Q: Why can I use old and new pennies for this project?
A: In 1982, the US Mint changed the composition of the penny from 95% copper to 97.5% zinc with only a thin copper plating (2.5%). In this project, the reactions that occur only affect the surface of the penny. Since the surface is copper for both old and new pennies, both old and new pennies will work well in this project.
Q: Why do I have to use copper test tablets?
A: The water and vinegar test tubes you prepared in this project serve as control solutions, which means that they show you what color the water and vinegar solutions are before copper from the pennies has been added. Without the control solutions, you would not be able to tell whether a sample actually has copper in it from the pennies or if there is contaminating copper from another source.
Q: Why did my pennies turn green after putting them in the vinegar?
A: Vinegar has a chemical called acetic acid in it. Acetic acid reacts with the copper oxide (the dirty stuff on the penny) as well as the copper of the penny itself. The acetic acid reacts with those copper substances to form another chemical, copper acetate, which is green in color. This process is actually why the Statue of Liberty is green!
Q: What if the pennies don’t look like they’ve changed at all after putting them in the vinegar?
A: There are a couple of reasons why the pennies might appear to be the same before and after putting them in the vinegar.
- The most easily visible change should be that the pennies get a bit cleaner from being in the vinegar. However, if you are using pennies that are already really clean, it's going to be difficult to see a large difference. It is still very possible that the copper of the penny reacted with the vinegar, which should be revealed using the copper test kit.
- It may take some time to be able to visibly see that the reaction has taken place between the copper in the pennies and the vinegar. After they are treated with vinegar, let the pennies sit on the paper towels for about an hour and then see if the pennies have changed.
- It is possible that that particular test just didn't work for some reason. This is why it is important to always repeat your procedure to have more data to corroborate, or back up, your claims. When repeating the project again, make sure that every step is followed carefully and see if the pennies look different!
If you have other questions about the procedure or need assistance troubleshooting your project or the Experimental Procedure, please post your question in the forum for this kit at Ask an Expert: http://www.sciencebuddies.org/science-fair-projects/phpBB3/viewforum.php?f=58. Our team of volunteer Experts is available to assist. We attempt to reply to questions within 24 hours. Please note that you will need a free Ask an Expert account in order to post questions.