Answer the question on basis of passage given below:
Lyophobic sols like those of metals (Au, Ag, etc) are unstable and are easily precipitated by addition of electrolytes. However, it is observed that the addition of certain lyophilic colloids like gums, soaps, gelatine, etc. to lyophobic colloids (like a metal sol) render lyophobic colloids difficult to coagulate by the addition of electrolytes. The process is known as 'protection' and the lyophilic colloids are termed as Protective colloids. It is believed that the protective action of the lyophilic colloids is due to the covering up of the particles of the lyophobic colloid by those of the lyophilic colloid. Evidently, smaller the gold number of a protective colloid, the greater is its protective action.
In which of the following sols, the colloidal particles will move towards the cathode during electrophoresis?
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → \(Fe(OH)_3\) sol
In electrophoresis the colloidal particles move under an applied electric potential towards the electrode of opposite charge. Positively charged particles move towards the cathode (negative electrode) and negatively charged particles move towards the anode.
Hydrated metallic oxides such as \( \ce{Fe2O3} \)·xH2O (ferric hydroxide sol) adsorb \( \ce{Fe^{3+}} \) ions from the medium and form a positive sol. So the particles of the \( \ce{Fe(OH)3} \) sol move towards the cathode.
Why the other options are incorrect
Option (2) \(Au\) sol — Incorrect. Metal sols (copper, silver, gold) are negatively charged, so the gold particles move towards the anode.
Option (3) Starch sol — Incorrect. Starch sol is a negatively charged sol; its particles move towards the anode.
Option (4) \(As_2S_3\) sol — Incorrect. Metallic sulphide sols adsorb \( \ce{S^{2-}} \) ions and are negatively charged, so the particles move towards the anode.
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 – …
