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Cartesian divers rise and sink in water that fills a capped bottle when there are changes in pressure, volume, temperature, and buoyancy. The dropper that functions as the diver is partially filled with water so its bulb just floats. When pressure is increased in the bottle while temperature remains constant, the volume of the air bubble in the dropper is reduced. Additional water enters the dropper, the dropper's buoyancy decreases, and it sinks. When the pressure decreases, the process is reversed, and the diver floats to the top of the bottle again. When temperature is decreased while pressure remains constant, the volume of the air bubble in the dropper is decreased. The dropper's buoyancy decreases, and it sinks. When the temperature increases, the process is reversed.
Cartesian divers are a quick and simple way to illustrate relationships among pressure, volume, temperature, and buoyancy. The Activity could be used in connection with the concepts of gases and liquids and discussions of Boyle's, Charles's, and the ideal gas laws. A biological application is that some fish have a small sac containing an air bubble inside their bodies. They control the size of the sac to alter their buoyancy in the water. The optional extension could be a chemistry recruitment exercise or a year-end engineering project with considerable room for student inventiveness.
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