Key Concepts
Chemical reactions, acidity, copper oxidation

Introduction

‘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.

Background

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. 

Materials

  • 1 set of measuring spoons 
  • 10 very dirty pennies (the dirtier the better!)
  • 2 tablespoons Baking Soda
  • Water
  • 4 teaspoons salt
  • 2 tablespoons White Vinegar
  • 2 tablespoons freshly squeezed Lemon Juice
  • 2 tablespoons Ketchup
  • 2 tablespoons Tabasco Sauce (or other hot sauce)
  • 11 plastic cups (small Tupperware containers or bowls will work as well)
  • Paper
  • Pen or pencil
  • Dishwashing gloves
  • Permanent marker
  • Clear tape
  • A timer or clock 
  • Paper towels
  • Access to a sink
  • Camera (optional)

Preparation

*Note: This activity works best if you use 10 pennies that are equally dirty. 

  1. Make a paste from your baking soda and water. Pour ¼ cup of baking soda into one of your plastic cups. Slowly add water, 1 tablespoon at time, until you’ve created a paste.
  2. Use your permanent marker to label your remaining plastic cups like this: Ketchup 1, Ketchup 5, Tabasco 1, Tabasco 5, Vinegar 1, Vinegar 5, Lemon 1, Lemon 5, Control 1, Control 5. If using bowls or Tupperware, write your label on clear plastic tape and gently attach it. 
  3. Put 1 tablespoon Ketchup into each ‘Ketchup’ cup. Put 1 tablespoon Tabasco into each ‘Tabasco’ cup’. Put 1 tablespoon of Vinegar and 1 teaspoon of salt into each ‘Vinegar’ cup. Put 1 tablespoon of Lemon Juice and 1 teaspoon of salt into each ‘Lemon’ cup. Leave the control cups empty.
  4. Use your paper and pencil to create a table like the one below. Make each cell (square) of the table large enough to place your penny in it after your testing is complete. 

 

Cleaning Method

1 minute

5 minutes

Ketchup 

   

Tabasco Sauce

   

Salt & Vinegar

   

Lemon Juice & Salt

   

Control

   

Procedure

  1. Place a penny on each square of your table, heads side up. If you have a camera, take a picture of the pennies. If you don’t have a camera, look closely at the color of each penny. This will allow you to make a better judgment of how well each method cleaned, by looking at the pennies before and after their treatment.  What do you notice about each penny? Can you clearly see the date on the penny, or is it too dirty? Can you clearly see Abraham Lincoln?
  2. Start with the cups labeled 1. Line these cups up in a row. Take the penny from the ‘Ketchup,1 minutes’ square of your table, and place it in the ‘Ketchup 1’ cup. Gently swirl the cup so that the penny is completely covered with Ketchup. 
  3. Start a 1 minute timer. 
  4. Place each of the pennies in the 1 minute column into their corresponding cup. In each case, make sure the penny is completely covered by the contents of the cup.
  5. Place your ‘Control, 1 minutes’ penny into the ‘Control 1’ cup. Do not add anything to the cup. This is your control. 
  6. Put on your dishwashing gloves. 
  7. When 1 minute has passed, rinse each cup with water until the inside of the cup and the pennies are clean (be careful not to lose your penny!). Keep the pennies in the cup.   
  8. Once all of the pennies have been rinsed, use your fingers to gently scrub the pennies one at a time with your baking soda paste (keep your gloves on for this part!). Scrub each penny for 10 seconds, then rinse it again with water and place it back into its cup. 
  9. Set these cups aside for now. 
  10. Repeat steps 2-8 with the pennies in your 5 minutes column. Start by placing the ‘Ketchup, 5 minutes’ penny into the cup labeled ‘Ketchup 5’. Again, make sure the penny is completely covered by Ketchup. 
  11. Start a timer for 5 minutes. 
  12. Continue to place each penny in the 5 minutes column in it’s corresponding cup. 
  13. Once all the pennies are in the correct cup, use the remaining time to observe the pennies, and the contents of the cup. Do you notice any changes in the appearance of the pennies? What is changing? Do you notice any changes in the contents of the cups – is anything bubbling, or changing color? 
  14. Once 5 minutes has passed, rinse each cup with water until the inside of the cup and the pennies are clean (be careful not to lose your penny!). Keep the pennies in the cup.   
  15. Once all of the pennies have been rinsed, use your fingers to gently scrub the pennies one at a time with your baking soda paste (keep your gloves on for this part!). Scrub each penny for 10 seconds, then rinse it again with water and place it back into its cup. 
  16. Go back to the table you drew. Use your paper towels to gently dry off each penny, then place it in the corresponding square of your table. 
  17. Compare the 1 minute and 5 minute pennies in each row. How does time affect the cleanliness of the pennies? Was 1 minute or 5 minutes more effective in cleaning the pennies overall? Can you think of why time would be important? Were there any cleaning methods where the time didn’t matter, so the 1 minute and 5 minute pennies are equally clean? Why do you think this might be? 
  18. Compare all of the pennies to the control pennies. Which penny is the cleanest, compared to the control? Which is the least clean? 
  19. If you took a picture in Step 1, go back and compare each penny’s cleanliness before and after being cleaned. Which penny is the cleanest, compared to its before picture? Which is the least clean?

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

Credits

Teisha Rowland, PhD, Science Buddies

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Key Concepts
Chemical reactions, acidity, copper oxidation
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