Science Experiment: How to Clean Pennies
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Have you ever wondered how to clean pennies? Cleaning copper pennies can be a lot of fun for kids! You can turn the coin from a dull brown to bright and shiny. Get out your tarnished coins & let’s get going!
I decided to turn this into science experiment with my kids to test a few different substances and see what works best to clean old coins.
I remember accidentally doing this one time with a packet of hot sauce from Taco Bell when I was in high school. That’s where this experiment idea came from! My friends & I thought it was so cool that the penny got so shiny.
Why Do Pennies Turn Brown?
Pennies are made of copper. When copper is exposed to oxygen it will corrode (turn brown) because of chemical reactions. The oxidation creates a brown coating called patina. These are copper oxide molecules and they actually protect the metal underneath. Over time, copper transitions from its shiny brown color to a darker brown shade. There are also layers of dirt and grime that build up on coins.
How to Clean Pennies:
If you are genuinely wanting to know how to clean pennies, I have learned that a mixture of vinegar and table salt will get them really clean and shiny. Soak them in the solution for a couple of minutes. Then gently scrub the pennies with a soft-bristled toothbrush. Obviously you’ll want to use an old toothbrush.  😉  Rinse the coins with warm water and use a soft towel to wipe them dry. Be careful not to create any scratches or abrasions on the surface of the coins as you clean them.
Why clean copper coins?
Coin collectors often want to clean coins. Collectible coins are typically displayed, so their owners want them looking good! You can clean your everyday coins, too if you want. But usually you only keep they for a short time. Rare coins need to be kept clean.
But for the sake of this experiment, we wanted to try some different substances to see what would work best for us. I let my kids help pick the different things to test.
We filled some cups with a small amount of different substances in our kitchen to see which one would get our dirty pennies the cleanest. I had my kids each take a guess at which thing would get the pennies the cleanest. They were way off, so it was fun to see which really did get them clean. We let the pennies soak for a time, then put them on a paper towel to dry. Next wipe or scrub the penny clean with paper towels.
Substances We Tested for Cleaning Pennies
When experimenting with how to clean pennies, we tested a variety of items, here are some ideas:
Lemon or lime juice
Water
Soy sauce
Dish soap
White vinegar
Ginger ale
Ketchup
Salt water
Hot Sauce
Mustard
Milk
Baking soda (we did not try this, but it would be a good option to add in!)
Pour a tablespoon or so of each substance in a a bowl, jar or cup, enough to cover the penny, then allow it to sit for a time. We left ours for 30 minutes.
Our Results:
We took them out and wiped each one off on a paper towel. You can see the surface of the pennies change and become shinier. You can then rinse them with soapy water and set them on a towel to dry. You may have to wipe off some of the built-up dirt on some of them.
Vinegar and lemon juice both work really well for cleaning pennies. But, ketchup was the clear winners for our pennies, with soy sauce next, maybe with a little more time they would have gotten a little bit cleaner.
This is a fun experiment to try with younger kids! My kids of all ages were interested in the results, though. So if you want to do it with older kids, just go deeper with the science!
Use my Scientific Method Worksheets to extend this!
See even more STEM projects for kids.
Now just for the sake of preserving history, here is our original image from the first time we tried this experiment about 10 years ago. 🙂
Perfect! This is just the activity we need this week!
Apparently, it is the salt + acid combo that works so well…that’s why hot sauce works better than just vinegar or lemon juice…and ketchup should work just as well as hot sauce! Thank you so much for linking up with the Smart Summer Challenge!
This would be my son science project for this year, than k you!!!!
Yeah! You are most welcome!
on the last penny to the lower right “clean-pennies.jpg” what did you use?
The one on the bottom row to the right (I think that’s the one you mean)- That one was soy sauce.
Thank u soooooo much! i needed to find a science project for schoool and it is due tommorow! luckily i found this!
What a fun experiment!! I did it with my husband for a date night. He predicted the vinegar and I predicted the salt solution. We used dish soap, soy sauce, vinegar, lime juice, ketchup and salt solution. But our results were different. All the pennies were very clean afterward. No brown ones left, except a stubborn stain here and there. To my surprise, the shiniest one was the one soaked in dish soap. No brown left at all!. I thought the dish soap would have performed the worst! Thanks for sharing thisa