Green Pennies

Happy St. Patrick’s Day, Friends! Today we are celebrating with a chemical reaction that will turn pennies green.

You’ll need: pennies, vinegar (acetic acid), salt (NaCl), paper towel, and a bowl

1. Pour 1/4 cup vinegar into a bowl and add 1 tsp salt. Stir to dissolve.
2. Put pennies in the bowl and let sit for 5-20 minutes (they should get shiny)
3. Take your shiny pennies out of the solution (don’t rinse!) and set on a clean paper towel.
4. Let sit and check back to observe over the next few hours/overnight. (Picture bottom middle is pennies flipped over after about 15 hours)
5. You can put the pennies back in vinegar for a few minutes to clean them off – just be sure to rinse with water this time!

Here’s what’s going on: You are seeing a reactions between the copper (Cu) in the pennies and the oxygen (O₂) in the air. The first thing that we observe is that the dingy pennies get shiny. When a penny is first produced it is shiny because the Cu on the surface hasn’t reacted much with the air yet. Over time, the pennies start to turn brownish black. This is from the formation of copper oxide (CuO). In the first part of the experiment, we see the vinegar and salt react with the CuO. This reaction turns the pennies closer to how they were when they were first produced – when the Cu hadn’t reacted with the air yet. When you take the pennies out of solution and let them sit in the air, though, they start to oxidize again. This time they are oxidizing in the presence of chlorine (NaCl from the salt that we didn’t rinse off) and acid (C₂H₄O₂ from the vinegar). This leads to different oxidation reactants that have this greenish tint.