The crushing can experiment shows that air is not empty space, because it can push with a strong pressure in all directions. In this project, a small amount of water is boiled inside an aluminum can, then the can is quickly flipped into cold water by an adult helper. The can suddenly crumples because the pressure outside becomes much greater than the pressure inside.
This makes a dramatic classroom demonstration of air pressure, temperature, and changes of state.
When the water boils, liquid water changes into steam and pushes much of the original air out of the can. When the hot can is turned upside down into cold water, the steam cools and condenses back into liquid water. This leaves fewer gas particles inside the can, so the inside pressure drops quickly.
The higher outside air pressure then squeezes the thin aluminum walls inward.
Key Facts
- Air pressure at sea level is about 101,000 Pa.
- Pressure is force per area: P = F/A.
- Heating water makes steam, which takes up much more space than liquid water.
- Cooling steam makes it condense into liquid water, reducing the amount of gas inside the can.
- The can crushes when outside pressure is greater than inside pressure: Poutside > Pinside.
- Always use an adult helper, heat-safe tongs, goggles, and a clear workspace when heating the can.
Vocabulary
- Air pressure
- Air pressure is the push caused by tiny gas particles colliding with surfaces.
- Steam
- Steam is water vapor formed when liquid water gains enough energy to become a gas.
- Condensation
- Condensation is the change from a gas to a liquid when particles lose energy.
- Atmospheric pressure
- Atmospheric pressure is the pressure caused by the weight and motion of the air around Earth.
- Pressure difference
- A pressure difference happens when one side of an object has more pressure pushing on it than the other side.
Common Mistakes to Avoid
- Thinking the can is crushed by the cold water, which is wrong because the main crushing force comes from the higher air pressure outside the can.
- Forgetting to boil the water long enough, which is wrong because enough steam must form to push most of the air out of the can.
- Waiting too long before flipping the can into cold water, which is wrong because air can leak back in and reduce the pressure difference.
- Trying the experiment without adult help or safety gear, which is wrong because the hot can, steam, and burner can cause burns.
Practice Questions
- 1 A can has a surface area of 0.030 m2 exposed to outside air. If outside air pressure is 101,000 Pa and the pressure inside is very low, what approximate force does the outside air exert on that area? Use F = PA.
- 2 If the outside pressure is 101,000 Pa and the pressure inside the cooled can is 30,000 Pa, what is the pressure difference pushing inward on the can?
- 3 Explain why the can crushes only after it is flipped into cold water, even though it was already hot before being flipped.