Tarnishing is a chemical change that occurs when certain metals are exposed to air or other sources. The change, known as oxidation reaction, is what leaves behind the tarnish.
Tarnish is most commonly associated with silver, but it can also be found on other metals, such as aluminum and copper.
Silverware, jewelry, and serving sets tend to succumb to tarnish over time. Just as a chemical reaction caused the tarnish, another reaction can remove the tarnish and leave silver pieces looking nearly as good as new. A mixture of boiling water and sodium bicarbonate is an effective way to remove tarnish from silver items.