Tom Kerrigan, age 12, of Vienna, Ohio, for his question:
Why does helium make a balloon float?
A helium filled balloon rises aloft for the same reason that a cork bobs on top of the water. Both are boosted upstairs by the force of buoyancy, or floatability. Archimedes of Greece figured out how it works, way back in 200 B.C. The rules of buoyancy keeps ships afloat and makes oil float on top of a puddle. They also hoist packages of certain gases up through the air.
Buoyancy is a force that pushes an object up through a solid, liquid or gas. It depends on density, which is the mass of material packed into the size of the object. A dense rock is heavier than a porous cork of the same size. Rock sinks to the bottom of a pond because it is denser than water. The cork is boosted to the top by buoyancy because water is denser than cork.
In ancient Greece, Archimedes tested buoyancy by dunking different metals in water. He found that gold displaces less water than the same amount of silver because gold is denser than silver. He figured that when an object is immersed in water, it is buoyed up by a force that is equal to weight of water it displaces. This same force buoys up a helium filled balloon and lifts it aloft through the air.
The air is a mixture of lighter and heavier gases and a litre of the mixture weighs about 1.2 grams. Its weight is related to its density. Helium is less dense and therefore lighter than the airy mixture. However, heat makes all gases expand and lose density. Pressure makes them gain density. Hence, the comparisons apply only when temperature and pressure conditions are similar for both the air and the helium for example on an ordinary day somewhat above sea level.
Then the surrounding air is considerably denser than the captive pocket of helium inside the balloon. The size of the bulging balloon displaces an equal volume of air. And this is what creates the buoyancy boost. The displaced air may weigh. about a gram more than the helium in a toy balloon. This surplus weight is the force that pushes the balloon up through the air where the breezes waft it on its way.
The lightest of all elements is hydrogen and it is the most buoyant lifting gas But it is highly flammable and unsafe for use in balloons and blimps. Helium is used because a spark does not ignite it, though it is a little denser and less buoyant. It has 7 per cent less lifting power than hydrogen which is a small price to pay for fire proofing.
In order to lift a balloon, a gas must be lighter and less dense than the mixture of gases in the air. Nitrogen is a little lighter, but hardly enough to lift the weight of a balloon's string. Oxygen cannot buoy up a balloon because it is heavier than air. And a balloon filled with heavy carbon dioxide stays right on the ground.