When pennies are made, the copper on their surface is bright and shiny. Over time, the copper oxidizes and becomes dull and dark. Sometimes they even turn green. In this polishing pennies experiment, we will test different liquids to determine if any of them can make the dull pennies shiny again.
1. Choose 3 dark, aged pennies and one new shiny penny to compare them to. Make a note of the color of each.
2. Pour 3 different liquids into small cups. We used vinegar, ketchup and milk. You might consider lemon juice or soda. Place one penny into each liquid.
3. After 5 minutes, remove the pennies and make a note of the color.
4. Put the pennies back and wait another 10 minutes. Remove the pennies and make a note of the color.
5. Wipe each penny with a cloth and, once again, make a note of the color.
Acids found in various liquids react with the copper oxide and cause it to dissolve revealing the shiny copper underneath. All the liquids we tried in our experiment contain an acid in varying forms and strengths. Try this experiment at home with various liquids to see which acid polishes pennies the best. Use this printable to record your findings.