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Colloidal gold can be prepared by which of the following methods?

1. Bredig's arc method
2. Reduction of AuCl3
3. Hydrolysis
4. Peptization

Correct answer: Reduction of AuCl3

Solution

Colloidal gold (gold sol) is most commonly prepared by the reduction of a dilute gold salt solution (AuCl3 or HAuCl4) using a mild reducing agent such as formaldehyde, SnCl2 (Faraday's method), or tannin. Bredig's arc method can also produce metal sols but is less specifically associated with gold. Hydrolysis is used for metal hydroxide colloids (e.g., Fe(OH)3 sol), not for gold. Peptization breaks up precipitates into colloids, not applicable here.

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