The Cartesian Diver

Cause a ketchup packet to float or sink on command!

For this experiment you will need:

• 1 ketchup or soy sauce packet from a restaurant or a Milky Way mini candy individually sealed
• water
• a 1 or 2 liter clear plastic bottle

Place the ketchup or sauce packet, or the candy, in a bowl or cup of water to see if it will float. For this experiment you will need a packet that just barely floats.

Take a packet that barely floats and put it in the clear plastic bottle (you may need to fold it in half lengthwise to get in through the opening). Fill the bottle to the brim with water and screw the cap on tight. Squeeze the sides of the bottle. What happens?

The packet or candy has a small bubble of air trapped in it. When you squeeze the outside of the bottle, you increase the pressure inside the bottle. This will compress the air inside the packet, which changes the overall density of the packet. When the air is compressed enough, the density of the packet will be greater than the density of the water in the bottle, and the packet will sink. When you release the pressure on the outside of the bottle, the air in the packet will expand, increasing the buoyancy of the packet, and the packet will rise to the top. If you are using a clear soy sauce packet, you may even be able to see the size of the air bubble change as you squeeze on the bottle.

Alternate procedure: Other Divers

If you can't find a sauce packet that works well for you, you can also use a pen cap and some modeling clay or silly putty. Stick the end of the pen cap through a pea-sized ball of the clay or putty. You will have to experiment with how much material to put at the end of the pen cap to get it to just barely float. Carefully place the pen cap in the plastic bottle so that there is an air bubble trapped inside the pen cap. Fill the bottle to the top, screw the cap tightly onto the bottle, and squeeze the outside. If your pen cap doesn't sink, take it out and put a little more material on the end of the cap. When you have it right, you will be able to send your "diver" to the bottom with just a small squeeze of the bottle. Experiment with your diver. Can you squeeze the bottle just enough to keep the diver suspended in the middle of the bottle?


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