Question

Gold sol can be prepared by

• A. hydrolysis of gold(III) chloride
• B. oxidation of gold by aqua regia
• C. peptization
• D. treating gold(III) chloride with metallic zinc
• E. reduction of gold(III) chloride with formalin solution

Hint:

Remember that metal salts are reduced to metal sols, whereas non-metal sols (like Sulfur) are often formed via oxidation (e.g., \(H_2S + SO_2\)).

Answer 1

Answer: (E)

Concept: Lyophobic sols, like metal sols, are prepared by chemical methods which involve the formation of particles of colloidal dimensions.
• Chemical Methods: These include Double decomposition, Oxidation, Reduction, and Hydrolysis.
• Gold Sol Preparation: Gold atoms are produced from their salt solutions using reducing agents.

Step 1: Analyze the specific reaction for Gold Sol. Gold sol is commonly prepared by the reduction of aqueous solutions of gold chloride (\(AuCl_3\)) using reducing agents like stannous chloride (Bredig's arc is a physical method, but chemical reduction is standard). The reaction with formalin (formaldehyde solution) is: \[ 2AuCl_3 + 3HCHO + 3H_2O \rightarrow 2Au (\text{sol}) + 3HCOOH + 6HCl \]

Step 2: Evaluate other options. Aqua regia (B) dissolves gold to form soluble complexes, it doesn't create a sol. Hydrolysis (A) is typically used for hydroxide sols (like \(Fe(OH)_3\)). Reduction is the primary chemical route for transition metal sols.