Liquid science: Cartesian diver

Aug 2 / Bhavani
A Cartesian diver is a classic science experiment which demonstrates the principle of buoyancy (Archimedes' principle) and the ideal gas law.

The Cartesian diver demonstrates the concept of density as a function of mass and volume. In the bottle's uncompressed state, the medicine dropper floats because there is not enough mass in the volume of the medicine dropper to make it sink (i.e. its density isn't great enough). It is mostly air, and this air is much less massive than the water surrounding it. The mass of the dropper and the small amount of water inside of it doesn't contibute to the total mass (and, as a result, the density) enough to make the diver sink.

Objective: Sink the Cartesian diver.

  • Plastic water or soda bottle
  • Plastic straw that bends
  • Modeling clay
  • Warm or room temperature tap water
  • Funnel
  • Scissors


Cut straw about 1.5 inches on either side of the bend. Fold straw at bend to make u shape. The ends of the straw should be equal. Secure both ends with a paper clip. Not to seal both ends of the drinking straw.
Place it in the water and see if it can float on water. If forcing a small push, it will fall straight down in the water, and then comes out of the water immediately, floating on the water again. Fill a transparent bottle with water.
Put the Cartesian Diver into the bottle and close the lid. When you give a force to the bottle the Cartesian Diver will dive and then rises up when you release the force.

Note: The longer the drinking straw, the more it the more paper clips is required)If it dives directly into water and never come back again, you need to make the drinking straw longer.

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Why it cannot dive into the water when I squeeze the bottle?

Check whether you have done what Step 2/3 require. If the requirements are met. Connect more paper clips to the drawing straw.
Why when I put back the lid the Cartesian Diver will directly dive into the water and never come back?

Reduce a little bit of water in the plastic bottle.
Why does the Cartesian Diver sink to the bottom after playing for a while?

It is too much water goes in the dropper. Take out the diver and empty the water in the dropper. Put it back.

How does it work? 

Question to Ask:

  1. Why can Cartesian Diver float on the surface?
  2. Why when you a force is put on the bottle the Cartesian Diver goes down?
  3. In addition to using a straw, what else do you think can make a Cartesian Diver?

The Cartesian Diver is balanced by the buoyancy of the water in the water and the gravity. Archimedes' principle states that the upward buoyant force that is exerted on a body immersed in a fluid, whether fully or partially submerged, is equal to the weight of the fluid that the body displaces and acts in the upward direction at the center of mass of the displaced fluid.

When the Cartesian Diver is placed in the water at first, its buoyancy is almost equal to gravity, just slightly larger than gravity, so the Cartesian Diver can just float on the water. When the bottle is forced hard, the pressure of the water is increased, and the air in the drinking straw is compressed, in another word, the volume of the drinking straw becoming smaller so that the buoyancy of the Cartesian Diver is reduced.

The gravity of the Cartesian Diver is greater than that of its buoyancy. The Diver will sink. When the force is released, the pressure of the water returns to its original state, and the buoyancy of the Cartesian Diver also returns, so the Cartesian Diver will rise back.

More to do:

  • You can replace the drinking straw and paper clips unit with droppers and paper clips unit.
  • You can make the Cartesian diver into a marine creature to create a sea world inside the bottle.
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