Physics

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Video

Materials

• A 0.5 L (16.9 fl oz) plastic bottle
• 1 bowl
• Food coloring (optional)
• Water

Short explanation

As the air in the bottle cools, its pressure on the water surface decreases. The air outside the bottle then has a comparatively higher pressure on the water surface, and pushes water into the bottle.

Long explanation

When a gas (such as air) is heated, the particles (atoms or molecules) that make up the gas begin to move faster. Temperature is in fact a measure of this kinetic energy of a substance's particles. Consequently, if the gas is kept within an enclosed space, the frequency and speed of the particles' collisions with each other and their surrounding also increase. In each collision, the gas particle pushes on what it collides with. So, when a gas is heated, its pressure also increases.

Because the particles in a gas repel each other all the time, gas escapes from its confined space if it can. When the bottle was emptied of water, it was immediately filled with ambient air. This air was quickly heated by the heat of the bottle and its pressure increased. Some of the air then escapes from the bottle again. But this "escape" ends as quickly as it began, and this is because the air pressure inside the bottle has now become equal to the air pressure outside. At this moment, the air inside the bottle has a higher temperature and lower density (the air particles are far from each other) than the air outside, which has a lower temperature and higher density (the air particles are close to each other). But their air pressure will be equal. The air outside pushes as much on the air inside the bottle as vice versa. As you dip the bottle in the water, imagine how the air pushes down on the water surface, both from inside the bottle and outside.

When the bottle is placed in the cold water, the air inside the bottle starts to cool down again. This causes the air pressure in the bottle to decrease, and it is now lower than the air pressure outside. Therefore, the air outside the bottle now pushes water into the bottle. This happens until the air pressure inside and outside the bottle is the same again. Nature hates pressure differences.

It is easy to talk about "suction" in this demonstration, but there is no kind of "suction force" or something like that - just pressure.

Some plastic bottles can crumble a little when filled with boiling water. This is because the plastic molecules start to curl when heated. You will then get a plastic bottle that is a bit smaller than before.

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 have a greater or smaller temperature difference between the hot and the cold water? Test boiling hot and ice cold water for a really clear result (use oven mitts).
• What happens if you use a larger bottle?
• What happens if you use a narrower bottle?
• What happens if you use a bottle with a larger opening?
• What happens if you put the cap on the bottle before putting it in the cold water (see below)?

Variations

Try putting the cap on the bottle before dipping it in the cold water. Then no water can be forced into the bottle and the air pressure in the bottle remains low. This means that the air outside will continue to push more on the bottle than the air inside the bottle does, thus compressing the bottle. After crumpling the bottle in this way, you can also try to restore it to its former luster by placing it in boiling water.

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