Penny Chemistry, part 1

Way back in the 70's, when I was working at the Memphis Pink Palace Museum, part of our Kitchen Chemistry program involved using packets of ketchup to remove the tarnish from pennies. You take a dull, brown, tarnished penny and rub it with some ketchup. In seconds, the penny is bright and shiny. Usually, the experiment stops there, but I thought we might take a look to see why it works. To try this, you will need:

  • ketchup
  • water
  • vinegar
  • salt
  • potassium chloride (salt substitute)
  • 5 small cups or bowls
  • 6 or more tarnished pennies
  • labels and a marker

Safety Warning

Before you go wild with pouring different chemicals together, remember to keep safety in mind. For the stuff in your refrigerator and spice cabinet, you can pretty much mix whatever you want. Tuna fish and grape jelly may not be tasty, but it will not explode or burn off your fingers. Outside your refrigerator, you need to be much more careful. Cleaning supplies and other household chemicals can be harmful by themselves, and if the wrong ones are mixed they can be deadly. Only use them for experiments that specifically call for them.

Experiment

A good place to start is with the original experiment. Put a little ketchup onto one of the tarnished pennies. Let is sit there for about 30 seconds, and then rinse it. What you should find is that the tarnish has been removed from the part of the penny that was in the ketchup. OK, so that works just as well as it did back in the 70's.

Next, take a look at the ingredients for the ketchup. Besides tomatoes, you will notice that two prominent chemicals are vinegar and salt. A little internet research will show you many other science experiments that use vinegar and salt for doing the same thing as the ketchup. If you want to be sure that the tomatoes are not responsible for cleaning the pennies, try using some tomato sauce that does not contain vinegar or salt.

After some experimentation, you will probably find that the vinegar and salt are the important ingredients but are they both necessary? Lets find out. Start with four small cups. Put about an inch of water in one. That will be our control. The control does not contain any of the chemicals that we are testing. If it cleans the pennies too that would tell us that the reaction happens, even without the vinegar or salt. Label this cup "Control."

In the second cup, put about an inch of vinegar. Label this one "Vinegar."

In the third cup, put about an inch of water, and then add a teaspoon of salt. Give it a quick stir to dissolve the salt. Label this one "Salt Water."

In the fourth cup, put about an inch of vinegar, and add a teaspoon of salt. Give it a quick stir to dissolve the salt. Label this cup "Vinegar and Salt."

Now, you are ready to do some testing. Lets start with the Control. Dip one of the tarnished pennies halfway into the water, and hold it there for 30 seconds. Remove it from the water, rinse it, and put it beside the Control cup.

Do the same for each of the other cups. Be sure to give each 30 seconds, and be sure to rinse the penny to remove any vinegar or salt. Place each penny beside the solution you used to test it.

Results

OK, now what did you find? If your results were like mine, you found that neither the water, the vinegar, or the salt water did much, if anything to the pennies. The mixture of salt and vinegar was very effective at removing the tarnish.

So what is happening? The tarnish on the penny is copper oxide, and a chemical reaction with the vinegar will actually dissolve it. Then why did the pure vinegar not work? With the penny and the vinegar, you get a series of chemical reactions that form a circle. One reaction removes the copper, but just as quickly, another reaction puts it back. In chemistry, this is known as an equilibrium reaction.

The trick is to add something that will interrupt that equilibrium. You want a chemical that will grab the copper before it can be put back, and the table salt does a very good job of that.

What is it about the table salt that grabs the copper? Table salt is sodium chloride. When you put it in water, it separates into sodium ions (charged atoms) and chlorine ions, but is it the sodium or the chlorine that grabs the copper. An easy way to test that is with a different kind of salt. One of the common salt substitutes is potassium chloride. You can find it beside the regular (sodium chloride) salt at the grocery. In a fifth cup, put about an inch of vinegar and stir in a teaspoon of potassium chloride. Does it work the same as the table salt? If so, then it is the chlorine that grabs the copper. If not, then it is either the sodium, or the combination of sodium and chlorine.

You can look deeper into the vinegar as well. Will it work with other acids? Try using lemon juice (citric acid and ascorbic acid) or carbonated drinks (carbonic acid). Carbonated colas also contain phosphoric acid. Again, remember safety. Look for acids from your refrigerator and spice cabinet, not from other household chemicals.

