Use this famous experiment to illustrate the changes in density of gases and liquids when compressed

This is an experiment named after René Descartes (1596–1650). Descartes was a French scientist and philosopher. The Cartesian diver can be used to illustrate the behaviour of gases and liquids when compressed. In this experiment, a Cartesian diver is constructed and some of the properties observed.

## Equipment

### Apparatus

• Plastic disposable pipette, 5 cm3
• Hex nut, 11 mm across face to face
• Clear plastic soft drink bottle, 2 dm3, 1.5 dm3 or 1 dm3
• Plastic beaker, 250 cm3
• Scissors

• Water

## Procedure

1. Screw the hex nut onto the base of the pipette until it is held tightly in place.
2. Cut off all but 1 cm of the pipette stem. (This is the diver.)
3. Place the diver in a beaker of water. Squeeze the bulb of the pipette to force air out and release to allow water up into the diver. Repeat this until the diver is about half full of water.
4. If adjusted properly, the diver should barely float in the water. If it sinks, squeeze a little water out.
5. Carefully transfer the diver to the soft drink bottle that is full to the brim with water. Take care not to lose water from the diver. Place the cap on the bottle.
6. Use both hands and squeeze the bottle.
7. Watch the diver sink when the bottle is squeezed, or float when pressure is released.

## Questions

1. What happens to the air in the diver when the bottle is squeezed?
2. Why does the air behave in this way?
3. Write a sentence that explains how the Cartesian diver works

1. Air is compressed and the volume of air is reduced.
2. When compared with those in liquids, the gas particles which make up air are a large distance apart. It is therefore easier to squash them closer together, thus reducing the volume.
3. When the bottle is squeezed, pressure in the water pushes on the pocket of air inside the diver. The volume of air is reduced and this allows more water into the diver. This makes the diver more dense, and it therefore sinks. When the pressure is released, the air expands, taking up a larger volume. Water is pushed out of the diver, which becomes less dense and therefore floats in the water.