Compressibility and incompressibility demonstrated with soda bottles and ketchup


I teach in a civil engineering department so we pretty much only deal with incompressible flows. However, this is a really simple demonstration to illustrate the compressibility of gasses and the relative incompressibility of liquids that uses stuff you can pick up at a fast food restaurant and recycling bin. I found it in a number of different books on science experiments for kids.


  1. 2 liter soda bottle with cap
  2. a small ketchup (or other condiment) packet that floats (test this before you wedge it in to the bottle).
  3. water

Photo Jan 12, 4 09 06 PM


  1. Stick the ketchup packet into the soda bottle and then fill the bottle with water until there is only a small volume of air below the top of the bottle.
  2. Tightly screw on the cap so that the bottle is sealed. The ketchup packet should be floating.
  3. Squeeze the bottle firmly with your hand and the ketchup packet should sink.
  4. release the bottle and the ketchup packet will float back up to the surface.


This is effectively a cheap way to make a Cartesian diver. The demonstration relies on the water being effectively incompressible and the air being compressible. When you squeeze the bottle it is the air pocket at the top of the bottle that is compressed by the change in volume. This increases the pressure in the water but does not compress it so the water density stays the same. However, the ketchup packet has a small air bubble in it which also compresses. This reduces the volume of the bubble enough that the net density of the packet changes from being less than that of water to greater than that of water so it sinks. This process is reversed when you stop squeezing the bottle.

You can also get the packet to sink just by leaving the bottle out in the sun. In this case the water and air both heat up. however, given the finite volume, as the water expands slightly from heating, the air is compressed and the packet sinks.

An index of all the demonstrations posted on this blog can be found here. Don’t forget to follow @nbkaye on twitter for updates to this blog. If you have a demonstration that you use in class that you would like to share on this blog please email me ( I also welcome comments (through the comments section or via email) on improving the demonstrations.


2 thoughts on “Compressibility and incompressibility demonstrated with soda bottles and ketchup

  1. Pingback: Video of “Compressibility and incompressibility demonstrated with soda bottles and ketchup” | Teaching Fluid Mechanics

  2. Pingback: Other fluids education resources IV: Science puzzles and fluids lab books | Teaching Fluid Mechanics

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