Why do elements that are lighter than air float

Why do elements like Helium and Hydrogen float exactly? Why doesn’t gravity pull everything to the earth. Is the cutoff point for floating being heavier than Oxygen, O2, Nitrogen or N2?

Gravity DOES pull everything down. Heavier things push lighter things out of the way, and the lighter things are left on top. That’s what floating is.

Say you define an arbitrary region in the air, a 1-ft cube. The top surface of the cube is at 1-ft higher altitude than the bottom surface, so the air pressure on the top surface is slightly lower than on the bottom surface. This difference in air pressure pushes this cubical region upwards. If this region is filled with air, the weight of the air is exactly the right amount needed to counteract the differential pressure, and there’s no flow of air. If this region is filled with helium or hot air, it would not be heavy enough to counteract the differential pressure. So it moves upwards.

A helium baloon ‘floats’ for the same reason styrofoam floats on water.

It’s the same reason as why liquids with lower densities and solid with lower densities go upwards in water. Mix some oil and water together in a container, and shake them up. Then set it down and watch: both the oil and water are pulled toward the earth by gravity, but the water weighs more, so it is pulled more, and moves down to displace oil below it. But that’s just another way of saying that the oil weighs less, so it moves up to displace water above it. Because oil and water don’t mox, you finish up with a layer of lighter oil on top and heavier water on the bottom.

The same thing happens when you make hydrogen or helium not mix with the air by containing the lighter gas in a balloon. The heavier air above the balloon is pulled down by gravity more than the gas in the balloon, so it moves down, displacing the ballloon which then moves upwards. You can’t see the air, so all you can see is the balloon moving upwards, but it happens because invisible air is moving downwards, being pulled down by gravity.

Try a different perspective – Gravity pulls all things downward, but things react to it differently according to their density, with denser things pulled to the bottom and less dense things remaining above them. So if you drop a rock or a lead pellet in water, it falls to the bottom of the water, because it’s denser than water. Balsa wood, on the other hand, floats, not because it’s being repelled by the Earth – it’ll fall to the ground quite nicely on dry land – but because it’s less dense than water.

Air is little different than water in this regard (though of course the gas/liquid difference does play a part, but not in density questions). A very limited number of substances are less dense – “lighter” – than air is, and will rise relative to air, which is actually that they sort themselves out to be at the top of a pile consisting of rock, soil, water, air, and, say, hydrogen.

Take some Wesson Oil or Puritan Oil and pour it on the ground – it will fall to the ground, but will not make any pebbles it surrounds float. If you pour it onto water, it will float on the water.

Now fill an imaginary balloon with hydrogen, and do likewise – and it will rise, because it is less dense than air.

Now, another thought experiment. Fill a chamber with hydrogen, and fill a balloon with air and release it in the hydrogen chamber. It’ll drop to the ground.

To ask the question is to answer it.
This should have been posted on MPSIMS in the thread titled: “There ARE no stupid questions!” Well, that sounds like a challenge to me. (game)

Hydrogen and Helium floating has nothing to do with any other gases in the atmosphere. They would both float if there was no other gases.

They are lighter than air because each molecule or atom is lighter than any other gas molecule or atom. Each molecule or atom therefore (by the ideal gas law) moves faster and has enough velocity to escape the earth’s gravity and therefore floats.

Check out the Ideal Gas Law and Daltons law of partial pressures in an introductory chemisty text.

So a helium balloon would float on the moon? No, don’t think so. As others have said earlier, it’s all about comparative density. Balsa wood floats on water, not on air. Air bubbles rise out of water, but sink in a container of hydrogen. Helium rises in this atmosphere, but would sink in an atmosphere of pure hydrogen.

The reason a helium balloon floats is because the average density of the balloon is less than that of the surrounding air. However, it is also true that helium cannot remain long in Earth’s atmosphere and quickly escapes into space, because helium easily exceeds the escape velocity dictated by Earth’s mass.

It doesn’t work that way because the molecules/atoms are moving randomly. Although each molecule does indeed exceed escape velocity each molecule is also moving a random and thus different direction. Therefore at any point in time just as many molecules will be sinking as rising, thus no floating will be observed. The only reason these gases float is because, as others have said, they are forced upwards by the pressure of the atmosphere.

Think about it for a moment monsterbob. You say these things float, as in rise upwards, upwards because they exceeded escape velocity. So what happens on a body like the moon with 1/16th the escape velocity? If they exceed the Earth’s escape velocity they sure must exceed the moon’s, and thus they will rise upwards on the moon as well.

That’s OK, but what happens when they are on the surface of a space probe weighing 100 grams? If they exceed the Earth’s escape velocity they sure must exceed the probe’s which will be almost immeasurable. So you are saying that they will move upwards relative to the surface of the probe. But what provides the energy for that motion? A molecule can’t move without an input of energy, and yet what you say would require these molecules to move with no energetic input. On Earth they move upwards because Brownian motion due to heat energy combines with pressure from the overlying gases to produce a net upwards movement. But what causes the net upward movement on an airless probe?

And more importantly monsterbob, when do you think they stop floating? 10 km above the Earth? 100? 10 light years? And whatever answer you give, why did they stop moving there?

The easiest way to think of it is that it’s not the lighter gases that float; it’s the heavier ones that sink more and push them out of the way. Like cream on milk.