A cartesian diver works in the same principle as a submarine does. Ninth graders may have fun creating cartesian divers within minutes using a variety of materials. Here’s how.
Supplies Needed For A Cartesian Diver
- Medicine dropper or pipette
- Hex nut
- Plastic soda bottle with cap
- Large clear plastic cup
Variations on cartesian divers
- Ketchup or soy sauce packet from takeout
- Paperclip diver: paperclip, rubber band, and 6” plastic straw
What Mystery Are We Solving?
Why does the cartesian diver float and then dives when you squeeze the bottle? Why does it float to the top when you release the pressure on the bottle? Why does it behave as it does?
There’s nothing particularly dangerous about this experiment. Have an adult present to help, and use common sense.
- Make the diver by screwing the hex nut to the base of the eye dropper or pipette. Cut off the stem but leave a ¼ of inch stem.
- Fill the cup with water.
- Place the diver in the water and notice how it floats.
- Now squeeze the dropper so that air is pushed out and water sucked in. Fill the diver about half with water. The diver should barely float (The diver is almost covered in water but is still floating). If it sinks squeeze water out.
- Fill the soda bottle to the brim with water.
- Carefully transfer the diver to the soda bottle. Be mindful not to lose any of the water in the dropper.
- Screw the cap on the soda bottle.
- Squeeze the bottle with both hands. When you squeeze hard enough, the cartesian diver sinks to the bottom and when you release the pressure, it floats.
Paperclip and straw diver
- Bend the straw.
- Tie the rubber band at the ends to hold the two ends together.
- Pull the outside of the paperclip slightly away and clip it to the rubber band. Make sure the open end of the paperclip is facing downward.
- Add as many paperclips (weight) necessary to have the diver barely floating.
Troubleshooting the diver
- If the diver doesn’t dive, it doesn’t have enough weight (add more water or paperclips).
- If the diver sinks to the bottom and stays there, it is too heavy (remove some water or paperclips)
- If the ketchup packet diver doesn’t sink, it means there’s no air trapped in the packet.
What Just Happened Here? Our Cartesian Diver Explanation
The science behind the diver is related to air. Although you can’t see air, it has weight, occupies space, and exerts pressure.
When the bottle is squeezed the water level inside the diver (dropper) rises and the pressure increases. Gas is easier compressed than liquid. The diver becomes less buoyant because water fills the dropper and the air bubble (gas) is compressed into a smaller space. The diver dives to the bottom.
The opposite is also true. When you stop squeezing the bottle, the pressure releases and the diver floats to the top.