“Mad Science” Mini-Lessons

6. Cartesian Divers


Pressure; Pascal’s principle; compressibilities of liquids and gases; buoyancy


For each student group: plastic soft-drink bottle with a tight recloseable lid, preferably 2-L; glass vial or test tube, narrow enough to put into the soft drink bottle, with equally-spaced lines written in permanent marker; bucket or sink; pitcher of water; posted example of data table


  1. On your own paper, draw a data table like the one displayed.
  2. If your test tube or vial is not already marked, draw lines at 1-cm intervals from the bottom (closed end) to the top (open end).  Holding the bottom up, number the marks, starting with “1” at the closed end and increasing toward the open end.
  3. Fill a plastic bottle with water.  Also fill the test tube or vial with water.  Quickly invert the test tube into the mouth of the plastic bottle so that a small bubble of air is trapped in the test tube.  If you wish, allow more air into the test tube by briefly lifting it above the surface of the water and then lowering it back down.  The object is to have enough air in the tube for it to float.  Let go of the test tube.  If it sinks, take it out of the bottle (as tidily as possible) and repeat the process, this time allowing a larger air bubble into the test tube.
  4. With the test tube floating in the plastic bottle, top off the bottle so that it is as full of water as possible.  Cap the bottle tightly.
  5. Squeeze the bottle in your hands.  If you squeeze hard enough, you should be able to make the test tube sink.  If you cannot, remove the test tube, refill it with water, and again float it in the bottle using a smaller air bubble.
  6. Stop squeezing the bottle.  Record the “air level” in the test tube in the “no squeezing” column of the table.
  7. Squeeze the bottle so that the test tube “diver” is just at neutral buoyancy: a little more squeezing makes it sink, and a little less makes it rise.  Record the test tube’s “air level” in the “neutral buoyancy” column of the data table.
  8. Now squeeze the bottle as hard as you can.  Record the “air level” in the “maximum squeezing” column of the data table.
  9. Remove the test tube form the bottle and re-float it as before, using different amounts of air in the test tube.  (That means different air levels in the “no squeezing” column.) Record the “neutral buoyancy” and “maximum squeezing” air levels in the table as before.

Questions to consider

  1. What happens to the volume of the air bubble when you squeeze the bottle?  What happens when you squeeze harder?
  2. How are the “neutral buoyancy” air levels related to each other?
  3. What changes when you squeeze a capped 2-L bottle?
  4. What happens to the amount of air in the test tube when you squeeze the bottle?
  5. What happens to the amount of water in the test tube when you squeeze the bottle?
  6. Why does squeezing the bottle make the test tube “diver” sink?

Data table: test tube “air levels”

 no squeezing   neutral buoyancy   maximum squeezing 

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Copyright © 2004, Richard Barrans
Revised: 21 December 2016; Maintained by Richard Barrans.
URL: http://www.barransclass.com/madsci/MadSci06_Diver.html