Do you have a collection of pennies somewhere in your house? More than likely those pennies don’t have a nice coppery shine but instead are dull, dark and possibly tinted with green. This month you will learn about a great technique to clean off this coating and reveal the true shiny copper color below. You will also learn that this is not just scrubbing off dirt, but it is a branch of science called chemistry.
20 or so dirty pennies
Two small glass bowls
Lemon juice or orange juice
Access to a sink
Examine the pennies, notice the grimy layer on them. Try to scrub this off with water… you won’t be able to get it shiny!
Pour ½ cup of vinegar into the bowl and add 2 teaspoons of salt to it.
Hold a penny halfway into the liquid for about 15 seconds and remove it. Make some observations.
Put the rest of the pennies into the vinegar/salt solution and wait 10-15 minutes.
Take half of the pennies out and rinse them really well with water. Take some time to scrub them a bit to make sure no salt/vinegar remains on them.
Take the other pennies out and just leave them on a paper towel WITHOUT rinsing them off. Wait until they dry and make some observations.
The world around us is made of little pieces of matter called elements. These elements combine with one another to make many different types of materials. This is chemistry! Most metals, like copper and iron, will chemically join with oxygen in the air after a while. This is called “oxidation,” and makes the metal “rust.” This is why a bike left out in the rain too long will turn rusty brown, and why the Statue of Liberty has turned green. It was once a shiny copper statue, just like your brand new cleaned pennies.
The “dirt” on the pennies was a new chemical substance called copper oxide. It doesn’t easily come off, as you found out by trying to scrub them with plain water. When you mix the salt and vinegar, however, this makes a new substance, too; a type of acid. This acid is really good at breaking down the dirty pennies. If you rinse it off afterwards, no further reaction will happen and they stay clean.
You notice, however, the pennies that were not rinsed off have become green and gross. The chemicals left behind on it make the copper react even more to form a new substance.
There are plenty of things to experiment with this idea. Are there other substances that might work as well as vinegar and salt? Does lemon juice and salt work? Look at the ingredients of ketchup. Will that work? Will the same thing happen if dipped in plain vinegar or plain salt water?
Dirty pennies work this way, what about other coins? How about different types of salt? Different amounts of salt? Wow, there are a lot of things to test with, and you have them all in your kitchen!
I hope you enjoyed this simple experiment. If you have more questions about this, specifically around the science fair aspect of this experiment, contact the author.
Steve Davala is a high school chemistry and physics teacher who likes to write and work with Photoshop. He’s got two kids of his own and subjects them to these science activities as guinea pigs. Follow him on Twitter, on www.stevedavala.blogspot.com or email him at firstname.lastname@example.org.