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.
Understanding The Crushing Can Air Pressure Experiment
Pressure comes from countless moving gas particles hitting surfaces. Each collision gives a tiny push. In a room, air pushes on the outside of a can from every direction.
Gas inside the can pushes outward at the same time. When these pushes are balanced, the can keeps its shape even though the forces are large. The metal is thin, but it does not need to resist the full outside push by itself.
The gas inside provides most of the support. This balance of inward and outward pressure is the key idea behind the result.
The rapid change is important because water vapor behaves very differently from liquid water. A cloud of vapor contains particles spread far apart, so it fills the can. When those particles lose energy on cooling, many collect into liquid droplets.
Liquid water takes up far less volume. The can is not left with a completely empty interior. Some vapor remains, and some air may remain too.
Still, there are far fewer gas particle collisions with the inner walls. The water around the opening helps stop outside air from quickly flowing back in, which allows the pressure difference to develop.
A useful way to think about the size of the effect is that force equals pressure multiplied by area. Even a moderate pressure difference acting over the broad side of a can creates a strong inward force. The can usually does not shrink smoothly from all sides.
Thin metal can buckle. A tiny dent or weak spot bends first, then the changed shape makes that area easier to bend further.
This is why the collapse can happen suddenly and make sharp folds. Engineers must consider the same buckling effect when designing tanks, pipes, drink containers, aircraft parts, and vacuum equipment.
Several details can change the outcome of the experiment. Too little heating may leave too much ordinary air inside. Waiting too long before cooling lets air enter the opening.
Water that is not cold enough slows condensation. A can with thicker walls may resist buckling longer. Students should separate what they observe from what they infer.
The visible crumpling is an observation. The lower internal pressure is an explanation based on particle behavior and temperature change.
Record the water temperature, heating time, can type, and delay before cooling if comparing trials. Heating metal and handling hot water can cause serious burns, so an adult should control the hot can with suitable tools.
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.