Which of the following will not cause coagulation of a colloid?

The correct answer is Option (2) → Addition of peptizing agent

A lyophobic sol is stable because its particles carry the same charge and repel each other. Coagulation (precipitation) is the settling of colloidal particles; it happens whenever this charge is removed or neutralised, so that the particles come together and settle down.

Peptization is the opposite process. A freshly prepared precipitate is shaken with the dispersion medium in the presence of a small amount of an electrolyte (the peptizing agent). The precipitate adsorbs one of the ions of the electrolyte, its particles become charged, and they break up into colloidal size. For example, freshly precipitated ferric hydroxide gives a red sol with a little \( \ce{FeCl3} \):

    \( \displaystyle \ce{Fe(OH)3 (precipitate)} \xrightarrow{\text{small amount of }\ce{FeCl3}} \ce{Fe(OH)3/Fe^3+ (positively charged sol)} \)

So adding a peptizing agent makes a colloid; it does not coagulate one.

• Option (1) Electrophoresis — Causes coagulation. The charged particles move to the oppositely charged electrode, are discharged there and precipitate.

• Option (2) Addition of peptizing agent — Does not cause coagulation. The peptizing agent gives the particles of a precipitate a charge, so it forms a sol instead of destroying one.

• Option (3) Boiling — Causes coagulation. Boiling increases collisions with the molecules of the dispersion medium, which disturbs the adsorbed layer and reduces the charge on the particles.

• Option (4) Persistent dialysis — Causes coagulation. Prolonged dialysis removes almost all the traces of electrolyte that keep the sol stable, so the colloid becomes unstable and coagulates.

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

Peptization