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Penny cleaning for science fair

Q. Hi,

I am doing a project for a seventh grade science fair, and I would like to know what liquid cleans pennies best. Also, I was wondering how I should do my experiment and what liquids I should use. Thank you!

Olivia M [last name deleted for privacy by Editor]
- Arlington, Virginia

"Naked Eggs and Flying Potatoes: Unforgettable Experiments that Make Science Fun"
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A. Hi, Olivia! You're not alone on this one :-)
This question has been asked enough times that we wrote an FAQ on Cleaning Pennies. I think that you'll find it helpful and interesting. Good luck!

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

A. Hi I am Aimee. I am in 6th grade and I just did an experiment on cleaning pennies and I can give you a little information on which liquids clean best. Okay, I tested 4 different liquids: Diet Coke, vinegar [in bulk on eBay or Amazon], Lemon Juice (from a bottle), and just water to tell the difference. So Lemon Juice came in 1st, Diet Coke came in 2nd and in 3rd place came Diet Coke. So I think you should test lemon juice.

Aimee [last name deleted for privacy by Editor]
- Riverview, Florida, USA
November 13, 2012

Q. My First Grader is doing a science fair project on cleaning pennies and we have our cleaning products but what we cannot seem to find out is how does the penny seem to get so dirty? Is it because of the copper and people holding it and touching it? Compared to all the other coins it seems the penny gets dirty faster than the rest. If you could help I would appreciate any info you could give.

Margaret S [last name deleted for privacy by Editor]
- Moreno Valley, California

A. Hi, Margaret. It's not "dirt", which you can prove to yourself by unsuccessfully trying to clean it with detergent. Rather, all metals except precious metals like gold, tarnish. That means the metal on the surface reacts with the oxygen in the air to form corrosion products (oxides of the metal). For example, when iron or steel corrodes, the reaction product is iron oxide (rust), and I'm sure you've seen plenty of that. But copper, which pennies are coated with, tarnishes easier than the nickel that the other coins are made of. Also, copper tarnish is a dark and easily distinguishable brown color whereas the slightly yellowish tarnish on the nickel-based coins is not nearly as obvious.

So, pennies don't get "dirtier" than other coins, their oxidation products or tarnish forms faster and is darker. Good luck.

Ted Mooney,
Ted Mooney, P.E.
Striving to live Aloha - Pine Beach, New Jersey

Q. I am an 8th grade American girl who is currently studying in International School. I chose five different liquids for my science fair experiment. As the result, baking soda worked the best. The problem is, I do not know why it happened. I searched on the Internet, but there wasn't any scientific information for that. I really want to know why baking soda worked best for my penny. Help!

Sherry Kwon
- China, Beijing
April 4, 2015

A. Hi Sherry. You didn't tell us what the other four liquids were, so we can't concur about baking soda being superior to four unknown solutions, let alone why. But there are many solutions that are better for cleaning pennies than baking soda -- including vinegar & salt, lemon juice & salt, and ketchup. Best of luck.


pic of Ted Mooney
Ted Mooney, P.E. RET
Striving to live Aloha - Pine Beach, New Jersey
April 2015

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