My kids have been collecting change for a few years now. Recently they’ve been telling me that their pennies look dirty. I told them that I think we can clean them, but not with soap – instead with items from the kitchen! They were excited to try it out.
This is another fun and super easy STEAM activity to try at home with your kids this summer. My summer theme for our STEAM projects is the easier the better, so you can count on all our activities being simple and easy to execute.
Check out our YouTube video showing the full experiment, then follow along with our step by step instructions below.
- 4 old/dirty pennies
- 4 small cups or bowls
- Place one penny in each of the four bowls
- Add ~1/2 to 1 cup water into the first bowl (the water bowl is optional – it won’t do anything to the penny but I liked to include it for a baseline and to show how much cleaner the other pennies become)
- Add ~1-2 Tbls ketchup to the second bowl (enough to completely cover the penny)
- Add ~1-2 Tbls salt to the third bowl (enough to completely cover the penny), then add ~1/2 to 1 cup vinegar
- Add ~1/2 to 1 cup vinegar into the fourth bowl
- Let the pennies sit in the liquids for about 5-10 minutes
- Pull the pennies out of each bowl. Dry the pennies off with a towel, and then compare how clean they got in each liquid. Which penny was the cleanest for you? Ketchup worked best for us!
STEAM Concepts Learned:
The pennies are dull due to oxidation. Overtime, the copper in the pennies react with oxygen forming copper oxide on the pennies giving them the dull and brownish appearance.
The vinegar and ketchup are both acidic. The acetic acid in the vinegar and ketchup breaks down the the copper oxide on the pennies in a chemical reaction. That’s why the ketchup, salt & vinegar combo, and vinegar all worked well.
But we noticed the ketchup and salt & vinegar combo worked better than vinegar alone. That’s because the salt (which is an ingredient in the ketchup), makes a separate chemical reaction forming copper chloride which releases more copper oxide ions from the pennies.
Both the acid and salt make chemical reactions with the copper oxide to remove it from the pennies. Ketchup works great because it contains both the acid and salt!
It’s easy to expand this experiment by trying to clean the pennies with even more kitchen items such as: soap & water, soy sauce, mustard, mayonnaise, Coke, Sprite, lemon juice, bubbly water, hot sauce.
My kids and I were surprised that the ketchup worked the best for us! I had read the science behind this activity before we started, so I knew the ketchup would work, but even I didn’t expect it to work so well. It was a fun surprise and we all learned a lot.
My kids were also excited about how clean their pennies became and now they want to use ketchup to clean all our pennies. I said they can slowly work through cleaning their pennies using their leftover ketchup after our meals. 😉
Have you tried cleaning pennies with kitchen items before? Which liquid worked best for you?