By Colie Haahr, CMNH Educator

We were surprised by how excited kids got about cleaning pennies when we offered this experiment as a drop in STEAM activity at CMNH! It does seem like a magic trick when you first see the pennies getting shinier almost instantly. 

This experiment is a good way to practice making predictions and observations, and it’s an easy one to do with materials that are available at home! For kids learning about money and currency, this is a great way to practice counting and recognizing the different coins as well. 

Materials:

  • Table cover of some kind (a trash bag works!)
  • Several pennies
  • Other coins- quarter, nickel, dime, coins from other countries if available 
  • Vinegar 
  • Salt
  • Bowl of water
  • Bowl for vinegar and salt mixture
  • Q-tips
  • Paper towels 
  • Optional: lemon juice, soapy water, other liquids you would like to experiment with

Directions:

  • One way to set this experiment up is to use a muffin tin or small bowls to offer several different liquids for kids to try. A small pallet for paint works well, too! 
  • Put the coins on a surface you do not mind getting messy- plastic plate, paper towel, or directly on the table covering, but you will want some paper towels or dish towels nearby
  • Allow kids to experiment with the materials to see what would work best to clean the different coins by dipping the coins in the liquid, and brushing them with a q-tip
  •  Ask what they think will work best, and why?
  • After some experimentation, guide them toward the vinegar or lemon juice mixed with salt. The salt works as a mild abrasive, and the acid in the vinegar and lemon juice react with the pennies to make them shiny and new looking
    • Important: rinse the pennies in water after cleaning them if you would like to keep them nice and shiny! They will oxidize and turn green otherwise. 
    • Fun fact: this is what happened to the statue of liberty, and why it looks green today! Kids did not dip it into a bowl of vinegar, but the statue is copper, and it turned green because of oxidation over time. 
  • Ask why the acidic liquids cleaned the pennies, but not the other coins? What is different about them? The pennies are a different color because they are made of a different material: copper!

What’s Happening? 

 Pennies are made of copper, and copper is shiny, but over time it becomes tarnished and appears black or brown because of copper oxide, which is created when copper bonds with oxygen. Acids mixed with salt help break down the copper oxide on the penny. Vinegar may remove actual dirt from the pennies and other coins because it works well as a cleaner! 

Extensions: 

Science Bob offers a few more experiments you can try using the same materials.

Scientific American offers some ideas for different liquids to use, and a more in depth explanation and method for advanced experiment-ers.