Why Do Pennies Turn Green?

You’ve probably noticed that some of the old pennies sitting in your piggy-bank are covered in dirt and this mysterious blue-green substance. That is copper oxide! Copper naturally turns green over time as it reacts with oxygen in the air. This chemical reaction is known as oxidation. The oxygen and some water in the atmosphere react with the surface of the copper. That causes the copper and oxygen atoms to bond to create copper oxide. That is the green or blue coating on copper. Over time, the layer gets thicker and thicker! The Statue of Liberty in New York is a great example of copper oxidation! The statue was originally made of copper, but the oxygen in the air and the water around it caused oxidation to happen, which turned the statue green! The same thing happens with pennies.

You can clean pennies by using vinegar and salt, or even ketchup! It dissolves the copper oxide layer, making the penny look clean. Make sure to get an adult’s help when trying this at home!