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Ever wonder what to do with all those dull pennies that may be lying about the house? Under the bed, between sofa pillows, in the hamper or a long-forgotten coin box? Here is a nifty experiment that introduces you to some of the secrets of copper, one of the oldest and most important elements in human history (think the Bronze Age and the beginnings of urban civilization).

• 20 dirty, dull pennies
• 1/4 cup of white vinegar
• 1 teaspoon of salt
• A clear, shallow bowl
• Paper towels

1. Pour the vinegar into the bowl, and add the salt.
2. Now, dip one penny halfway into the liquid. Hold it there for about 10 seconds, and remove it. See a difference?
3. Put all of the pennies into the mixture.
4. After 5 minutes, take out half of the pennies and put them on the towel to dry.
5. Rinse the other half of the pennies in water, and then put them on another paper towel. Wait for a while. See any difference between the groups?

What Happened:

Copper is made of one type of atom. Sometimes other molecules join them to form new molecules. When copper meets oxygen, it forms copper oxide, and creates a dull finish on the surface of the copper.

But, when you immerse the dull copper pennies in a weak acid mixture (vinegar and salt), it dissolves the copper oxide, and the pennies shine again.

If you don’t rinse off the acid, the copper atom will join with the oxygen from the air and the chlorine from the salt to form a blue-green compound called malachite. So now you know!