Things that Float: The Cartesian Diver

Things that Float:

The Cartesian Diver

Materials Needed for this Experiment:

2 liter soda bottle
Plastic pipette
Brass nut (non-corrosive)
Enough water to completely fill the bottle
Note: it is possible to do this project with mustard packets (or other condiments that have a pocket of air at the top!)
Click on the photos above to be linked to be linked to a website where you can purchase the materials for this experiment!

How to Construct your own Cartesian Diver:

Wedge the nut onto the pipette (slide it back far enough so that it will not come off easily!)
Cut the pipette so that there is about 1/2'' of plastic that goes past the outer edge of the nut
Next, we need to make the diver just a little bit less dense than the water (we want it to just barely float, so that we compress the bottle it will become more dense than the water, and will sink!)
Fill pipette up with some water (to where the water goes to not quite half way up the bubble on the pipette). We don't want the pipette to float very well!
Tip: Turn the pipette over and tap on it to remove some of the extra air into the tube of the pipette. Next, squeeze the bubble of the pipette to remove some of the water. Draw up some more water into the pipette. When you add more water to the pipette, there will also be more air, which you want to move up to the top of the pipette where there is a bubble of air.
Now, your diver should just barely float at the top of the water's surface.
It is time to place your diver into the soda bottle!Make sure that before you put the cap on your bottle, that there is absolutely no are between the edge of the water and the bottle cap. You don't want to have to compress that air as well as the air in your diver!
All you have to do now to make your diver move down in your bottle is to compress the bottle slightly, which makes the contents of the soda bottle more dense (there is the same amount of stuff inside of a slightly smaller space when we compress the bottle!)
You may need to adjust the amount of water in the bottle if it is difficult to move your diver, and you may need to adjust the amount of water in the pipette if it becomes more dense and won't raise up from the bottom of the bottle!
Want a visual of how to make a cartesian diver? Click this button to be redirected to a Youtube video of Jeff Goodman's construction process:

So, how does this this experiment work?

The bubble of air at the top of the plastic pipette makes the diver (pipette+nut+air+water) just slightly less dense than the water that is sealed inside of the soda bottle.
Because the diver is just a little bit less dense than the water, it floats!
But, when you squeeze the sides of the soda bottle, you also compress the contents of the diver, which makes the diver slightly more dense than the water that it is submerged in.
Because there is the same about of material inside of the diver, but more pressure around it, the diver is more dense than the rest of the contents of the bottle, which makes it sink!
When you release the pressure from the soda bottle, the diver becomes less dense than the water once again, and will rise back up to the top of the bottle.

Once you have all of your materials gathered, you are ready to make your very own Cartesian Diver!

Project Extensions:

Want to blow your students away when you introduce then to the Cartesian Diver and density? Begin your Cartesian Diver demonstration with a prank! Allow for your students to critically think about what makes the diver move up and down, using distractions like magnetism, electricity, and even sound, and don't introduce the activity by telling your students that this is a density demonstration.

Here is a video of Jeff Goodman tricking our class into thinking that the diver is moved with sound:

You can also push your students to critically think about what in the world is happening inside of that soda bottle by making them think that the diver is moved by electricity, or even magnets! Get creative! Once your students find out what was really going on inside of the bottle (changes in density!) they will be much more engaged in the process of learning about density!

This week, we learned about what makes thinks float and sink. The Cartesian Diver is a great demonstration to illustrate what density looks like in the real world, and can be done easily and with materials that can be gathered without much hassle! Enjoy!

Fun fact: Sperm whales regularly dive to depths of at least 1000m where the water pressure is on the order of 100 atmospheres! In the process, their rib cages fold and collapse and their lungs compress down to one per cent of their size at the surface. Just like the cartesian diver, the whale's insides become denser, so that they are able to survive deep under the ocean's surface.

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