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Cartesian Diver


An object can be made to float or sink inside a bottle of water by pressing on the sides of the bottle.


The Cartesian diver is a classic demonstration which effectively demonstrates Pascal's Principle, which states that Pressure applied to an enclosed fluid is transmitted undiminished to every part of the fluid. In this case, the pressure excerted on the sides of the bottle causes the air inside the 'diver' to compress, making it less buoyant.

Equipment typical required (per demonstration):

  • 2l drinks bottle with cap
  • water
  • diver
The diver can be simply made up before the demo. It may consist of a dropping pipette, a pen-cap with plasticine weight added or for simplicity, a ketchup sachet. Whichever type you choose, the item should contain a small amount of trapped air and it should float just beneath the surface of the water in the bottle.

The object needs to be placed inside the bottle and and the bottle then filled completely with water and capped. It is essential that there is no air left in the bottle otherwise when squeezed, this air will also compress.

When the sides of the bottle are squeezed, the pressure is applied to the water but because water cannot be compressed, this compression is transferred to the air inside the diver. This air compresses and the object becomes denser until it reaches a point where it cannot float. It should sink some way down the bottle.

Some trial and error may be required because ketchup sachets vary greatly from one to the next, some containing more air, some less. If you are making your own diver, plasticine may be added to get the correct buoyancy to begin with. Students may want to experiment with different shaped divers or different materials which to make one with.

This demonstration may be conducted to show how air can be compressed while water cannot and also as an introduction to Pascal's Principle and hydraulics.


 CautionThe contents of this page are for information only. Please refer to CLEAPSS, SSERC or ASE safety advice and/or publications before undertaking any preparation, practical experiment or using any equipment featured on this site or any other.