Cartesian Diver

Density and pressure effects are explored using a simple visual demonstration.


  • large plastic bottle (~2L) with lid, filled with water
  • glass filled with water (for testing the ‘diver’)
  • straw
  • BluTack
  • scissors


  1. Cut the straw to ~4cm in length.
  2. Use small blobs of BluTack to seal each end of the straw.
  3. Check that the sealed straw just floats in the glass of water. It should slowly rise to the top of the water after you drop it in. If it sinks, remove some of the BluTack. If it floats too easily add some more BluTack.
  4. Drop the straw 'diver' into the bottle.
  5. Fill the bottle with water from the glass if necessary.
  6. Screw the cap on the bottle.
  7. Squeeze the sides of the bottle (fairly firmly). The straw diver will sink through the water. Let go of the bottle and the diver will float back up to the top.

How Does it Work?

The straw diver contains an air bubble sealed inside. The combined density of the straw, air and BluTack is slightly lower than that of the water, so it floats. When you squeeze the sides of the bottle you increase the pressure pushing on the air bubble, making it compress into a smaller space. This decrease in volume causes the air bubble to increase in density. Therefore the overall density of the straw diver will now be greater than that of the surrounding water, making it sink. Releasing the pressure (by stopping squeezing the bottle) allows the air bubble to expand back to it's normal size, and so the straw diver will float again.

Tips for Success

Don't be scared about pushing hard on the sides of the bottle – they can take a lot of force. It may be easier to rest the bottle on a flat surface and then use both hands to push the sides.

If your diver keeps floating then take it out and add more BluTack, making sure to test it in the glass of water to be sure that it doesn't immediately sink – you'll find it easier to get it out of the glass than out of the bottle if it does sink!

Serving Suggestions

This is a good visual demonstration when you have a small crowd. However it works best if everyone can have a try for themselves, so it's not recommended for large crowds.

Did You Know?

Sperm whales regularly dive to depths of at least 1000m where the water pressure is on the order of 100 atmospheres. In the process, the rib cage will fold and collapse and the lungs will compress down to one per cent of their size at the surface. The associated change in buoyancy is essential to keep the animal at such a great depth.

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