles.
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.”