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Cartesian divers are one of the oldest and most interesting toys you can build at home. While they are easy to construct, there is a lot of science behind the workings of this deceivingly simple toy. A Cartesian diver is an object whose density changes with pressure. In fact, most Cartesian divers become denser as pressure is increased. By constructing a Cartesian diver carefully, it is possible to make a diver that floats in water at atmospheric pressure, and sinks when the pressure is increased.
Water has a density of about 1 gram/ml. Objects that have a density of less than 1 gram/ml float, while objects with a density greater than 1 gram/ml sink. As pressure is increased, a Cartesian diver's density might increase from about .8 grams/ml to 1.2 grams/ml. When this happens, the diver sinks in water. Cartesian divers often change their density by changing the amount of water they displace (i.e., changing their volume). When the pressure is increased, the air inside the diver is compressed. This compressed air takes up less space, and thus displaces less water. As less water is displaced, the density of the diver appears to increase and the diver sinks.
There are literally hundreds of experiments you can try! For instance, try crumpling up a piece of aluminum foil into a small ball. Place this in your bottle. See if you can sink it by squeezing the bottle... how about pumping it?
Try numbering your divers and see if you can make them sink in order. Note that your divers are not yet sealed, and so they can be adjusted as many times as you like (colored water will leak out of them until they are sealed).
Q & A