Clean Coins
What is the difference between a dirty coin and a clean coin?
That may seem like a simple question, right? It is actually pretty interesting when you find out more! Maybe it's not just dirt on that coin.
Materials Required
• Old, dirty pennies
• Vinegar
• Salt
• Containers
• Paper towels
Procedure to clean pennies
1. First, examine the surface of an old penny. Why dos it appear to have a different color than a new penny? Why do old pennies all have similar color and appearance?
2. Using vinegar and a little salt, wash the dirty pennies.
3. Dry off your cleaned pennies. How has the surface changed?
Question 1. What do you think the vinegar and salt did to the surface of the coin?
Question 2. How deep was the "dirt" on the penny if the chemicals seemed to remove it?
Discussion
Old pennies are made of copper. When copper is exposed to air, it combines with oxygen. It forms Copper Oxide. Pure Copper Oxide is black in color.
If your penny is old, it can develop a thin film of copper oxide on the outside. This very thin black layer makes the coin look dull. Your penny has undergone a chemical reaction.
If you try to wash your penny with just water, nothing happens. Water won't make it "clean" because the dirty look doesn't come from dirt, but rather a chemical change of the copper. If you add a mild acid, like vinegar, the copper oxide can dissolve. The salt makes this reaction go faster.
Extensions
Sometimes pennies can turn green
If you take a copper penny and put it on a paper towel soaked in vinegar they will turn green. It may take some hours or days, but they will change. Mild acids can work to clean a penny, but when they are exposed to water and carbon dioxide and oxygen at the same time they form a different chemical composition called malachite. Malachite can be found in nature as a mineral:
Copper can combine with several other elements and compounds to form different minerals. Here some examples:
Copper and sulfur form Copper Sulfate. You can see this in some commercial products that are advertised as root killers.
Copper and Chlorine can combine to form Copper Chloride.
When copper oxide is heated, it takes on more oxygen. It also changes color:
