Shine Bright Like a….Penny!
‘See a penny pick it up, all day long you’ll have good luck!’ – maybe you’ve heard this phrase before, and maybe you’ve even stopped to pick up a lucky penny off the sidewalk. But sometimes those pennies you see on the ground look anything but lucky. They look brown or black, and sometimes they’re so dirty looking that you can’t even tell whether they’re pennies!
In this activity we’re going to explore why pennies don’t stay bright and shiny, and test different methods to bring the shine back!
This activity is not appropriate for use as a science fair project. Good science fair projects have a stronger focus on controlling variables, taking accurate measurements, and analyzing data. To find a science fair project that is just right for you, browse our library of over 1,200 Science Fair Project Ideas or use the Topic Selection Wizard to get a personalized project recommendation.
All pennies start out the same color – beautiful and bright copper. In fact, new pennies are so pretty that it’s considered a compliment to say that something is as ‘shiny as a new penny’. But somewhere along the way, those pennies lose their luster. What happens?
The answer is pretty simple – before 1982, pennies were made almost entirely from copper, and modern pennies are made of copper-plated zinc. While copper is a beautiful metal, it is also reactive. The negatively charged oxygen atoms in our air are attracted to the positively charged copper atoms in the penny. When oxygen binds with copper, they form a new molecule known as copper oxide. Copper oxide is brownish or sometimes black in color (depending on other things in the penny’s environment). This is why most pennies you see look dirty or tarnished, it’s not actually dirt, but copper-oxide that makes them look so dull.
Luckily, there are a number of ways to clean pennies and make them look bright and shiny again, using chemistry! In this activity, we’re going to test a few different methods for cleaning pennies, and determine which gets our pennies the cleanest.
*Note: This activity works best if you use 10 pennies that are equally dirty.
Extra: Experiment with even longer periods of time with each of the cleaning methods. What is the optimal time to leave the pennies in the cleaning solution?
Extra: Test whether these cleaning solutions work on other types of coins. What do your results tell you about the special properties of copper pennies?
Extra: Can you think of other safe household acids you could test? What about soda? Oranges?
Observations and Results
In this activity you experimented with 4 different cleaning methods to determine which was the most effective at cleaning pennies. You should have observed that each cleaning method made the pennies cleaner, but some methods may have been more successful than others. Which method works best is determined by the amount of acid present, and the presence of salt. Different brands of Ketchup and Tabasco or hot sauce will have different amounts of salt and acid in their ingredients, so your results may be different than someone else doing this activity at their house. However, in each case, the cleaning method that worked best for you is the one that has the highest concentration of acid.
As they are exposed to the environment, pennies become coated with a layer of copper-oxide, making them look tarnished (dull, brown and dirty appearance). Copper oxide dissolves in a mixture of weak acid and table salt. Vinegar and lemon juice are both acids. Check the ingredient list for your Ketchup and Tabasco sauce – some of the labels might list vinegar or citric acid, while others might say tomato puree. Tomatoes contain naturally occurring citric acid, another acid that is generally slightly weaker than vinegar or lemon juice. Therefore, each cleaning method you tested contained acid and salt, but the Ketchup and Tabasco sauce may have had a slightly weaker acid. In this case, you may have noticed that the Ketchup and Tabasco pennies were not quiet as well-cleaned as the pennies that were in the vinegar and lemon juice.
You also may have noticed that the longer the pennies stayed in the cleaning solution, the cleaner they became! The chemical reaction that dissolves the copper oxide on your pennies is an ongoing process. Leaving your pennies in the cleaning solution longer gives the reaction more time to continue, and gives you shinier pennies!
More to Explore
Teisha Rowland, PhD, Science Buddies
Science Buddies |
Chemical reactions, acidity, copper oxidation
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