Theory

Aldehydes such as glucose are reducing agents and will reduce Ag (aq) ions to metallic silver. They themselves are oxidised to carboxylate ions. The reaction that occurs is:

CHOH(CHOH)CHO (aq) + 2Ag() (aq) + 3OH (aq)

2Ag(s) + CHOH(CHOH)CO(aq) + 4(aq) + 2HO(l)


Extensions

Try using an ordinary aldehyde instead of glucose, and show that the reaction does not work with a ketone such as propanone.

Further Details

The silver can be removed from the silvering flask using concentrated nitric acid. Work in a fume cupboard because nitrogen dioxide is formed. There are reports of silvered flasks being kept for several years as ornaments.


Safety

Wear eye protection.
There have been a few reports of alkaline ammonical silver nitrate solutions exploding after standing for some time. This rare occurence is thought to be caused by the formation of silver nitride or silver fulminate. To avoid this risk, the ammonical silver nitrate solution should not be made up before the demonstration and any silvering solution left after the demonstration should not be placed in a silver residues container but should be washed down the sink with plenty of water. The silvered flask should be rinsed thoroughly with water and the washings washed down the sink as soon as the silvering has finished.