balloon in a bottle experiment

andieje
andieje used Ask the Experts™
on
Hi

I don't fully get this basic experiment:

Empty a 1L pop bottle and make a small hole in the side.
Place a balloon inside and wrap its open end around the top of the bottle.


Apparently the balloon won't inflate if you cover the whole in the bottle with your finger because the air in the bottle is exerting a pressure on the balloon. I would have thought that you could still inflate the balloon and the air in the bottle would be compressed?

Also once you have inflated the balloon (by uncovering the hole) the balloon will say inflated when the bottle is removed from your mouth (if you keep your finger over the hole). Why does it stay inflated?

Why will it then deflate when you move your finger off the whole?

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You can still inflate the balloon, it would just take more air pressure inside the balloon to overcome the air pressure in the now sealed bottle, probably hard to do with just your lungs (unless you fancy doing yourself some damage). A bike pump for instance could inflate it.

The balloon stays inflated for the same reason that it is hard to inflate. The air pressure in the sealed bottle, when you place your finger over the hole after inflating the balloon, will keep the balloon where it is. The pressure inside the balloon is greater than the pressure outside (in the bottle), therefore it stays where it is. Think of it this way, if the balloon were to deflate, what would fill the space in the bottle? If you uncover the hole then air can come in and keep the pressure outside of the balloon equal or greater than the pressure inside, allowing it to deflate.
An easy visual example of this would be to forget the balloon and the bottle and use a clear drinking straw.
If you immerse one end of the straw in water with the other end open, water will fill the straw to the normal water level. (Hole in the bottle uncovered).

Cover the upper end of the straw with your finger and immerse the straw again, the air pressure prevents the water from entering the straw. (Inflating with the hole covered)

Immerse the straw until there is some water in it, now cover the top end with your finger. The straw can be lifted clear of the water container and the water in the straw remains inside as the air pressure outside is greater than the air pressure inside, or is greater than the air pressure inside would be if it expanded to let some water was to escape. (deflating with the hole covered).

Author

Commented:
thanks for your replies

I'm still not sure why the balloon stays inflated. The air pressure outside the balloon is greater than the pressure in the bottle pushing on the balloon? But the bottle isn't sealed. The pressure in the balloon isn't greater than air pressure so why wouldn't it exit via the top

Author

Commented:
sorry, i didn't write that very well as I'm on the phone at the same time!!

I don't get why the balloon stays inflated. All i know about pressure is that if the pressure in the balloon was greater than air pressure the air would exit the balloon.

The pressure inside the balloon is greater than the pressure outside the balloon. Ok
BUt what about air pressure? Isn't it greater than air pressure so it would leave the balloon?

You say: Think of it this way, if the balloon were to deflate, what would fill the space in the bottle?

Why does anything have to fill the space in the bottle? Couldn't the pressure in the bottle just decrease?

Are there some basic rules about pressure I am missing?

Ironically i get the straw example but not the balloon one :)
Hi andieje,

For a complete explaination of th8is experiment, ave a look at this Internet site Experimentopia - Air Is Not Nothing:

The Ideal gas law states PV=nRT where P = pressure, V = volume, R = the ideal gas constant, T = temperature and n = moles of gas. In order to expand the balloon you fight against the elastic forces in the balloon PLUS the air pressure in the bottle, which increases as you decrease the volume in the bottle not taken up by the balloon. Once there is a hole in the bottle the air can escape, so the moles of gas “n” can decrease as you decrease the volume in the 2 liter bottle outside of the balloon.

Even with your finger removed from the hole in the side of the bottle, the system (outside air pressure, baloon) is in equilibrium. In order to remove the air from the baloon, you must increase the air pressure INSIDE the bottle by blowing into the HOLE IN THE SIDE of the bottle.

PaulS
>Why does anything have to fill the space in the bottle? Couldn't the pressure in the bottle just decrease?

