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:
- potassium chloride (salt substitute)
- 5 small cups or bowls
- 6 or more tarnished pennies
- labels and a marker
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.
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.
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.