This penny cleaning experiment a fun supplemental homeschool science activity.
Using a book like Lots and Lots of Coins by Margarette S. Reid, introduce your child to a bit of American history and some of our greatest presidents with a handful of coins. Read the book together, talk about the cool facts, observe the coins, and sort them.
Pull out a few pennies. Make sure there is a mix of old pennies and new ones. Ask your child to compare the old pennies to the new ones. The older ones will be dingy and dark, while the newer ones will be bright and shiny.
Depending on the age of your kids, you can talk to them about copper oxide. Explain that everything is made up of atoms, and sometimes atoms join with other atoms to make molecules. The copper atoms on the surface of the penny joins with oxygen atoms from the air to form copper oxide molecules, turning the pennies dark and dingy.
Ask your child what they think would be the best method for cleaning the pennies. Go with whatever they choose. My kids decided that they wanted to try plain water and a soap and water mixture. When that didn’t work, I told them that acids are often used to clean copper oxide, and since they often play with baking soda and vinegar, they immediately grabbed for the vinegar.
You can help your child come to that conclusion, or just pull out the vinegar yourself. Mix about 1/4 cup vinegar and 1 teaspoon salt together in a clear glass or plastic cup until the salt is dissolved. Then put a few pennies in the mixture.
You and your child will be able to observe the pennies cleaning right up. Take those pennies out after five minutes and rinse them well. Give them to your child to observe, then put another handful of dirty pennies into the mixture for five minutes.
Pull half of the pennies out, and rinse well, placing them on a paper plate or towel marked rinsed. Pull out the remaining pennies and spread out, unrinsed, on another plate or towel.
Leave the pennies alone for an hour, and then go back to have your child observe them again. What’s happened? Encourage your child to make observations. The rinsed pennies are still clean, but the unrinsed pennies should be dark blue-green (if not, leave them alone for a bit longer).
When the vinegar and salt mixture dissolves the copper layer on the pennies but is left on the pennies, the copper atoms from the penny join with both oxygen atoms in the air and chlorine atoms in the salt, forming a compound called malachite.
Just like the blue-green coating that covers the Statue of Liberty!
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