Ketchup Diver



Explore water pressure by making a Cartesian diver out of a condiment packet.


Condiment packets (ketchup, soy sauce, etc.), plastic bottle, cup or bowl, water


René Descartes devised this classic experiment which demonstrates the principle of buoyancy and the ideal gas law (PV=nRT).


  1. Place several condiment packets in a bowl of water. Look for packets that float vertically. Packets that sink or float horizontally are not well suited for making a Cartesian diver.
  2. Place one of your carefully selected packets in a plastic bottle and completely fill the bottle with water. Screw on the lid firmly.
  3. Squeeze the sides of the bottle and watch the packet sink to the bottom. Release the bottle and the diver floats back to the top.


A careful observation of your condiment packet will reveal that there is an air bubble trapped inside. When you squeeze the bottle you increase the pressure pushing on the bubble, which causes the bubble to shrink. When the bubble shrinks it gets denser, making the packet denser than the surrounding water. When you let go of the bottle you decrease the pressure on the bubble, allowing it to expand to its normal size and float to the top.

Sperm whales dive to depths of over 1,000m where the water pressure is 100 times greater than air pressure at sea level. Their rib cages are attached to the spine by cartilage, which allows them to collapse under high pressure, and their lungs shrink to a fraction of their normal size under such pressure.