This curious toy was first invented by René Descartes, an important scientist and mathematician in the early 1600's, and while his design has evolved into many different variations they all continue to amaze us while teaching some important scientific principles in a fun way.
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what you'll need
Pipette "Submarine" diver:
Balloon "Fish" diver:
Ketchup packet diver:
The pipette diver floats in the water at first because there is air trapped inside which makes it buoyant- just like the air inside a balloon or ping pong ball makes it float in water. When you squeeze the sides of the bottle you increase the pressure inside and that pushes more water into the pipette through the open hole at the bottom (you should be able to see it happen). This extra water makes it too heavy to keep floating (or more dense than the water in the bottle) and it sinks. When you stop squeezing the pressure drops and the extra water (and weight) flows back out of the pipette and it floats again.
The balloon "fish" also floats at first because their is a little air trapped inside, but this time when you squeeze the bottle there is no way to push any extra water inside (it's tied shut). But the balloon is made of rubber which is very flexible, so the pressure from the water outside squeezes the balloon and makes it a little smaller. The balloon still has the same weight, but now that everything inside is squished closer together (mostly the air molecules trapped inside) the balloon is more dense than the water and sinks. Stop squeezing and the balloon expands, becomes less dense and floats again.
More detailed explanation
The Earth's mass pulls other objects with less mass towards it's center (i.e. down). This is just the force of gravity that makes a ball fall to the ground, but the ball doesn't fall as long as you are holding it because your hand pushes up with a force that exactly balances the force of gravity pulling down. Of course if your hand pushes up with even more force you can make the ball move up. This is actually what's happening to our diver, we just need to understand what is causing these forces that make it move up and down. Obviously gravity is the force that makes it move down, but what force makes it move up? The surprising answer is actually gravity! But how can gravity make it move up and down?
Okay, gravity doesn't directly push the diver up, but it is responsible for the force that does, which is called the buoyant force or buoyancy. As we said, the Earth's gravity pulls everything with mass down, including the water molecules in your bottle, and the weight (which is the amount of force of gravity) of all these water molecules trying to squeeze to the bottom of the bottle creates water pressure inside that pushes back and stops them. The deeper you go in the water the greater the pressure at that depth. Think of it like the cheerleaders making a pyramid. The cheerleaders on the bottom are being squished by the weight of the others above them, and must push back to keep them up. The cheerleaders in the second row have fewer above them, don't get squished as much and don't need to push back as much to hold up those above them. And of course the lone cheerleader on top isn't squished or pushing back at all.
This water pressure doesn't only push on other water molecules, it pushes on everything inside the bottle as well as the bottle itself (and in every direction, not just up). So now we have a force that pulls the the diver down (its weight, which is equal to its mass multiplied by the acceleration due to gravity, i.e. W= M x g) and another force that pushes it up at the same time (its buoyancy, caused by the water pressure, which is also ultimately due to gravity). Whichever force is stronger determines the direction the diver will move. The weight of the diver is easy to measure, but how do we calculate its buoyant force? For the diver to move it must push any water molecules in its way aside or displace its volume (the amount of space it occupies). It turns out that the force needed to do this, or the buoyant force of the diver, is just the weight of the volume of water it needs to displace. This is called Archimedes Principle. To make things easier we can calculate the density, which is mass divided by volume (d=M/V), for both the diver and the water. If the density of the diver is greater than the density of water, then the weight of the diver is greater than its buoyant force in water and it sinks; if the diver is less dense than water the buoyant force is greater than its weight and it floats. The density of water doesn't change in this experiment (because water is incompressible- more on this later), so to move the diver up and down we must change its density instead.
[BTW- water pressure pushes in all directions, so why does the diver go up if it's less dense than the water around it? Couldn't the water pressure just as easily push it down or sideways? Remember that pressure increases with depth, so at any depth the pressure on one side of the diver is always balanced by an opposite pressure push on the other side of the diver, i.e. they cancel out. But the water pressure pushing down on the top of the diver is always just a little less than the pressure pushing up on the bottom (because it's deeper), thus there is always just a little more force pushing up on the bottom than down or sideways and this is enough to insure that the diver (or any buoyant object) always floats upwards.]
For the pipette diver we change its density by adding or removing mass while keeping its volume the same. When you first placed the pipette in the bottle of water it floated, so its density was less than the density of water. The pipette had only air inside and thus very little mass because there were very few air molecules (molecules in a gas are very far apart). When you squeeze the bottle it tries to squeeze the water inside, but the water molecules are already as close together as they can possibly get, i.e. we can't compress them any more, they're incompressible (which is effectively the case for all liquids). Gasses, however, are compressible since their molecules are very far apart and can easily be squeezed closer together. Thus the force of your squeezing the bottle increases the water pressure inside which pushes more water inside the pipette through the open hole in the bottom. The added water molecules increase the total mass of the pipette while the total volume inside stays the same (the air just takes up less volume now to make room for the added water volume), so the pipette density increases. Once enough water goes in to makes the pipette more dense than the water, it sinks. If you stop squeezing everything reverses- water flows out of the pipette, the air expands to fill more of the volume inside decreasing the overall density and the pipette floats once again.
