What's in those balloons that float up into the sky? Helium! What's so special about helium, that it makes balloons lighter than air?
It's all about density. Density is defined as the weight of a substance divided by its volume. For example, the density of iron is 491 pounds per cubic foot (a cube 12 inches on each side), or 7870 grams per liter. It's pretty obvious that, if you had a piece of iron smaller than a cubic foot, it would weigh less. But the ratio of the weight of the smaller piece to its volume would be the same as the larger piece.
If you mix substances with two different densities, the more dense one will sink, or looking at the the other way around, the less dense substance rises. If you dropped a chunk of iron (whether large or small) into water, it would sink. But if you put a piece of wood (most kinds, at least) into water, it floats because its density is less than that of water. Helium gas is less dense than air (0.011 pounds per cubic foot or 0.18 grams per liter for helium versus 0.078 pounds per cubic foot or 0.078 grams per liter for air), so balloons filled with helium float!
It's all about density. Density is defined as the weight of a substance divided by its volume. For example, the density of iron is 491 pounds per cubic foot (a cube 12 inches on each side), or 7870 grams per liter. It's pretty obvious that, if you had a piece of iron smaller than a cubic foot, it would weigh less. But the ratio of the weight of the smaller piece to its volume would be the same as the larger piece.
If you mix substances with two different densities, the more dense one will sink, or looking at the the other way around, the less dense substance rises. If you dropped a chunk of iron (whether large or small) into water, it would sink. But if you put a piece of wood (most kinds, at least) into water, it floats because its density is less than that of water. Helium gas is less dense than air (0.011 pounds per cubic foot or 0.18 grams per liter for helium versus 0.078 pounds per cubic foot or 0.078 grams per liter for air), so balloons filled with helium float!
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