Kids, do you think you could make pennies change from dull to shiny to green right before your eyes? Over the course of two months, we’ll learn about metals using pennies, nails, and a few simple household ingredients to explore some of the properties of metals.
Please note: All chemicals and experiments can entail an element of risk, and no experiments should be performed without proper adult supervision.
For this month you will need: 10-20 dull pennies, 1/4 cup white vinegar (dilute acetic acid, CH3COOH), 1 teaspoon salt (sodium chloride, NaCl), a shallow, clear glass or plastic bowl (not metal), water, plastic gloves, and paper towels.
For “Shiny Clean Pennies”, pour the salt and vinegar into the bowl. Stir until the salt dissolves. Dip a penny halfway into the liquid and hold it there for 10-20 seconds. Remove the penny from the liquid. What do you see? Dump the rest of the pennies into the liquid. The cleaning action will be visible for several seconds. Leave the pennies in the liquid for 5 minutes.
After the 5 minutes, take half of the pennies out of the liquid and place them on a paper towel to dry. Remove the rest of the pennies and rinse them well under running water. Place these pennies on a second paper towel to dry. Write labels on your paper towels so you will know which towel has the rinsed pennies. Allow about an hour to pass and take a look at the pennies you have placed on the paper towels. Are they different?
Pennies get dull over time because the copper in the pennies slowly reacts with moist air to form hydrated copper carbonate (which is greenish). When you place the pennies in the salt and vinegar solution, the acid from the vinegar dissolves this “patina”, leaving behind shiny clean pennies. The copper from the patina stays dissolved in the liquid. You could use other acids instead of vinegar, like lemon juice. Rinsing the pennies with water stops the reaction between the salt/vinegar and the pennies. They will slowly turn dull again over time, but not quickly enough for you to watch.
On the other hand, the salt/vinegar residue on the unrinsed pennies starts another reaction between the copper and the oxygen in the air. Let the pennies dry overnight; the resulting blue-green copper substance is called “verdigris”, which is one of several copper acetates (e.g. Cu(CH3COO)2·CuO·6H2O). You may even see this stain on the towel. (Since most copper salts, including the carbonates and acetates here, are considered to be irritants, an adult partner should handle the verdigris pennies with gloves and dispose after use).
TRIVIA: Some copper carbonate minerals (CuCO3) in nature are malachite and azurite. Copper acetate (verdigris) is used as a dye, and it can form on copper pots that are used to cook acidic foods such as tomatoes.
Keep the liquid that was used to clean the pennies stored safely until next month. We will use it in Part II to make “Copper Plated Nails”.
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Kathleen Carrado Gregar, PhD, Argonne National Labs
[email protected]
March 2006
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Reference:
Dr. Anne Marie Helmenstine at:
http://chemistry.about.com/cs/demonstrations/a/aa022204a.htm and http://www.scorecard.org/chemical-profiles/html/coppercompounds.html
