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Why is the pressure inside a soup bubble greater than the pressure outside?
Question Date: 2012-02-23
Answer 1:

When things get hot, the volume generally increases. For example, if you have a sealed can of something and you heat it up, you have to be careful it doesn't explode. Take, for example, any of the pressurized spray canisters (like hairspray or spray paint) and you'll see there's an explosion warning! If they heat up, the gas inside wants to expand, and the pressure will increase until the container explodes to violently release this gas.

Boiling (like in a soup) is no different. As you create gas bubbles, the hot gas wants to expand, so the bubble rises until it reaches the surface of the soup, where it pushes against the surface and the atmosphere and usually bursts, releasing the water vapor (gas) into the atmosphere.

Answer 2:

Good question. Think of the soap film of the bubble as a balloon, it keeps the air inside the bubble from mixing with the air outside of the bubble and it stretches as it grows. Now to blow up a balloon you need to force air in to the balloon, but air on the outside of the flat balloon is already there and needs to be pushed out of the way so that the balloon filled with air can take its place. Since air molecules on the inside of the balloon can not ask the air molecules on the outside of the balloon to please move out of the way they need to push them out of the way. The force the air molecules on the inside need to push the outside air away is related to the pressure of the inside gas. ( Pressure * Surface area = the force).

Now once the balloon is filled the pressure is very close to the outside pressure*, but slightly higher. The reason for this is because the walls of the balloon ( or bubble) are rubbery and need to stretch out to hold the air. Stretching is a force and the balloon or bubble want to return to a less stretched out state, but it is the air inside keeping them stretched out. The walls of the balloon then slightly squeeze the are and increase the pressure.

* The reason we know that the pressure on the inside of the bubble is close to the pressure outside has to do with the balancing of forces. The force of the air outside pushing in on the balloon has to be equal to the force of the air on the inside pushing back on the air outside for the balloon to stop expanding.

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