SCIENCE with Ms Lun
Magic Fish aka Cartesian Diver
Materials
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Plastic bottle (500ml to 1L) (this is the diver bottle)
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Soya sauce fish (sauce packets may also work)
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Larger plastic bottle or plastic jug
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Water
Method
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Fill the diver bottle almost to the top with water.
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Place the soya sauce fish in the bottle.
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If the fish sinks to the bottom, tip the water out into the larger bottle or plastic jug.
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Retrieve the soya sauce fish. Unscrew the cap and squeeze out one drop of soya sauce, Recap.
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Repeat steps 1-4 until the fish just floats on at the top.
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Top the bottle up with water. The magic fish will not work if there is an air bubble in the bottle
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Test the diver by squeezing the bottle. The fish should sink.
Difficulty:
Time: 5 minutes
Adult supervision: some
Dangers: minimal
The Science
It's all about density. Density is how much mass something has in a certain volume. Increasing the pressure will decrease the volume (it's occupying a smaller space), increase the density but mass stays the same. When the soya sauce fish is floating at the top of the bottle, its density is less than the surrounding water. However, when you apply pressure to the bottle, this changes the density of the soya sauce fish. The molecules of air and sauce are forced closer together and they occupy a smaller volume, becoming more dense. The fish sinks. As the pressure is released, the density decreases as the molecules can expand again and the fish can rise.