Demonstration: Cartesian Diver

by Kenneth Fuller
copyright information
May 06

The Cartesian diver demonstration can be valuable in a number of teaching contexts.  It can be used to introduce the lab activity, to review and consolidate learning after the lab activity, as an inquiry activity, or, if you must, to illustrate a lecture.  It is probably best used after the class has studied density, archimede's principle, and knows something about why things sink or float.

Theory:  Archimede's Principle; When an object is placed in a fluid ( a liquid or gas), if it is less dense than the fluid it will displace an amount of fluid with the same mass (weight) as the object (it will float), if it is more dense than the fluid it displace an amount of fluid with the same volume as the object (it will sink).  At more advanced levels, this can be understood in terms of gravitational pressure and the kinetic molecular theory of matter.

To get the diver to float, we must make its density less than the density of the water.  To get the diver to sink, we must make its density greater than the density of water.
NOTE: The density of an object is the average density of all its parts.  In the case of our diver that includes the container plus the air and water inside the container.
To increase the density of our diver, we put more water into it.  To decrease the density, we force some water out of it.

Because the bottle of water is airtight, by pressing on it, we increase the pressure on the water inside.  The increased pressure of the water on the air bubble in the diver causes the bubble to get smaller (the air in the bubble is compressed), so more water is pushed into the diver adding to its weight, thereby increasing its density, so it sinks.  Releasing the force on the bottle reduces the pressure on the water inside.  With less pressure of the water on the bubble, the bubble gets bigger (the air in the bubble expands) and pushes some of the water out of the diver reducing its weight, thereby decreasing its density, so it floats.

In submarines, instead of changing the water pressure on the outside, they change the air pressure on the inside (they use compressed air to push water out of the ballast tanks, and let air out to get more water in).

Preparation:  The preparation of the soda bottle diver is given in the instructions for the lab activity.   For demonstration purposes I have used gallon size glass bottles that chemical reagents are sold in, or at least were.  These instructions can be adapted to large flasks or other rigid containers. (If the container is too deep, water pressure may be enough to prevent the diver from rising again.)  They might work with soda bottles, but would require more careful handling.

For a diver I use a very small test tube, or small glass vial.  A dropper such as used in the student lab works well, but the bulb hides most of the air bubble.  Using a marker or something to give the diver transparent color greatly increases its visibility.

Materials:  A large (30 - 40 cm deep) container, a rubber stopper for the container, a very small glass vial for the diver, a beaker deep enough to float the diver, a dropper, water.

1. Fill the container and the beaker with water, let it stand over night.  This will allow the extra air to come out of solution, otherwise after an hour or so bubbles will form inside the container and on the diver, and the diver may refuse to sink.

2. In the beaker, fill the diver with water then hold it bottom up with its mouth below the surface of the water.

3. With the dropper full of air, place its tip under the mouth of the diver and squeeze out some bubbles into the diver.

4. Lower the diver into the water and see if it floats.

5. If it sinks, add another bubble of air.  If it floats too high, tip the diver to allow a bubble to escape.

6. The diver should be adjusted so that it just barely breaks the surface of the water.

7. With the stopper in the bottle, turn the bottle over and hold it with its mouth below the surface of the water in the beaker.

8. Remove the stopper and place the diver under the mouth of the bottle, when it floats into the bottle, put the stopper back.

9. Stand the bottle upright, the diver should float to the top.

10. Trap as little air as possible, when placing the stopper in the bottle.  A small bubble won't hurt.

11. Pressing on the stopper should cause the diver to go down, releasing the stopper should cause the diver to rise.  Some adjustment may be necessary.
12. The speed of descent and rise is controlled by the amount of pressure on the stopper.

Some variations:diver with solid stopper
Over the years I have used some variations on the above instructions, depending on the immediate objective I have in mind.  When I want to make my manipulation of the diver less obvious (like magic), I use the solid rubber stopper.  By grasping the top of the bottle with my palm over the stopper, and the other hand under the bottom, I can carry it in a natural manner supposedly to give students a closer look.  Without noticeable motion I can vary the pressure on the stopper, causing the diver to rise and sink with the magic words. 
diver with dropper
For most purposes I find placing a dropper (filled with water) through a one hole stopper is most convenient.  This is the form I use for inquiry lessons.  Squeezing the bulb increases the pressure to cause the diver to sink.
diver with rod
To determine whether the pressure can be adjusted so that the diver's density is exactly the same as the density of water, so that the diver can be made to remain motionless in the middle of the jar, I use a glass rod inserted through a one hole stopper.  By sliding the rod up and down, the pressure can be adjusted in very small increments, then maintained indefinitely, until the temperature changes.

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