Cartesian Diver

Age

Elementary School, Middle School

Format

Hands-on

Materials

   2-Liter Soda Bottle
   The Cartesian Diver
   Water 

Safety Precautions

Science Theatre demonstrators must keep the safety of themselves and their audience in mind at all times. All Science Theatre demonstrators must have read through the Safety Training page. The ST Safety Box with first aid kit, fire extinguisher, etc. should always be available to demonstrators. Always wear safety gloves, glasses, and a labcoat if handling chemicals; always perform potentially dangerous demonstrations at a safe distance from the audience; and always keep a very close eye on any volunteers you call from the audience. Add safety information

Preparation

See #Demonstration

Demonstration

The Cartesian diver experiment is set up by placing a "diver"—a small, rigid tube, open at one end, very similar to an eyedropper—in a larger container with some flexible component; for example, a two liter soft drink bottle. The larger container is partially filled with water, as you can see in the demonstration, and must be made airtight when closed. The "diver" is partially filled with a small amount of water, just enough to allow it to contain enough air so that it is nearly neutrally buoyant, but still buoyant enough that it floats at the top while being almost completely submerged. (This demonstration should be completely put together.)

What to Say

There is just enough air in the diver to make it positively buoyant. Therefore, the diver floats at the water's surface. As a result of Pascal's law, squeezing the airtight container increases the pressure of the air, part of which pressure is exerted against the water that constitutes one "wall" of the airtight container. This water in turn exerts additional pressure on the air bubble inside the diver; because the air inside the diver is compressible but the water is a incompressible fluid, the air's volume is decreased but the water's volume does not expand, such that the pressure external to the diver a) forces the water already in the diver further inward and b) drives water from outside the diver into the diver. Once the air bubble becomes smaller and more water enters the diver, the diver displaces a weight of water that is less than its own weight, so it becomes negatively buoyant and sinks in accordance with Archimedes’ principle. When the pressure on the container is released, the air expands again, increasing the weight of water displaced and the diver again becomes positively buoyant and floats.

Why it is

See #What to Say