Which of the following is an example of negatively charged sol?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → \(As_2S_3\)
A colloidal particle gets its charge mainly by preferential adsorption of ions from the medium. The common sols fall into two groups:
Positively charged sols: hydrated metallic oxides (\( \ce{Al2O3} \).xH2O, \( \ce{Fe2O3} \).xH2O), basic dyes (methylene blue), haemoglobin (blood), oxides such as \( \ce{TiO2} \).
Negatively charged sols: metals (Cu, Ag, Au), metallic sulphides (As2S3, Sb2S3, CdS), acid dyes (eosin, congo red), sols of starch, gum, gelatin, clay, charcoal.
An arsenious sulphide sol is made by passing \( \ce{H2S} \) through a solution of \( \ce{As2O3} \); the particles adsorb sulphide ions (\( \ce{S^{2-}} \)/\( \ce{HS^{-}} \)) and become negatively charged. So As2S3 is the negatively charged sol.
Why the other options are incorrect
Option (1) \(Al_2O_3.xH_2O\) — Incorrect. A hydrated metallic oxide; it adsorbs \( \ce{Al^{3+}} \) ions and forms a positively charged sol.
Option (3) \(TiO_2\) — Incorrect. An oxide sol listed among the positively charged sols.
Option (4) Blood — Incorrect. The colloidal haemoglobin of blood is listed among the positively charged sols.
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 – “… A list of some common sols with the nature of charge on their particles is given below:”
| 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. |