1. Cartesian Divers
How do they work?

2. Ask: What makes the diver sink and float?
Get ready
Fill a 1 or 2 liter soda bottle with water.
Use a cup of water. Fill the dropper with enough water so that the dropper barely floats upright in the glass.
Place the dropper in the bottle and screw the cap on tightly.
Explore
Find out how to make the dropper sink, float, or hover in the water.
What’s going on?
Try a ketchup packet instead of a dropper
Try other materials -marshmallow, foam peanuts, paper clips, etc.

3. Cartesian Divers
A medicine dropper is partially filled with water and placed in a 1 or 2 liter bottle of water. The bottle is capped and the sides of the bottle are compressed and released. What happens? Why?
after compression ?
before compression

4. What’s happening?
The Cartesian diver demonstrates the concepts of density. In the bottle the dropper floats because it does not have enough mass for its volume. It contains mostly air which is less dense than water. When the bottle is compressed by squeezing, the pressure inside the bottle increases and compresses the air and decreases the volume of air inside the bottle and the dropper. When the volume of air decreases, the space left behind is filled with water. As a result, the dropper becomes heavier. The mass becomes heavier but the volume has not changed. Therefore, it is now more dense than the water and sinks to the bottom. When the pressure on the bottle is released, the process is reversed.
The same thing happens with the air inside the ketchup packet. However, this time the packet is soft and the volume of the packet is made slightly smaller by the pressure of the water.
To make a successful diver with other materials you need at least one material that is compressible such as marshmallows, packing peanuts, or something containing air.