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:
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.
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.
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.
