Which of the following sols are correctly matched with their corresponding charges?
(A) \(Cr(OH)_3\) sol: negatively charged sol
(B) \(TiO_2\) sol: positively charged sol
(C) \(CdS\) sol: positively charged sol
(D) Gum: negatively charged sol
(E) Silver sol: positively charged sol
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (B) and (D) only
The charge on a sol depends on the type of substance (and the ions it adsorbs):
(A) \( \ce{Cr(OH)3} \) sol : negatively charged sol — Incorrectly matched. Hydrated metallic oxides such as \( \ce{Cr2O3.xH2O} \) and \( \ce{Fe2O3.xH2O} \) form positively charged sols.
(B) \( \ce{TiO2} \) sol : positively charged sol — Correctly matched. \( \ce{TiO2} \) is an oxide sol and carries a positive charge.
(C) CdS sol : positively charged sol — Incorrectly matched. Metallic sulphides (\( \ce{As2S3} \), \( \ce{Sb2S3} \), CdS) form negatively charged sols.
(D) Gum : negatively charged sol — Correctly matched. Gum, starch, gelatin, clay and charcoal give negatively charged sols.
(E) Silver sol : positively charged sol — Incorrectly matched. Metal sols of copper, silver and gold are negatively charged.
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 (Some common sols with the nature of charge on the particles)
| Positively charged sols | Negatively charged sols |
|---|---|
| Hydrated metallic oxides, e.g., Al2O3.xH2O, CrO3.xH2O and Fe2O3.xH2O, etc. | Metals, e.g., copper, silver, gold sols. |
| Basic dye stuffs, e.g., methylene blue sol. | Metallic sulphides, e.g., As2S3, Sb2S3, CdS sols. |
| Haemoglobin (blood) | Acid dye stuffs, e.g., eosin, congo red sols. |
| Oxides, e.g., TiO2 sol. | Sols of starch, gum, gelatin, clay, charcoal, etc. |