Silver to Black - and Back

Silver and silver-plated objects react with sulfur and sulfur compounds to produce silver sulfide (Ag₂S), or tarnish. Contact with materials that contain sulfur compounds, such as hardboiled eggs, mayonnaise, mustard, and rubber bands can cause tarnish. In air, a silver object can tarnish owing to the reaction of silver with hydrogen sulfide (H₂S). This is a gas found in the air as a result of some industrial processes and the decomposition of dead plants and animals. The reaction of silver with hydrogen sulfide to form tarnish is as follows:

2 Ag(s) + H₂S(g) ---> Ag₂S(s) + H₂(g)

The tarnish on silver can be removed with commercial silver polishes. This method usually removes the tarnish through abrasion. As a result, each time tarnish is removed, some of the silver is lost. However, tarnish can also be removed chemically through the reaction of aluminum with the tarnish. The aluminum is a more active metal than silver, so it reacts with the tarnish to chemically convert the tarnish back to silver. The reaction is as follows:

3 Ag₂S(s) + 2 Al(s) ---> 6 Ag(s) + Al₂S₃(s)

This activity allows students to remove tarnish from silver using the reaction of tarnish with aluminum.