GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY GIPHOTOSTOCK / SCIENCE PHOTO LIBRARY
Cleaning tarnished copper. A 1981-minted US penny (one cent) coin has been half-submerged into saturated citric acid in a Petri dish. After several hours, the coin was removed from the acid. Before 1982, US pennies were minted from brass (95% copper, 5% zinc). Brass coins tarnish or corrode over time. Tarnish appears as a dull, dense film on the surface. It protects the underlying metal from further corrosion. Tarnish consists of various oxides, sulphides, sulphates, etc. A common way to clean copper (i.e. remove tarnish) is to treat it with an acid, as demonstrated here.
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