For one of our 31 days of STEM activities, of course we had to turn pennies green as a science experiment.
I love science experiments for kids that are easy but have impressive results like the penny trick.
However… our penny experiment actually took much longer than we expected! Our hypothesis was that since pennies have a bit less copper now, perhaps that is why they took longer to react. The other hypothesis was that it was user error.
The Science Behind Turning a Penny Green
Pennies are largely made of copper, which means they oxidize just like many other metals. However, rather than rusting, pennies simply get covered in a coating of green that can be polished off. Turning a penny green does not eat holes into the penny.
When you add vinegar and salt to pennies, it dissolves the top copper-oxide layer of the penny. This causes the copper atoms to mix with oxygen in the air and chlorine in the salt. This creates a blue-green substance known as malachite. It’s chemistry in action!
Turning a Penny Green Science Experiment
- Shallow tray
- Paper towels
- Pennies (we used a mix of old and new to see if there was a difference in when or how they would turn green)
Soak a paper towel in vinegar and line the bottom of your tray.
Place the pennies onto the paper towel. Sprinkle the pennies with salt.
Allow the pennies to sit until they start to turn green. This could take as little as 24 hours, but in our case, it took over 2 weeks. However, we did have the pennies soaking in a deep solution at first. When we changed it out for a vinegar-soaked paper towel, the pennies turned green the next day.
I think our original set-up didn’t get enough oxygen, which is essential for turning a penny green.