American pennies may not be especially valuable as currency, but they can be an invaluable teaching aid in the classroom. Simple science experiments that turn pennies "shiny," take the shine off pennies or turn pennies different colors can be used to explain different types of chemical reactions.
Making a Shiny Penny
All that's needed for this simple experiment are "dirty" pennies, salt, vinegar and a non-metal container, such as a glass or ceramic bowl or cup. Mix 1 teaspoon of salt with about 1/4 cup of white vinegar. Place the pennies in the bowl or cup and pour the mixture over the pennies. The salt-and-vinegar mixture should fully cover the pennies. Wait 10 seconds, then remove the pennies and rinse them with cold water. After they're dried, the once-dirty pennies should appear shiny and new. The pennies are now clean because the salt and vinegar combined to form an acid. This acid reacted with the oxidized copper on the surface of the penny, transforming the copper oxide into a new compound that was washed off by the water.
Making a Green Penny
This experiment can be carried out in tandem with "Making a Shiny Penny" or can be conducted on its own. The steps are the same as the previous project, except that the pennies aren't rinsed after they are removed from the salt-and-vinegar mixture, but instead are placed on a towel to air-dry. The pennies should turn from their original coppery-brown color to a dark green. This dark green substance is the new compound formed when the copper from copper oxide reacts to the oxygen atoms in air and the chlorine atoms in salt. This substance is called malachite, a common type of mineral.
Turning Pennies Silver
Unlike the previous two experiments, this experiment requires specialized chemicals and safety equipment. Place 25 milliliters of 3 M NaOH (more commonly known as lye or sodium hydroxide) in an evaporating dish, then add 1 gram of zinc dust, being careful not to inhale any zinc. Heat the mixture over a Bunsen burner until it is hot but not boiling. Use tongs to place a penny in the solution and remove the penny when it appears silver. The pennies are now "silver" -- what has really happened is that they are now covered in silver-covered zinc. For the best result, use clean, pre-1983 pennies, which contain more copper than newer pennies. The pennies can be cleaned using the salt-and-vinegar solution used in other experiments.
Turning Pennies Gold
The natural next step of the "Turning Pennies Silver" experiment is to turn the "silver" pennies "gold." Holing the silver penny in tongs, place the penny in the flame of the Bunsen burner, being careful to avoid inhaling fumes. The penny should slowly turn gold. Once the penny is golden, drop it in water so it will be cool enough to touch. The "gold" pennies are really now brass pennies. When exposed to the heat, the zinc in the "silver" coating reacted with the copper in the penny, creating a mixture of metals called an alloy.