Bright and Shiny Pennies

Nothing's nicer than a shiny new penny, but it doesn't take long for those pennies to become old and dull.  However, by following the simple easy steps below, you can make your old pennies shiny and like new again...fast!  You can also learn a little science while you're doing it.

What you need:  A few old pennies; 1/4 cup white vinegar; 1 teaspoon table salt (sodium chloride); a non-metal bowl; paper towels.

What you do:  Combine the salt and vinegar into the bowl and stir well.  Place the pennies in the bowl for approximately 30 seconds. Don't leave them in there too long (or do, and see what happens). Remove the pennies, rinse them with water and dry.  In less than a minute, you've gone from old dull penny to new shiny penny...now where's that gumball machine?

How does it work?  Pennies are made from the metal copper.  Over time, the copper reacts oxygen in the air to form a chemical compound called copper oxide on the face of the penny.  Copper oxide gives old pennies that dull, greenish color.  Vinegar is an acid which reacts with copper oxide, causing it to dissolve away from the penny.  The salt increases the solubility of copper oxide, which means it allows more copper oxide to dissolve in the vinegar solution.  If the pennies are left for too long, a light blue-green color will form on their surface.  This compound is called malachite, and it is caused by the reaction between copper, oxygen, and vinegar.  So, just the right amount of time in the vinegar will give you that shiny penny you desire.

This is a great activity for pre-K up to 5th grade students.  The chemical change at the surface of the penny is striking, and it happens so quickly that it is impressive.  Put some pennies in water for a comparison experiment that doesn't do anything.  Leave another set of pennies in a vinegar solution for longer to see the formation of malachite.  Thanks to Patricia Wong of U.T. Arlington for contributing this activity to DISCUS.  Patricia cites a chemistry blog by Anne Marie Helmenstine on about.com for inspiration for this activity.  That blog entry can be found here.