Effect of air pressure

Demonstration

A can collapses when evacuated.

Apparatus and Materials

• Polythene or PET drink bottle or large can (5-litre size) (see Technical notes)
• Vacuum pump
• Rubber tube and bung to fit bottle or can
• Length of pressure tubing, 1 m

Health & Safety and Technical Notes

Wear eye protection and use a safety screen to protect observers.

Take care when fitting the glass tube through the bung.

Read our standard health & safety guidance

Care is needed in choosing a can because the original contents of the can may damage the pump. A 5-litre olive oil or vegetable oil can is suitable, but try to ensure that it is clean and dry before use.

Procedure

1. Fix a rubber bung with a glass tube through it in the neck of the bottle or can.
2. Using pressure tubing, connect the bottle to the pump.
3. Switch on the pump. The air should be removed from the bottle (or the can) so that it collapses.

Teaching Notes

• Using a soft plastic bottle shows the effects of removing the air, as it collapses. When the bottle is opened air rushes in and the bottle is restored to its normal shape. This demonstration is normally done to show the effects of air pressure when the pressure on one side is reduced.
• Evacuating an oil can shows what happens when the metal is unable to return to its original shape.

This experiment was safety-tested in July 2007