By Mark W. Meszaros, PhD
Vice President, Carolina Biological Supply Company
What unseen force can instantly crush a can as it is being submerged into water? The answer is all around us—air pressure. Use this surprising and entertaining demonstration to introduce the concept of pressure.
National Science Education Content Standards
Grades K–12, Unifying Concepts and Process
- Evidence, Models, and Explanation
Grades 5–8, Physical Science
- Motion and Forces
Grades 9–12, Physical Science
- Structure and Properties of Matter
- Motion and Forces
- Conservation of Energy and Increase in Disorder
Use heat-resistant tongs to pick up the hot can. The hot can and steam may cause burns; handle with care. Wear protective safety eyewear.
- Aluminum Beverage Can, 12 oz
- Bunsen Burner or Other Heat Source
- Heat-Resistant Tongs
- Plastic Bowl, Tray, or Terrarium
- Rinse out several empty 12-oz aluminum beverage cans.
- Fill the plastic container almost to the top with tap water.
- Clear off the area around the plastic container to avoid getting anything wet.
- Set up and light the Bunsen burner or other heat source.
- Add 10–15 mL of tap water to the aluminum beverage can.
- Heat the can until the water begins to boil and condensed water vapor can be seen coming out of the can opening.
- Continue heating the can for another minute.
- With tongs, pick up the can, quickly invert it, and plunge it into the plastic container of water.
- The can will be instantly crushed when its opening is submerged.
- Repeat the demonstration and ask your students why the can is crushed so quickly and thoroughly.
- Your students may think that air is very light and that air pressure cannot be very strong. In fact, at sea level, air pressure is very strong, pushing against everything at 14.7 lb/in2.
- Your students may think that steam is the visible droplets coming out of the beverage can. In science discourse, “steam” refers to water in the gas phase, which (like many gases) is clear, colorless, and invisible. What is visible above the can is the mist of water droplets formed as the escaping gas cools and condenses (much as a visible cloud condenses from gaseous water in the atmosphere). In science discourse, both “steam” and “vapor” refer to gases, as opposed to visible clouds. “Vapor” may refer to any gas, whereas “steam” refers only to water vapor (in particular, to hot water vapor). As with any terms frequently used in both scientific and everyday conversation, make sure that your students understand the underlying science well enough so that they can clarify their intended meaning, if need be.
This activity demonstrates the power of atmospheric pressure and how a pressure change inside a container may cause a dramatic result.
Atmospheric pressure, commonly called air pressure, is 14.7 lb/in2 or 101 kPa at sea level. When any container, such as the aluminum beverage can, is open to the atmosphere, the pressure outside the can equals the pressure inside the can. When the water in the can is heated to boiling, the can fills with hot air and steam. As long as the can is open to the atmosphere, the gas pressure inside the can is the same as atmospheric pressure.
When the can is inverted and placed in the cold water, the steam inside the can condenses into liquid water, which has much less volume than steam. The water immediately seals the opening of the can to the atmosphere creating a closed system, and the pressure inside is greatly reduced. The pressure outside the can remains 14.7 lb/in2, and the pressure difference crushes the can.