Simple Science: Let’s Clean Pennies! *

Not This One. It's Too Clean.

*Note: Cleaning Pennies is not at all the same as Laundering Money.  I just thought I’d make that clear.

As of 4 February 2013, the government of Canada phased out the penny from Canadian coinage. Now, when you pay or receive change in cash, the amount is rounded up or down to the closest five cents’ worth (amounts ending in 1 or 2 go down to 0, 3 or 4 go up to 5; 6 or 7 down to 5; and 8 or 9 go up to the next ten.) The Canadian Mint has estimated that it will save about $11 million dollars per year by eliminating the penny.

There are still plenty of pennies around, of course — one estimate puts the number of pennies still in circulation within Canada as some 35 billion.  And here’s a simple science experiment that you can do at home that uses the lowly penny to explore the concepts of atoms, molecules, electrons, and chemical compounds.

That's Better.

That’s Better.

What You’ll Need:

  1. A handful of pennies (the grungier the better — you don’t want shiny new coins for this experiment)
  2. White vinegar
  3. Table salt
  4. A non-metallic bowl
  5. 2 shiny new steel nails, if you can find them around the house

Using a glass or plastic bowl, (ideally a clear bowl, so that you can see everything better), mix 1 teaspoon of table salt into 1/4 cup of white vinegar. Stir it till it’s dissolved.

The Dirty Pennies

The Dirty Pennies

Hold a penny so that it is submerged half-way in the solution while you count to 10, then pull it out. What do you see?

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After 10 Minutes. Notice the Penny That’s Only Halfway in the Vinegar?

Now take the rest of the pennies and dump them into the bowl of vinegar solution. Leave them for about 10 minutes. Do they look different?

Scoop out about half of the pennies and rinse them under running water (use a colander or sieve so you don’t drop any down the drain), then lay them on a paper towel to dry.  Write “rinsed” on the paper towel with a marker. Fish out the remaining pennies, but don’t rinse them. Just put them onto another sheet of paper towel (you can mark that one “unrinsed”, if you want to). Leave the two groups of pennies to dry for about an hour.

Meanwhile, drop one of the nails into the same bowl of vinegar solution that you just took the pennies out of, so that it is completely submerged. Prop up the other nail in the bowl so that it’s half-in, half-out of the vinegar. Leave those for 10 minutes, just like the pennies. How do the nails look? And what’s going on with those pennies?

What Is Going ON?

The short version:  everything is made up of tiny bits called atoms. Pennies are made using an metal called copper, which is made up of atoms of copper. Copper atoms combine with atoms of oxygen in the air to form a compound called copper oxide, which has a greenish colour.  As pennies (or anything else made of copper) are used, handled, and exposed to the air, the surface of the penny becomes coated with copper oxide.  Copper oxide can be dissolved using a weak acid solution, like the salt-and-vinegar that you used to clean the pennies. This is a chemical reaction.

It's Not Easy Being Green

It’s Not Easy Being Green

Take a look at the pennies on their paper towels.  The rinsed pennies should still be bright and shiny — but what about the pennies that were not rinsed? The copper atoms on the surface of the unrinsed pennies have joined together with the atoms of oxygen in the air and the atoms of chlorine in the salt to form a compound called malachite, which is a distinctive blue-green colour. The surface of the Statue of Liberty, pictured above, is actually made of copper plates which have naturally weathered to the distinctive blue-green through the surface formation of malachite.

Of course, if you leave the pennies on the paper towels for long enough, even the rinsed ones will start to show signs of oxidation, and they’ll get all grungy-looking again. How long do you think it would take for the shiny, cleaned pennies to turn all brown and dirty again?

What About the Nails?

When the acidic vinegar-and-salt dissolved the copper oxide (the compound that originally made the pennies look dirty), it left atoms of copper floating in the solution in the bowl.  These copper atoms have what is known as a positive charge, since they left two of their negatively-charged electrons behind when the molecular bonds of the copper oxide were broken.

Back to the steel nail. Steel is a mixture of the element iron, carbon, and some others. The acidic vinegar solution also dissolves the chemical bonds in the steel a bit, so that there are iron atoms floating around in the solution. Like the copper atoms, these are also positively-charged ions, having left negative electrons behind.

So, then, the steel nail has a net negative charge, and the ions of copper and iron in the vinegar solution have a net positive charge — and you know what they say! Opposites do attract.  The copper ions are more strongly attracted to the nail than are the iron ions, and so a film of copper is deposited onto the nail.  Neat!

Summary:

  • Suggested ages: 7 years and up
  • Time required: approximately 1 hour
  • Experiment cost: less than $1 — and that includes the pennies!
  • Science knowledge required: minimal
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