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Why do helium balloons float?



 If you have ever taken an air-filled ball into the pool you may have noticed that it popped up to the top of the water and floated. That's because air is lighter than water. Even when you add the weight of the ball itself, it is lighter than the amount of water that would fit in the same space. This is called buoyancy. 

Helium is even lighter than air. A liter (about a quart) of air weighs about 1.25 grams (much less than an ounce) -- that's not very much. But a liter of helium weighs even less -- only 0.18 grams (a tiny fraction of an ounce). A balloon that's 1 foot (30 centimeters) in diameter holds about half a cubic foot of helium, which is the same as 14 liters. This means the balloon will weigh about half an ounce (14 grams) less than the same sized balloon filled with air.  So buoyancy goes to work, and just like the ball in the pool, the balloon floats upward.