Which of the following statement is incorrect?

The correct answer is Option (3) → Mixing two oppositely charged sols neutralizes their charges and stabilises the colloid.

A lyophobic sol is stable because all its particles carry the same charge and repel each other. When two oppositely charged sols are mixed in about equal proportions (for example positive \( \ce{Fe(OH)3} \) sol and negative \( \ce{As2S3} \) sol), the charges neutralise and the sols are coagulated.

• Option (1) Brownian movement stabilises sol. — A correct statement. The zig-zag motion has a stirring effect that does not let the particles settle.

• Option (2) Both emulsifier and peptising agents stabilises colloids but their actions are different. — A correct statement. An emulsifier forms an interfacial film round the droplets, while a peptising electrolyte gives charge to the particles of a precipitate.

• Option (3) Mixing two oppositely charged sols neutralizes their charges and stabilises the colloid. — The incorrect statement. Neutralising the charges removes the repulsion between the particles, so mixing coagulates the sols instead of stabilising them.

• Option (4) The stability of lyophobic colloidal particles is due to preferential adsorption of ions on their surface from the solution. — A correct statement. The adsorbed common ions give all particles the same charge, and the mutual repulsion keeps them apart.

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):
Coagulation or Precipitation – “… (ii) By mixing two oppositely charged sols: Oppositely charged sols when mixed in almost equal proportions, neutralise their charges and get partially or completely precipitated.”

Properties of Colloidal Solutions – “… 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.”