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Which of the following reactions can result in the formation of a colloidal solution?

1. Reduction of AuCl3 with HCHO in the presence of water.
2. Reduction of CO2(g) using Mg to give carbon and MgO.
3. Acid-catalysed hydrolysis of an ester.
4. Oxidation of NH3(g) into nitric oxide in Ostwald's process.

Correct answer: Reduction of AuCl3 with HCHO in the presence of water.

Solution

Reduction of AuCl3 with formaldehyde (HCHO) in water produces gold nanoparticles (colloidal gold/gold sol) — particles of size 1-1000 nm dispersed in water. The other options involve gas-solid reactions or homogeneous reactions that do not produce colloidal systems.

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