# Gas Bubbling Through Water

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...really descriptive title, I know.

I was wondering about the physics of devices in which air bubbles pass through a liquid and then out again.

I got to thinking about a straw, and how the water is moved through the tube. You have two openings, one in the cup which is really big, and then the end of the straw which is really small. If you reduce the pressure on the straw end the water gets pushed up the tube by pressure on the other end.

But what if you reduce pressure on the cup side? It seems that the air will bubble through the liquid and out. Is this because there isn't enough pressure to push all the water out?

My question is how you can tell whether the air will bubble through or whether the water will move, and if it will bubble, how much pressure will be required to make that happen.

I hope that my question is clear enough. I'll try to clarify as requested.

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assuming the pressure in the straw is kept constant the pressure will force the water down and then escape at the edges and "bubble" out.

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right, while the air does'nt reach the end of the straw, it will push the liquid, this being pushed up in the cup.

when it reaches the end of the strw it will always bubble. Not sure but I think that the condition for a bubble to appear is that the gas inside the liquid is at a higher pression that the one at the surface. as this is the case, the air will always bubbles!

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