Physics

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Video

Materials

• 1 empty soda can
• 1 pair of grill tongs
• 1 bowl
• Cooktop
• Water

Short explanation

When the water boils, the can fills with water vapor. When the can is then immersed in the cold water, the water vapor cools down and condenses into water droplets. This leads to a void inside the can and the air outside can compress it without resistance.

Long explanation

When water boils, it changes from a liquid to a gas. 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 can and pushes out the air that was there before.

You might think that water vapor looks like the smoke you're used to seeing above boiling water. But the smoke is actually water vapor that has once again cooled down and condensed into small drops of liquid water. Water vapor is in fact transparent.

When you dip the can in the cold water, the water vapor cools down and condenses to liquid water. This means that the volume of water in the can decreases 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 down the can is difficult to know - this is because the volume of water vapor largely depends on the temperature - but it will likely be at least 1 000 times smaller.

So when the water in the can condenses, and its volume decreases drastically, a void arises in the can. The gas (water vapor) that was just there and pushed on the inside walls of the can is gone. On the other hand, the gas (air) outside the can remains, and it pushes on the outside walls of the can, and it now compresses the can completely.

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 everything around them. So when the water molecules of water vapor collide with the can, they push on the inside walls of the can. Likewise, the air particles (mainly nitrogen and oxygen molecules) push on the outside walls of the can.

When a gas cools down, the particles that make up the gas begin to move more slowly. Temperature is a measure of the kinetic energy of a substance's particles. They then also end up closer to each other, because their collisions with each other are milder and they do not repel each other as much anymore. This also means that the gas' pressure on its surroungins decreases. If the gas is cooled down sufficiently, it becomes liquid. Then the particles are so close to each other that they have close contact all the time, and the substance behaves in a completely different way - a substance in a gaseous state expands and fills the entire container it is in, while a substance in a liquid state only adapts its shape to its container but retains an almost constant volume. And when it comes to the pressure in the can, liquid water overall exerts much less pressure than water vapor.

You may also notice that some water from the bowl is forced into the can. This is the ambient air that pushes on the water surface and thus pushes water into the can. Had there been gas of equal pressure inside the can, this would not have happened. So the air both compresses the can and pushes water into it.

You might mention "suction" in this demonstration, but there is no kind of "suction force" or anything like that - just pressure.

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 no water in the can, but instead heat the air in it before dipping it in cold water?
• What happens if you use a larger metal can?
• What happens if you have lukewarm water in the bowl?
• What happens if you don't turn the can upside down, but instead dip it bottom first in the water?

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