Trick Balloons

HI! How come when you blow up a balloon in a jar it doesn’t work but when you put a straw in the bottle and blow up the balloon it works? Bye! hope u answer my question!
- Susy (age 13)
Forman Middle School, London, England

When you blow up a balloon, you push air inside. Then, as the balloon gets bigger, it pushes the air around it to make room for itself. When you try to blow up a balloon inside a bottle, one of the first things that happens is that the balloon expands to touch the neck of the bottle and it makes a seal, locking in the air which is inside of the bottle but not inside the balloon. There’s nowhere for this air to go, so its pressure increases as you try to blow up the balloon and it becomes rapidly very difficult to force more air into the balloon.

If you put a straw in between the balloon and the bottle, it opens a pathway alongside the balloon in the neck for the air in the bottle to escape. The air in the bottle that’s not in the balloon now just is at atmospheric pressure, so it is not that hard to blow up the balloon.

Of course, it may be possible to crush the straw between the balloon and the bottle’s neck, and then the first situation applies (the seal may be leakier though).


Adam (and Tom)

(published on 10/22/2007)