Science Project: The Dancing Penny

The Dancing Penny

• The setup for the dancing penny experiment is simple, and the materials are inexpensive enough that each student in a classroom could do the experiment himself. Take a small, empty bottle and smear the rim with oil or water. Place a penny across the mouth of the bottle. It must completely cover the opening, with the water or oil forming a weak seal. The student then places his hands on the bottle, warming it. The penny eventually pops off the top of the bottle, releasing air from inside and appearing to "dance."

Younger Students

• For younger students, this experiment lends itself to learning about the scientific process. The experiment works better if the bottle is put into a freezer or refrigerator first. Students can find out if the penny "dances" more vigorously if the bottle is left in for 10, 20 or 30 minutes, what happens if the penny is dry, what happens if there is a hole in the bottle or any other variation. Try dividing students into groups and asking them to come up with a variable to see what factors influence the penny's behavior. The unmodified experimental setup is the control group that you compare your variations to.

Older Students

• For older students, the dancing penny is an interesting way to visually demonstrate some principles of how gases work. When you place your hands on the bottle, it warms the air inside. Heat causes air molecules to move more rapidly and causes gases to expand. The expanding gas builds up enough pressure to push against the penny, causing it to jump. This is why the experiment works better if the bottle is cold to begin with, because the cold bottle will have compressed air molecules inside. As soon as the bottle starts to warm up, the air expands and the results are visible for a longer time than if you use a room temperature bottle. In addition, this is why the penny needs to be wet. If the penny is not wet, there is no seal to break and the air does not force the penny upward. The air will merely escape in the gap between the penny and the lip of the bottle.

Egg in the Bottle

• Another classic experiment that works very similarly to the dancing penny is the egg in the bottle experiment. This is slightly more dangerous and works well as a demonstration. Light a small piece of paper and put it into the bottle. Wait a moment, then put a hard-boiled egg over the mouth of the bottle. The egg must be about the same size as the mouth of the bottle without being able to fall in. After a few seconds, the egg falls into the bottle. The flame heats the air molecules, causing the air to expand and escape the bottle. When the flame goes out, the air cools. This causes a partial vacuum, as there are fewer air molecules inside the bottle than there were before the start of the experiment. The pressure of the air outside the bottle pushes the egg inside. This is the same principle as the dancing penny experiment.