No, the pressures inside and outside the balloon must be equal else the balloon will inflate/deflate until they are equalised. Inflate/deflatebare probably the wrong words here, they describe what the balloon looks like but not what is really happening. The balloon is a flexible airtight membrane separating two spaces in the bottle. these spaces have just one exit each, the neck ofvthe bottle and the drilled hole. Increasing the pressure in either of these spaces over the other will causevthe balloon membrane to flex and equalise the pressure difference. If you seal either hole and release the other one you create a new space, separate from the outside air. Now the balloon membrane will try to equalise the pressure between thevoutside and this new space. If you prevent airventering the bottle by covering the hole it is similar to covering the straw when water is inside and the water  can't escape. The water in the straw is acting as the membrane here and will move up and down according to any pressure differences.

Author

Commented:
I'm struggling with this partly because I'm trying to think of a simple way to explain it. At school children are taught that gases are easily compressed. For example you can easily compress air in a syringe. So let me take a step back...

why can't you inflate the balloon (easily) unless your finger is removed from the hole? Without the balloon i should just be able to blow into the bottle and increase the pressure (as more molecules in the same volume if we are using PV = nRT). Right?

So it would seem the crux of the issue is with the balloon. For the balloon to inflate you have to 'add' enough air molecules to create sufficient force to overcome the elastic forces in the balloon and the force of the air pressure in the bottle (which will increase as the volume gets bigger). It is not possible to do this from simply blowing so the balloon does not expand.

So while i blow the balloon up with the hole uncovered is it correct to say the pressure inside the bottle is the same as the pressure inside the balloon? And is the pressure inside the balloon the same as air pressure?
Using Robin's notion that the balloon membrane will flex to equalise the pressure difference,
That is correct as long as you take into account the elastic forces of the balloon.
With the hole in the bottle uncovered you can ignore the bottle altogether, the balloon will blow up just the same as if it wasn't in a bottle.
With the hole covered, you are not only trying to overcome the elastic of the balloon, but you are trying to compress a bottle-sized volume of air (outside the balloon and inside the bottle) by blowing into it. It will be about as easy as trying to inflate a bottle that doesn't have a balloon in it.
I think you will be able to get the balloon to expand a little with the hole covered, but with the hole uncovered you will may be able to get the balloon to fill the bottle. The difference between 'expand a little' and 'fill the bottle' are so extreme that for a simple experiment designed to get people to think about pressure differences, 'can't blow the balloon up' is probably a fair explanation.

Author

Commented:
It's probably doing my box in a bit because i don't have a bottle and a balloon to try it. Having blown up balloons its hard to imagine the balloon staying inflated :) I did look on you tube but got distracted by imploding coke cans
Your question didn't say it was a glass bottle, and making a hole in the side of a plastic one would be a lot easier. I think I have a couple of balloons somewhere so I could try this and post a picture, my only two concerns are whether the neck of the balloon will fit over the bottle, and whether I have enough puff to inflate a balloon even if the hole isn't sealed. Last time I tried to blow a balloon up without a pump I think I fainted.
Youtube! Here's half your experiment, so tantalisingly close to showing all of it, I'll keep looking.

http://www.youtube.com/watch?v=4bknXfNh1JY
Here it is with just one unexplained part, why did he put water in the bottle?

This is your whole experiment and quite impressively done

http://www.youtube.com/watch?v=MufVVLYgt4U

Author

Commented:
Thanks you are a star! It still freaks me out seeing it though. i suppose that's why it's called weird science :)
hmm, is this on a syllabus?  Seems like a somewhat complex arrangement.

I think it is fair to say that the balloon can never fill all the space in the bottle.  The air left over has to still be there.
>The air left over has to still be there

I've decided that's why he adds the half inch of water, it will reduce the air in the bottle to a minimum when the balloon expands and increase the effect. If there was a little more air in the bottle then it would expand a bit and let the balloon shrink perhaps noticeably. By adding the water it allows more of the air to be driven out by the expansion of the balloon and so there will be much less shrinkage of the balloon when the hole is sealed.

Author

Commented:
many thanks

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