Identify the sols whose charge is correctly given:
| Sol | Charge Present |
(A) | Blood | positive |
(B) | Congo red | positive |
(C) | Gum | negative |
(D) | Gold sol | positive |
(E) | \(Fe(OH)_3\) sol | negative |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
The charge on a sol comes from the ions its particles adsorb. Positively charged sols: hydrated metallic oxides (\( \ce{Fe2O3} \).xH2O, \( \ce{Al2O3} \).xH2O), basic dyes, haemoglobin (blood), \( \ce{TiO2} \). Negatively charged sols: metals (Cu, Ag, Au), metallic sulphides, acid dyes (eosin, congo red), sols of starch, gum, gelatin, clay, charcoal.
(A) Blood — positive — Correct. Haemoglobin (blood) is listed among the positively charged sols.
(B) Congo red — positive — Incorrect. Congo red is an acid dye and forms a negatively charged sol.
(C) Gum — negative — Correct. Gum, like starch and gelatin, forms a negatively charged sol.
(D) Gold sol — positive — Incorrect. Metal sols (gold, silver, copper) are negatively charged.
(E) \(Fe(OH)_3\) sol — negative — Incorrect. \( \ce{Fe(OH)3} \), made by adding \( \ce{FeCl3} \) to hot water, adsorbs \( \ce{Fe^{3+}} \) ions and is positively 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 – “… 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. |