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Question: Why does helium float?
This question is directly related to a physical property of the matter called density.
Density can be defined as the ratio between the mass and the volume of a substance or compound at a given temperature and pressure. Qualitatively, for gases, density can be understood as the measure of the relative "heaviness" of gases occupying the same volume.
Let's consider the gases involved in this problem: air and helium. A mixture of gases composes air; the two major components are nitrogen and oxygen gases, one liter of air weighs roughly 1.28 grams. One liter of helium gas weighs approximately 0.18 grams. Comparing the masses, we conclude that the helium gas is lighter than the air; therefore, helium gas can float in air. The principle is the same when ice cubes are added to a glass of soda, the ice cubes float in the soda because they are less dense (lighter) than the soda. But we have to consider one more aspect of this question: if the helium gas is trapped in a balloon, would it make a difference is instead of the usual big and thin balloon we would put the helium gas in a 2-liter soda bottle? Would it still float? The answer is, again, based on the mass. The 2-liter soda bottle is too heavy for 2 liters of helium gas to lift it up. It would not float. In conclusion, in order for a helium balloon to float it is necessary to consider the balloon itself, it has to be big and made of a light material so the total mass of the helium gas plus the balloon is lighter than the air that it is being displaced.
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