Why is the plastic bottle cap flat when it is ready?

After a period of time, plastic bottles will flatten out.

This is because the temperature of the gas in the bottle is higher when the boiling water is just poured out. At this time, the lid was closed. With the continuous drop of temperature, the gas in the bottle is shrinking, the pressure of the gas in the bottle is decreasing, and the pressure difference between the gas inside and outside the bottle is getting bigger and bigger. Finally, atmospheric pressure can flatten plastic bottles.

This involves two physical facts:

1, thermal expansion and cold contraction

This gas expands when heated. Add hot water, shake well and pour out again. The gas in your bottle will expand. When you pour out hot water and continue to tighten the bottle cap, the gas will cool, resulting in a decrease in volume.

2. Atmospheric pressure

After the bottle cap is tightly closed, the gas temperature drops rapidly, making the pressure inside the bottle lower than that outside the bottle. Therefore, the bottle will deflate. The reason for this phenomenon is that the pressure inside the bottle will be lower than the atmospheric pressure outside the bottle, and the atmospheric pressure outside the bottle will cause the bottle to deflate.

Extended data:

The purpose of this experiment is to prove the existence of "atmospheric pressure":

1. Take an empty mineral water bottle and immerse it in the water. If you grab the bottom of the bottle and lift it, you will find that the bottle is always full of water before the bottle mouth leaves the water. If you drill a small hole in the bottom of the bottle and repeat the above experiment, you will find that the water surface in the bottle is always flush with the water surface outside the bottle, which can prove the existence of atmospheric pressure. ?

2. Take an empty mineral water bottle, inject a small amount of hot water into the bottle, shake it, pour it out, and immediately cover the bottle cap. After a while, I found that the bottle slowly sank inward, which can prove the existence of atmospheric pressure.

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