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Cleaning Pennies

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Supplies
  • 4 Dull Pennies
  • 3 Cups
  • Vinegar
  • Dish Soap
  • Water
  • Paper Towels

On average, a U.S. penny is in circulation for 40 years. That’s a lot of time for a penny to get dirty! Which liquid is best for making your copper cents shine again?

Instructions:
  1. Predict which liquid will make a dull penny shiny.
  2. Set one penny aside. This is called your control so you can compare the results.
  3. Put each of the other pennies into its own cup.
  4. In the first cup, pour enough vinegar to cover the penny.
  5. In the second cup, pour enough liquid dish soap to cover the penny.
  6. In the third cup, pour enough water to cover the penny.
  7. Wait at least 10 minutes.
  8. Remove the pennies, rinse them in water, and rub them with a paper towel.
  9. Compare all three pennies to your control penny. Which one is the shiniest?

The Science Behind the Experiment

Pennies made of copper become dull over time as copper on the surface reacts with oxygen from the air, forming dark-colored chemicals called copper oxides. The acetic acid in vinegar dissolves these chemicals and leaves the copper surface of the coin looking shiny. Soap can clean things like dirt and grease, but it can’t dissolve copper oxides.

Further Experimentation:

  • What would happen if you left the penny in vinegar for only 30 seconds?
  • What would happen if you left the penny in vinegar for an hour?
  • What would happen if you put only part of the penny in the vinegar?
  • Try other acidic liquids, like lemon juice or pickle juice. How do they compare?

Support the Museum

The Virginia Discovery Museum is a vital nonprofit community resource, but we will be closed through at least May 31 due to the COVID-19 pandemic. Please consider a donation to support us during this uncertain time. Donate Now