Try This: Cleaning Pennies

Some chemical reactions can take dirty objects and make them clean by removing the dirt. Detergents do this with the dirt on your clothes and your dishes, and the soap you use in the bath or shower does the same for your body. But what about metals? They are harder to clean.

Fun Facts

Oxides are what metals produce when they encounter oxygen. The most famous oxide we know is iron oxide, or rust.

Coins circulate for about 25 years on average.

QUESTION

How do you clean a penny?

MATERIALS

Vinegar

Glass jar

Dirty pennies

1 teaspoon salt

What size T-shirt do you buy for a 200-pound egg?

Eggs-tra Large!

Science Online

For a fun look at how money is made and how much money is made, check out the U.S. Mint's Web site at www.usmint.gov.

PROCEDURE

  • Pour vinegar into the jar until it is about half full.

  • Stir in salt until it dissolves.

  • Drop several dirty pennies into the vinegar.

  • After a few minutes, take out half of the pennies and lay them on a paper towel to dry.

5. Remove the other pennies and rinse them with water before letting them dry.

6. Note the differences between the two groups of pennies after they have been out of the vinegar/salt solution for a while.

WHAT'S HAPPENING

The vinegar/salt solution is able to loosen the residue on the pennies, which is called copper oxide. With this residue removed, the pennies are shiny once more. When you rinse them off, the cleaning stops and they remain shiny. The unrinsed pennies still have some of the solution on them, and when oxygen in the air hits them, a new reaction occurs, turning the pennies a bluish-green color.

FOLLOW-UP

Try this same experiment with nickels, dimes, and quarters. Do you get the same results?2

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Acid Bath

Oops! This young scientist tried to clean his friends' copper ID bracelets. But the acid was too strong and it removed part of each letter! Question: Can you add the missing lines to complete the letters and see to whom each bracelet belongs?

KIDS' LAB LESSONS

QUESTION How are metal things made shiny?

EXPERIMENT OVERVIEW Electroplating is the process of taking a metal and using it to coat something else. It is a complex process involving electricity and is difficult to perform in a home laboratory. This experiment is not electroplating in the true sense, but it does produce a transfer of copper from the pennies to the nails.

SCIENCE CONCEPT It's possible to remove atoms of a material (like copper) and have them float around in a liquid without you seeing them. To retrieve them, you simply need to make them want to attach to the metal you want to coat. In this case, the vinegar/salt solution removes copper oxide atoms (the “dirt” on the pennies), and when they dissolve, they form copper atoms that want to attach to another metal. The nails attract the copper atoms floating in the water, and the atoms stick to them, coating them with copper.

MATERIALS

Vinegar/salt solution prepared as described in the “Cleaning Pennies” activity on page 40

Two clean nails or metal paper clips

Dirty pennies

electroplating: a process that uses one metal to coat another metal.

PROCEDURE

  • Prepare a solution of vinegar and salt as described in the previous activity.

  • Soak the dirty pennies in the vinegar/salt solution until they are clean.

  • Remove the pennies and set them aside.

  • Place your nails into the remaining solution and let them sit for a few hours.

  • When you are ready to remove the nails, remove them carefully and look closely to see if they have changed color. If they have not done so noticeably, return them to the solution. If you want to speed up the process, place more dirty pennies in the solution along with your nails.

QUESTIONS FOR THE SCIENTIST

  • What is the coating on the nails? ______________________________________________

  • Why can't you see that coating in the solution before you place the nails in it?

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CONCLUSION

You are observing a pretty amazing process. The acid solution not only removes the dirt (which is a combination of copper and oxygen called copper oxide), but in doing so, leaves copper atoms floating in the solution. These atoms are so small you can't see them, but they will float in the acid solution until they find a negatively charged metal to stick to. When you place the nails in the acid, some of their atoms are also removed and they are left with a negative charge. The copper atoms are attracted to the nails and stick to them, giving them a slight copper tint.

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