Cartesian Diver Experiment

Floating and sinking, and the science behind it.

A few tools to demonstrate principles behind floating & sinking (and density).

Level: Middle to H.S.

Time involvement: 15 minutes.

This long time favorite of science teachers uses a small soda bottle, water, and an eyedropper to create a puzzling demonstration that seems to defy logic. Gently squeezing the bottle simultaneously increases the pressure inside the eyedropper too. A small air bubble is compressed thus increasing the density to a point greater than the surrounding water. The eyedropper sinks to the bottom but immediately floats to the top when the squeeze is relaxed. The action can be repeated. The demonstrator can cleverly use a “magic” tuning fork, spoon, magnet, etc. to make it appear that the outside object is causing the action inside the bottle. The truth, however, is that only the squeezing action is the cause. The presence of the tuning fork, magnet, etc. is a deliberately confusing distraction. An interesting discussion can follow about jumping to conclusions and making unsupported assumptions. The unique demonstration helps to develop careful observation and critical thinking skills.

The above photo contains several items that tease our minds about floating. The gray rock is pumice. Pumice is rock expelled from a volcano. It is very light due to its formation at extreme temperatures and the presence of hot gases. Amazingly, pumice is a floating rock. The red and yellow mini dug-out canoe shown illustrates how clay can float if shaped into a small boat.

Discussion: The underlying principle is the concept of buoyancy. It’s all about the principle of the density of the water compared to the variable density of the eyedropper. Students should be cautioned that in scientific investigations that mere simultaneous occurrences are not proof of cause and effect. The Density/Buoyancy principle is a key concept in understanding weather phenomena. The name Archimedes is famous in science history. Born in 287 BC the legend claims that in a flash of enlightenment, while bathing, the mathematician ran naked through the streets shouting “Eureka” or “I’ve found it.” The concept of buoyancy is easy to understand if you try to hold a ping-pong ball under water. Upon release it immediately pops to the surface. The example is visual and obvious. Harder to understand, but no less important, buoyancy is the underlying principle to understanding the global size patterns of air circulation around our Earth.

Notes: The size of the air bubble in the eyedropper is critical to a successful illusion. Too big a bubble will require an obvious forceful squeeze thus giving away the secret too easily!