Monday, May 24, 2010

Air Pressure: How to Blow up a Balloon in a Bottle

Can you blow up a balloon in a bottle?

Go ahead, give it a try. Place a balloon into a bottle (a 2 liter soda bottle works well), leaving a bit sticking out to blow into. Give it your biggest breaths. What happens?

You'll get about this far:

Why can't you get it any more inflated? There's still room in the bottle and the balloon can certainly stretch further than that.

What's wrong?

It's that pesky air pressure again.

The bottle is filled with air. When you start to inflate the balloon, it seals off the opening of the bottle. The air molecules that were in the bottle are stuck in the bottle - they can't get out. And, you're trying to push more air molecules into that space as you inflate the balloon.

Unless you've got lungs unlike any person I've ever known, you don't stand a chance pushing against those air molecules in the bottle.

But, what if you gave those molecules an escape?

Try again, but this time, stick a straw in the bottle with the balloon - the end of the straw needs to be sticking out of the bottle.

Now blow (on the balloon, not the straw!)...

That's more like it!

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  1. Very neat. I enjoy reading your blog even though a lot of it is over my daughter age at the moment. I remember the opposite experiment that we did in school - how to get a hard-boiled egg through the opening of the bottle (the bottle has to have a somewhat wider opening though).

  2. Scientific experiments have never been so fun, thanks for sharing the results with us!