A Deep Dive into Buoyancy: Build the Cartesian Diver Experiment

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Have your children ever seen submarines in some of their favorite cartoons? If so, it may have piqued their interest in how the submarine could sink to the bottom of the ocean and float back up. How can they sink down that far?

This experiment is exactly how submarines can sink down in the ocean and float on the surface!

The Cartesian Diver experiment demonstrates how changing the pressure on a gas within a liquid can alter the object’s buoyancy by modifying its density.

How to make the Buoyancy Deep Dive Cartesian Diver experiment

Supplies you will need

For this experiment, you’ll need:

Supplies needed for the Buoyancy Deep Dive Cartesian Diver experiment

Before you start

Be sure to have a towel handy for the inevitable spills!

Instructions

Here is how to do this experiment with your child:

Step 1: Wrap the pipe cleaners around the pipette

Wrap all four of the pipe cleaners around the pipette, right under the bulb at the top of the pipette.

The goal of adding the pipe cleaners is to add just enough weight that the pipette barely floats, meaning it is neutrally buoyant. Four was the magic number for us.

Pipe cleaners wrapped around the pipette

Step 2: Cut the bottom of the pipette

Next, remove the bottom portion of the pipette by cutting it off (we cut around the 0.5 mark that was etched on the pipette’s side).

The full-length pipette can potentially contain too much air due to the elongated straw, making it more difficult to get the right buoyancy to do the experiment.

By cutting off the bottom of the pipette, we are adjusting the amount of air the pipette can hold.

Cutting the bottle piece off of the pipette

Step 3: Gather water into the pipette

We gathered enough water into the pipette that the water line was almost halfway up the bulb at the top of the pipette.

Again, the goal is to make the pipette neutrally buoyant, meaning it barely floats, before we put it into the bottle.

If you completely fill the pipette, it will immediately sink to the bottom of your bottle. If you don’t add enough water, it will bobble around the top of the bottle and the change in pressure will not affect the pipette.

You can test this out in a glass of water, just make sure the glass is much taller than the pipette.

Showing the water level in the pipette’s bulb

Step 4: Place the pipette in the bottle and squeeze

Place the pipette with the water in it inside the bottle of water, the bulb of the pipette facing up. Securely screw on the top of the bottle.

Squeeze the sides of the 2-liter bottle and your pipette should sink to the bottom.

Why does it sink?

When the bottle is squeezed, the increased pressure compresses the air inside of the pipette. The compression reduces the air’s volume, and water enters the pipette. That added water dramatically increases the density, making the pipette sink to the bottom of the bottle.

When you let go of the bottle, the pipette rises back up to the top because the water that was forced in due to the increase in pressure leaves the pipette and is replaced with air, making it less dense.

Squeezing the sides of the 2-liter bottle to make the pipette sink to the bottom

The STEM behind the Buoyancy Deep Dive Cartesian Diver experiment

This experiment teaches:

How submarines sink and float
Buoyancy
Water pressure

How it works

This experiment is a great demonstration of how pressure, density, and buoyancy are related.

When the bottle filled with water is squeezed, the increased pressure transmits through the water, compressing the air inside of our pipette. This compression reduces the air’s volume, causing water to enter the pipette and increase its overall mass.

Since the pipette’s mass increases, the density surpasses that of the surrounding water, leading it to sink.

When we release the pressure on the sides of the bottle, that allows the trapped air inside of the pipette to expand, displacing the water inside of the pipette and decreasing its density. The decreased density means the pipette returns to its original state – neutrally buoyant!

How submarines sink and float

This is the same concept used in underwater vessels like submarines!

Submarines use these special compartments called ballast tanks that can be filled with water or air, depending on whether they need to sink or float up to the surface.

It’s essentially the same as our experiment, but instead of squeezing the sides of the submarines, they just add the water or air needed into the ballast tanks to change its density. Cool, right?

Buoyancy

Buoyancy is the ability of an object to float.

If an object’s average density is less than that of the surrounding fluid, it will float.

In the case of the Cartesian Diver experiment, we add just enough weight through the pipe cleaners and water in the pipette to make the pipette itself neutrally buoyant, which essentially means the object neither floats nor sinks. The density of our pipette is equal to the density of the water in the 2-liter bottle.

By squeezing the sides of the bottle, we are adding pressure to the bottle. That pressure transmits into the water, including the water inside of the pipette, as well as the air inside of the pipette (everything is affected!).

Since air is compressible, the increased pressure forces the air inside of the pipette to shrink, allowing more space for water inside of the pipette. When more water gets into the pipette, that increases its mass, which increases its density!

That means, uh-oh! We’re no longer neutrally buoyant; the pipette is now more dense than the water. That causes it to sink!

Water pressure

When you squeeze the bottle, you’re not just applying pressure to one spot. The pressure is transmitted equally throughout the entire volume of the water.

Since it’s hard to visualize changes in water pressure, the Cartesian Diver experiment is a great way to see how an increase in pressure can transmit through the water and into the pipette to change its density.