Link to Penny Chemistry, part 2

Jack-O wrote on Fri, 08/30/2013 - 19:06:

i loved this experiment but i left the vinegar and salt out and then saw little clumps of something at the bottom. is it leftover copper??

please answer,Jack-O

rkrampf wrote on Fri, 08/30/2013 - 21:49:

Excellent job! Can you tell me more about your observations? What did the clumps look like? What color? Was it just vinegar and salt, or were there still some pennies in it? The more you can tell me about what you observed, the better the chance that I will be able to answer your question.

Jack-O wrote on Sun, 09/01/2013 - 21:19:

well the clumps are brown and I did leave some pennies in there (on accident). I even have it with me at the moment so I have a lot of detail. There is also brown dust at the bottom of the cup.

Oh and the pennies that got left in there looked just as dirty when I put them in, in fact I left it there for about 4 or 5 days, and I cleaned like, 50 pennies if that has anything to do with it. well that's about all i can observe from looking at them, can't wait to hear back! Jack-O

rkrampf wrote on Mon, 09/02/2013 - 00:07:

From your description, the lumps are copper oxide. The vinegar and salt block the equilibrium reaction to dissolve it from the pennies. I suspect that over time, that reaction has slowed, allowing the copper oxide to come out of solution.

Anonymous wrote on Mon, 09/26/2011 - 22:51:

My kids and I did both experiments. Afterward, I cleaned all of the pennies with the leftover vinegar and salt solution. One was being stubborn, so I left it in longer. Possibly I had messed with it a little too much. As an extension, we put it in a small amount of peroxide. Anyway, I noticed as I was putting things away that the penny in the solution was doing something we hadn't already noticed. There was a column of something rising from the center of the penny. Hydrogen? Metal? Something from the copper acetate? We did see some bubbles, too, but I'm not sure the column was bubbles.

rkrampf wrote on Tue, 09/27/2011 - 09:07:

The hydrogen peroxide changes the reaction, dissolving the copper much faster. Pennies are no longer solid copper. Instead they are made of zinc, and then copper plated. You probably dissolved a hole in the copper, letting the chemicals react with the zinc.

dunnjo wrote on Wed, 09/14/2011 - 08:37:

Hi ,
I get sent your free experiments via e-mail but ,recently I've noticed that I just get directed back to this website ? I much prefered it when you sent the actuall experiment . Why have you reverted to this way of sending them ?
THanks.

rkrampf wrote on Wed, 09/14/2011 - 20:57:

Linking to the experiments lets me use photos, which makes explaining things much easier. It also lets me update the information, correct typos, etc. You can still copy the text from the page, and paste it into a text document if you want to print it.

Michele wrote on Fri, 06/10/2011 - 01:27:

I read today that it is copper in swimming pools that attaches to hair (blond hair to be specific) and causes it to turn green. Is this true and do you think washing the hair in vinegar and salt will remove the green?

A young lady I know would be very happy to learn about how to remove her newly acquired hair color!

rkrampf wrote on Sat, 06/11/2011 - 15:59:

Yes, it is the copper that causes the greenish color. I don't think vinegar and salt would get it out, but I do know that they make special shampoos that contain chelating agents, chemicals specifically for removing the copper from your hair.

Anonymous wrote on Thu, 06/09/2011 - 21:04:

Can you tell me why, when you save the vinegar/salt/penny-cleaning liquid, after the pennies are out, and add aluminum foil to it, you get a precipitate that must be copper? When I filtered it out and left it on the counter, it turned green again, like the "vertigris" (learned that word from you)if you don't wash off the pennies. So it must have combined with oxygen and turned green like the Statue of Liberty. But what's the reaction? I teach homeschool science to elementary kids, and I've tried to find the answer to this question.

rkrampf wrote on Thu, 06/09/2011 - 22:35:

We will start with that explanation in part 2, and continue on in part 3.

Anonymous wrote on Thu, 06/09/2011 - 14:03:

can you please write the chemical equation for this? thank you

Anonymous wrote on Thu, 06/09/2011 - 13:13:

I showed my students how to "write on copper" by mixing a solution of salt and lemon juice, then painting the solution onto a piece of copper laying flat on a table. Let it react for a little while,. When you wipe it clean, your paint strokes will be visible in the copper surface.