Why pennies are brown

When you cut an apple in half and expose its insides to air, you will notice that parts of the inside will turn brown. All these phenomena are simply the results of oxidation. This is also the same explanation why pennies turn brown in the long run. Copper does not generate any tarnish nor does it emit something. Oxidized copper will turn green in color and form patina. Patina is a coating of different compounds that serve as a protection to copper rather than corrode the metal.

In simple terms, aside from oxidizing copper attracts minerals from the dirt and everything around it. This happens when people handle the coins. As they are exposed to the environment, pennies become coated with a layer of copper oxide, making them look tarnished with a dull, brown, 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 whereas others might say tomato puree. Tomatoes contain some naturally occurring citric acid, another acid that is generally slightly weaker than vinegar or the amount of citric acid in 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 might have noticed the ketchup and Tabasco pennies were not quite as well cleaned as the pennies that were in the vinegar and lemon juice. You also might have also noticed the longer the pennies stayed in the cleaning solution, the cleaner-looking 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!

This activity brought to you in partnership with Science Buddies. Already a subscriber? Sign in. Thanks for reading Scientific American. Create your free account or Sign in to continue. See Subscription Options. Go Paperless with Digital. Materials Set of measuring spoons 10 very tarnished pennies The more tarnished the better—and the activity works best with pennies that are about equally tarnished.

Two tablespoons of baking soda Four teaspoons of salt Two tablespoons of white vinegar Two tablespoons of freshly squeezed lemon juice Two tablespoons of ketchup Two tablespoons of Tabasco sauce or other hot sauce 11 plastic cups Small Tupperware containers or bowls will work as well. Paper Water Pen or pencil Dishwashing gloves Permanent marker Clear tape A timer or clock Paper towels Access to a sink Camera optional Preparation Make a paste from your baking soda and water: Pour one quarter cup of baking soda into one of your plastic cups.

Use your permanent marker to label your plastic cups as described below. If using bowls or Tupperware, write your label on clear plastic tape and gently attach it. Use your paper and pencil to create a table with three columns and six rows that lists your five test substances ketchup, Tabasco sauce, vinegar, lemon juice and control in a column on the left.

Do you have a hypothesis about which substance will best clean a tarnished penny? Procedure Place a penny on each square of your table, heads side up. If you have a camera, take a picture of the pennies. 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? Gently swirl the cup so the penny is completely covered with ketchup.

Start a one-minute timer. Put on your dishwashing gloves. You can watch them change for the first few seconds. After that you won't see anything happen. After 5 minutes, take half of the pennies out of the liquid. Put them on a paper towel to dry. Take the rest of the pennies out of the liquid. Rinse them really well under running water, and put them on a paper towel to dry. Write "rinsed" on the second paper towel. Put a nail and a screw into the liquid.

Lean another nail against the side of the bowl so that only part of it is in the liquid After 10 minutes, take a look at the nails. Are they a different color than they were before? Is the leaning nail 2 different colors?

If not, leave the nails in the bowl and check on them again in an hour or so. What's happening to the screw? You may see lots and lots of fizzing bubbles coming from the threads. Is the screw changing color? Leave it in the liquid for a while and see what happens. After about an hour, look at the pennies on the paper towels. What's happened to the ones you rinsed?

What's happened to the others? What color is the paper towel under the unrinsed pennies? Everything around you is made up of tiny particles called atoms.

Some things are made up of just one kind of atom. The copper of a penny, for example, is made up of copper atoms.