Things That Float:
A Lesson on Density
The Cartesian Diver
The Cartesian Diver is a demonstration that allows students to experiment with density, but also explore explanations for science and creating curiosity.
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Here's what you need..
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Liter soda bottle (empty)
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Plastic pipette
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Erosion free nut (brass nut used in example)
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Water
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Scissors
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Put the nut on the pipette until it fits on nicely. Cut the end of the pipette only leaving about 1 cm or so at the end of the nut.
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Fill the pipette with water until about 1/2 of the head of the pipette is filled. This is shown in the picture to the right. ​​​
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Th​e video to the right will explain how to fill the pipette correctly. This explains what to do when the pipette needs to be filled more and less.
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Test the filled pipette in a box of water to see if it will barely float at the top of the water. Make adjustments as necessary.
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When that is done, fill the bottle completely with water to the top and place the pipette in there. Close the bottle!
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The pipette should move freely up and down the bottle when the bottle is compressed.
The Process...
Where the pipette should be filled
Fill the bottle and place the pipette in the water so it is floating on top.
The Cartesian Diver is a demonstration that allows students to experiment with density, but also explore explanations for science and creating curiosity.
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The finished product!
The Diver Demo in Action...
Sink, Float, or Magic Demo:
There are many ways to demonstrate density in the classroom. ​One way to do so is to have students experiment with different liquid densities.
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Fill three glasses with water. In two of the glasses stir in 4 tsp of salt. Off to the side have three eggs. Next students should predict if the eggs will float in the cups when placed in the water.
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The egg in the tap water glass will sink to the bottom. This indicates the egg is more dense than the water. However, in the other two glasses, the egg will float, indicating it is less dense than the water.
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In the third cup, slowly add more tap water and watch as the egg moves to the middle of the cup. The egg shows it not quite dense enough to sink because the salt water is still dense. However the egg is still denser than the pure water which is at the top of the cup. Therefore, it will sit below that water.
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For objects to float, they need to be less dense than the ​liquids they are in. The salt makes the water in the cup more dense. That is why the egg will float when in the salt water. It is the same amount of liquid as the first cup, but is more dense.
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This demonstration can be used to reinforce the idea that the density of the liquid is also important in making an object sink or float. By having three liquids with different densities, students are able to see the effect on the egg. The egg is the same density, but reacts differently in each cup. By adding fresh water to the third cup, there is also an element of mystery as to why the egg sits in the middle. The key is the liquids are not stirred. This can create discussion in the classroom.
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Other demonstrations:
9 Layer Dip
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By layering 9 liquids, and using an assortment of objects, the objects will rest in certain areas due to density differences. When the object passes through a layer, it shows it is more dense than that liquid and "sinks" until it finds a more equal dense liquid.
Why the Diver works??
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The pipette is currently less dense than the water it is sitting in. There is some air in the pipette allowing it to be less dense.
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When pressure is applied to the bottle, it increases the pressure on the pipette as well.
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The water in the bottle is then forced up into the pipette since the pressure has increased and the water needs to go somewhere to balance out the pressure. It goes inside the pipette because of the available air and room.
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The water that fills in the pipette makes the object more dense. It now becomes more dense than the liquid it is in causing it to sink.
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When the pressure is released (i.e when we stop pressing on the bottle) the water will leave the pipette making it float once more.
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A little tip:
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That is why it will be helpful to fill the entire bottle when placing the pipette in the liquid. If there is air in the bottle, when pressure is applied, the water will first move to fill the empty space before it moves into the pipette. This means it will take more pressure to move the pipette down. However when the bottle is full of water, the pressure will force the water to move into the pipette since it has nowhere else to go.
Floating in your Boat Demo
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There are many ways to demonstrate density in the classroom. ​One way to do so is to have students experiment with different liquid densities.
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Once they've made their boats, have them predict/gather data on how much they weigh. Then test them in a clear container filled with water. Their boats should float.
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Next students should predict if their boats will always float, no matter their shape, and conjecture why they gave the prediction they did. Following this discussion, have students crumple up their boats and predict/gather data on the weight of the ball. Place them in the water. The boats will sink.
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Even though the boat was the same weight as the ball, and the size of tinfoil was the same, only one floats. Other objects can be tested from the room. The ball in this experiment is much denser than the boat. ​
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For objects to float, they need to be less dense than the ​liquids they are in. While the boat and the ball are the same size, there is more stuff packed into a smaller space with the ball. However, the boat has a lot more surface area and the particles in the tinfoil of the boat are spread out further.
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There is also more water displacement occurring with the boat. The shape of the boat allows it to displace the water to equate for it's weight. This allows it to be less dense than the water. The shape of the ball is not conducive to it displacing the water. The ball is denser than the water so that when it is placed in the water, it will sink.
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This demonstration can be used to reinforce the idea that even though objects can be made of the same materials and weigh the same, their densities can vary. Objects that have a higher density than the liquid they are in will have sink. Objects with a lower density than the liquid they are in will float. This can be explored further with other objects.
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Source of demo:
For Cartesian Diver materials:
(nuts) https://www.teachersource.com/product/cartesian-diver-ballast-nuts/density-divers
(pipettes) https://www.teachersource.com/product/graduated-transfer-pipets-222/density-divers
(kit) https://www.teachersource.com/product/cartesian-diversions-class-kit-only/density-divers