Physics

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Video

Materials

• 2 matches
• A 0.5 L (16.9 fl oz) plastic bottle
• Water

Explanation

The head of a match is porous and contains small pores with air in them. This means that the density of a match head is slightly lower than water and that it thus floats. But when you squeeze the bottle, you force water into the pores and the air inside is compressed. This causes the density of the match head to be higher than water and it sinks.

Unlike air, water can only be compressed a tiny bit. You will notice this if you try to squeeze a bottle filled with just water.

This demonstration is named after René Descartes - the French philosopher, mathematician and author who is said to have invented it in the 17th century.

Experiment

You can turn this demonstration into an experiment. This will make it a better science project. To do that, try answering one of the following questions. The answer to the question will be your hypothesis. Then test the hypothesis by doing the experiment.

• What happens if you use a 1.5 or 2 L plastic bottle?
• What happens if you have some air in the bottle?
• What happens if you use shorter or longer match pieces?
• What happens if you use matches from another manufacturer?

Variations

Instead of squeezing the bottle, you can leave the cap off and press your thumb into the opening of the bottle. This only works if the thumb clogs the entire opening.

Instead of a plastic bottle, you can use a glass jar with a screw lid. The diver sinks when you press the lid.

You can exchange the matches for other objects with air in them. Small packets of ketchup or soy sauce from a restaurant can work. If the packet sinks, you can make it float by increasing the density of the water. Do this by dissolving salt in it.

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