If you squeeze the bottle just hard enough you can force the right amount of water into the pipette to makes its density exactly equal to the water density and thus stop and hold the pipette motionless at any depth you choose. Give it a try! Watch the pipette closely as you squeeze the bottle and you should be able to see the water go in and out. This is exactly how a real submarine dives, surfaces or controls its depth in the ocean. Submarines contain ballast tanks that they can fill with water to increase their mass (and thus density) when they want to dive. To surface they blow air into the tanks, forcing the water out, which decreases their density and they float upwards. With just the right balance between air and water in the tanks they can control the density to remain at any depth they choose.
The balloon "fish" also dives and sinks due to changes to its density, but instead of adding or removing mass we change its volume instead. Initially the balloon has just enough mass (mostly from the BB's inside) and volume (mostly due to the air inside) to make its overall density a little less than that of water, so it floats. When you squeeze the bottle and increase the water pressure inside it squeezes the balloon which then squeezes the air inside and reduces the volume of the balloon, i.e. it actually gets a little smaller. Since the balloon is closed nothing can go in or out (unlike the pipette diver), so the mass must stay the same even though the volume is smaller, and thus the density increases. If you squeeze the bottle and make the balloon small enough, the density will be greater than that of the water around it and it sinks. Stop squeezing and the balloon expands once again, its density decreases, and it floats back to the top of the bottle. Just as with the pipette diver, if you squeeze just hard enough you should be able to make the balloon stop and hover at any depth you like.
We call this a balloon "fish" not only because it kind of looks like a little fish, but because this is very similar to the way a real fish controls how deep it swims in the water. Fish have a bladder inside their body that is filled with air (kind of like our lungs). They don't use it to breathe, but rather to control their density. Fish have muscles that can squeeze this bladder to make it a little smaller (just like you squeezed the bottle to make the balloon smaller) which in turn makes their whole body a little smaller and therefore denser when they want to sink to go deeper. To float up they just relax the muscles around the air bladder to make their body a little bigger and less dense. To hold any depth they squeeze their muscles just enough to keep the bladder the right size to match the density of the water. Can you make your "fish" perform all these movements?
A ketchup packet usually contains a little air bubble inside so that it works just like the balloon in this experiment. The ketchup provides the mass (instead of BB's) and the flexible plastic or foil of the package allows it to change size as you squeeze the bottle.
Finally, gravity also creates buoyancy in air and other gasses, which explains how helium and hot air balloons can float due to differences in density. In fact, this is true for all fluids, which can be composed of liquids, gasses or even solids (like sand) as well as mixtures of each.
variations and related activities
More air inside the balloon increases the buoyant force, while adding more BB's makes it heavier, so by varying the size and weight of the balloons we can make it easier or harder for our diver to work. Try 2 or 3 different combinations in the same bottle. Another way to make it harder to operate is to leave a small amount of air in the bottle before screwing the top on. Now when you squeeze the bottle the increased pressure just compresses this air pocket rather than affecting the diver.
Temperature also affects the density of the water (cold water is more dense than hot water), so a balloon which floats in cold water may sink if the water bottle warms up too much, perhaps by sitting in hot sunlight [to a lesser extent the warm water also warms the air in the balloon, which causes it to expand and increase its volume, thus increasing its buoyancy, so there is a trade off to be considered before determining if the balloon will now sink or continue floating. Generally speaking, the decreased density of the water with warming usually dominates, and the balloon sinks]. On the other hand, if you leave some air in the bottle, then warming the water also warms this air pocket, increasing its pressure, and effectively adding more "squeeze" force to make the balloon sink. Galileo designed a thermometer" using these principles (see link below).
You can also make the balloon sink by removing the bottle cap and blowing very hard into the bottle with your lips tight around the neck (as though you're trying to inflate it). This works best if your diver is very easy to move (i.e. its density is already very close to that of water). "Fizz-Saver" pumps can be used the same way.
Buoyancy can turn raisins into divers, and you don't even need to squeeze the bottle. Buoyancy also makes Lava Lamps work. See the reference links below for more information.
There are many other diver designs, including some that spin as they go up and down and one that works in reverse (i.e. it floats when you squeeze the bottle and sinks when you stop squeezing). You can even create a game to retrieve sunken treasure (and maybe think of a way to use buoyancy to retrieve real sunken treasures). Try other objects, such as restaurant salad dressing or honey packets and even little pieces or orange or lemon peels. Can you create a design of your own? You can also decorate your divers to make them look more like fish or squids- get creative! Check out the many links below for ideas.
Can you use what you've learned in this experiment to design an instrument that measures the density of liquids?
references and links to more information
History of René Descartes and Cartesian Divers:
More about density, buoyancy and Archimedes' Principle:
Other Cartesian Diver designs:
Make a reverse Cartesian Diver:
Make a game to retrieve sunken treasures:
How submarines and fish use buoyancy to control their depth:
Make our cool Lava Lamp and raisins dance with buoyancy:
Make a hydrometer to measure liquid density:
Have a question or comment? Let us know at the bottom of the page.