Physics

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Video

Materials

• 1 glass bottle - preferably with a small opening
• 1 large bowl
• Microwave oven
• Water
• Safety equipment: 1 pair of protective goggles, 1 pair of protective gloves

Short explanation

When the water boils, the bottle fills with water vapor. When the bottle is then immersed in the colder water, the water vapor cools down and condenses into water droplets. This means that a void arises inside the bottle and the air outside can push water into the bottle without resistance.

Long explanation

When water boils, it changes from a liquid state to a gaseous state (it does so even at room temperature - but at the boiling point it happens much faster). The water vapor, i.e. water in a gaseous state, fills the bottle and pushes out the air that was there before. Water vapor and air are both transparent, so you will not see this happening.

A gas (such as air or water vapor) always exerts a pressure on its surroundings. This is because the particles (atoms or molecules) that make up the gas are constantly moving and colliding with each other and the surroundings.

The moment you dip the bottle in the water, there are two "gas bodies" that push down on the water surface in the bowl. One is the water vapor in the bottle and the other is the air in the atmosphere. The moment you dip the bottle in the water, both gas bodies push equally on the water surface. However, the water vapor in the bottle immediately cools down. And it cools down so much that it condenses to liquid water. This means that the volume of water vapor in the bottle is reduced many times over. Liquid water takes up much less space than water vapor, because the water molecules in liquid water are much more tightly packed. Exactly how much the volume of water decreases when you cool the bottle is difficult to know - this is because the volume of water vapor largely depends on the temperature - but it will most likely be at least 1,000 times smaller. The water vapor literally implodes.

So when the water in the bottle condenses, and its volume decreases drastically, a void arises in the bottle. Almost a vacuum. This means that the gas (water vapor) that was just there and pushed down on the water surface is gone. However, the gas (air) outside the bottle remains, and it still pushes down on the water surface. Therefore, the air outside the bottle now pushes water up into the bottle.

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

The key to success with the demonstration is that the water vapor doesn't escape from the bottle until you have had time to dip it in the water. Therefore, you should be quick to dip the bottle in the water, and choose a bottle with a small opening. The shape of the bottle probably also plays a role, where some bottles retain the water vapor better. Also remember to dip the bottle deep enough in the water, otherwise air will be sucked in when the water level in the bowl drops.

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 wait a while before dipping the bottle in the water?
• What happens if you have hot water in the bowl?
• What happens if you use a bottle with a larger opening?
• What kind of glass bottle works best?
• What happens if you fill half the bottle with water?
• What happens if you let the opening of the bottle just touch the surface of the water?

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