Penny Cleaning Science

Penny cleaning science projects are fairly popular at grade school science fairs. The reason is that the chemistry is pretty interesting and the project is easy to do. Basically, the experimenter tries to find out which solution will clean pennies best, eliminating the copper oxide that gives the surface a tarnished appearance.

Penny Cleaning Science Projects Work

To attempt the experiment, the budding scientist needs to gather at least 10 pennies that have a gray, green or brownish coating on the copper. Pennies made before 1982 work best, since they are made of solid copper. Newer pennies are made of zinc with a thin coating of copper on top. Luckily, it’s also the older pennies that are more likely to be dirty. (This is not to say that newer pennies won’t work at all. There’s no need to make a special trip to the bank.)

Once the child has a good pile of pennies, he or she should collect the cleaning solutions to use during the experiment. Most of these substances are not usually used for cleaning, which is what makes the process fun. Common contenders are:

• Soap and water
• Household bleach
• Lemon juice
• Vinegar and salt
• Coca Cola
• Ketchup or taco sauce

Forming a Hypothesis

Before beginning the actual test of penny cleaning science, the experimenter needs to form a hypothesis as to what he or she thinks will clean the pennies fastest or most thoroughly. At first glance, many think the soap and water or the bleach would perform the best, since they are actually meant for cleaning things. However, if the child knows more about the chemical composition of the copper before making an educated guess, it may change his or her opinion. The paragraph below will help.

About the Tarnish

The tarnish on pennies is actually not tarnish at all. It is simply a reaction between the copper in the penny and the oxygen in the air. When the two elements react with one another, they form copper oxide. To get rid of this dull and often greenish substance, an acid needs to bond with the oxide and effectively dissolve it. Hence, cleaning the pennies with a highly acidic solution will reveal the shiny copper that was there all along, letting it again have the chance to show through.

The Experiment

Now that the experimenter has a hypothesis, it’s time to start the experiment. He or she should put each solution in a separate bowl or Petri dish and dip one to two pennies in each and let them sit for five to ten minutes. Then the child should rinse the pennies with water and place them on a paper towel to dry. Observers may be surprised to learn that while soap and water do little to nothing to improve a penny’s appearance, the taco sauce can cause a vast improvement. The experimenter, however, will know better, since taco sauce contains highly acidic vinegar.

Documenting the Results

The best way to record the results of the penny cleaning science experiment is to take a photograph of each penny and label it with the solution used. The results for each penny will be noticeably different because of the variations in the strength of each acid. Some children like to declare ‘’a winner’’ and place it prominently in the middle of their science project display board. Others will prefer to take the actual pennies to school instead of just the pictures.

For a really dramatic effect, children can take a couple leftover pennies and dip half of each coin in the vinegar and salt solution while leaving the other half as is. This will give them a penny that is half dirty and half clean, which is a pretty cool example of the miracle of chemistry!.

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