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Hey everybody.  Squeaks and I were just out for a walk and Squeaks found a bunch of lucky pennies for his lucky penny collection.  He's pretty excited.  Oh, yeah, Squeaks, some of the pennies are new and shiny but some of them are kinda grimy and dull.  I wonder if there's a way that we can clean them up before we add them to your collection.  I think this calls for an experiment.

Okay.  We've gathered a few things that I think might help us clean our pennies.  If you want to do the experiment with us, here's what you'll need.  First, you'll need to grab a grown up and find five dull pennies.  Then, you'll need a little bit of dish soap, some brown cola, vinegar, a teaspoon of salt, lemon juice, and ketchup.  Finally, you'll need five cups to fill with our different cleaners.

Fill one cup with dish soap and a little water.  Fill the next cup with some brown cola.  Next, combine the vinegar and salt into another cup, and put some lemon juice in the next one, and finally, squirt a little bit of ketchup in the last one.  Okay, we're all set up here, but before we put any of the pennies in the cups, what do you think is going to happen?

Ohh, Squeaks says that he thinks the soap and water will work the best, since we use soap to clean lots of things.  Cola stains things if you spill it, so I'm not sure it'll clean these pennies very well, and ketchup?  I don't think that one will work at all.  I think the vinegar and salt will work the best, because some cleaning solutions are made with vinegar.

Okay, now we're ready to put the pennies in.  We'll let them sit for five minutes or until you start to see some of the pennies getting shiny.  I think they're ready.  Let's rinse them off and take a look.  

Hmm, the penny that was in the soap and water doesn't look any different to me.  It was a good guess though, Squeaks.  Let's look at the penny that was in the cola.  Wow, it's a lot shinier that I thought it would be and it's a lot cleaner than the soap and water penny.  Ooh, the penny from the lemon juice is even more shiny and the penny from the vinegar and salt is the shiniest of all, and the penny from the ketchup...whoa!  It's even shiny too.  That's awesome. 

Now we have our results, but why did some of our pennies get really clean and some of them didn't?  Well, pennies are made of a metal called copper.  Over time, the copper combines with oxygen in the air to make that dark coating on the outside.  That dark stuff is called copper oxide and it isn't shiny like copper is, which is why the penny looks less shiny, but under that dull layer of copper oxide, there's still shiny copper.  To make the penny shiny again, we need to use things that can dissolve or get rid of the copper oxide.  

So how do you dissolve that dark coating?  Well, you can use something called an acid.  Acids are great at dissolving lots of things, including copper oxide, and one thing that all the clean pennies have in common is that they were in stuff that was really acidic or had lots of acid.  You can tell that a food is acidic because it tastes really sour.  Lemons are acidic and they're super sour.  That explains why the lemon juice cleaned the penny so well.  Vinegar is an acid, too, which is why it tastes kind of sour and eventually, it would have cleaned the penny off all on its own, but when you add salt to vinegar, it turns into an even stronger acid that cleans the penny really fast.  The ketchup worked because it has salt and vinegar in it.  It's also made from tomatoes, which are really acidic, too, so that's why it cleaned the penny so well.  Cola has lots of acidic ingredients added to it, which is why it cleaned the penny and it's why you should brush your teeth after drinking it, because acid can hurt your teeth and cause cavities.  Water and dish soap aren't very acidic at all, so the dark coating of copper oxide didn't dissolve and the penny stayed dull.

You can try this experiment at home with lots of different things.  Try soaking the penny in different kinds of juices or even coffee or tea.  There are so many possibilities and the best part is that you'll have lots of cool shiny pennies when you're done.  Squeaks, what do you say we clean up the rest of our pennies for our collection?  

Thanks for joining us for today's experiment and thanks to Google Making Science for helping us make this episode.  Do you have any questions about today's experiment?  Let us know.  Leave a comment down below or send us an email to  Thanks for joining us and we'll see you next time here at the fort.