Not Your Usual Pop!

Students learn how to crush a can with only air pressure.

Learning Objectives

  • Students learn that air takes up space and exerts pressure.

Materials

For each demonstration:

  • One 12-once soda can (empty)
  • Water
  • One bowl of ice water approximately 6” deep or greater
  • One pair of tongs
  • One hot plate or electric burner
  • Safety glasses

Directions

  1. Have students stand or sit approximately five feet from the demonstration. Put on your safety glasses.
  2. Fill an empty 12-ounce soda can with one or two tablespoons of water.
  3. Place the soda can directly on a hot plate or electric burner until steam comes out of the can, the water boiling.
  4. Lift the soda can off the burner with tongs. Immediately immerse the can upside down into a large bowl of ice water. Make sure that the opening of the can is underwater. The can will crush.

Ask students the following questions

  1. What was the force that crushed the can? (Air pressure!)
  2. What happened to the air inside the soda can when it was heated? (Some of it escaped as it was heated and the molecules spread out.)
  3. What happened to the water inside the can when it was heated? (Some of it became a gas - water vapor.)
  4. Did anything escape from the can as it was heated? (Air and steam)
  5. What happened to water vapor inside the can when it cooled in the ice water? (It became liquid water.)
  6. What happened to the air inside the can when it cooled in the ice water? (The molecules slowed down and didn't take up as much space.)
  7. With less air and water vapor in the can, air pressure outside the can was greater than inside the can. Do you think this would work with other materials like plastic, thick metal, or wood? (The thin metal of a soda can is easier to crush than many other materials.)

 

Background

Although air is invisible, it still takes up space and has weight. In this experiment, when the air in the soda can is heated, the air inside the can will rise and some will escape. When the water in the can is heated, it begins to evaporate becoming water vapor, a gas. It fills much of the newly created space left by the escaping air. When the can is placed in the tub of ice water, the water vapor instantly condenses back into liquid water.

What takes the place of the water vapor and steam? Nothing! For a brief second, the inside of the can has much lower air pressure than outside. Air exerts 14.7 pounds of pressure per square inch at sea level (1 kg per square cm). If the air inside the can was at the same air pressure, then the can would remain intact. But with more pressure outside the can than inside, the can is crushed in seconds.

This demonstration works best when students have prior understanding that air and water are made of molecules, and understanding of how water evaporates and condenses.