How does a helium balloon float?
Helium is lighter than air and just as the heaviest things will tend to fall to the bottom, the lightest things will rise to the top.
Helium weighs 0.1785 grams per liter. Nitrogen, which makes up 80% of the air we breathe, weighs 1.2506 grams per liter.
Basically, if you were to fill a soda bottle with helium and another with air, the one filled with helium would weigh a gram less than the bottle with air. It doesn't sound like a lot, but that's usually why blimps and balloons are usually really big, the one-gram difference really adds up in large volumes. Helium balloons follow the same principle as you do when you float in the water; the law of buoyancy. If the water you displace weighs more than you do, you will float.
Helium isn't the lightest element, hydrogen, weighing a mere 0.08988 grams per liter, is.
Wondering why we don't use that instead of helium?
Well, hydrogen balloons used to be really popular, but it is extremely flammable. The slightest spark will set off a huge explosion. The Hindenburg catastrophe was a direct result of that.
Did you know?
* Helium is a colorless, odorless, tasteless inert gas at room temperature and makes up about 0.0005% of the air we breathe.
* Helium's principal source is natural gas wells where it is extracted from the crude natural gas stream and purified and that it can be stored and shipped either as a gas or a cryogenic liquid.
Subject: How does a helium balloon float? 
Thu Feb 26 2009, 12:07
