I know this sounds crazy, but I wonder if the stuff they put in “helium-filled” balloons is not just pure helium.
I had a birthday recently, and the balloons used for the decorations eventually lost their helium. My eldest child asked me a simple question, “OK, if the reason why it no longer floats is because it’s lost its helium, why is it still inflated?”
Why indeed? Is it because there is still helium in the balloon, but not enough to float the balloon? Or is it because they don’t put pure helium in the balloon in the first place, but instead mix it with air: thus when the helium escapes it leave ordinary air behind?
I’d suspect that even if the balloon is filled with pure helium, as it slowly “leaks” (maybe “diffuses out” would be better), there reaches a point where there just isn’t enough He to sustain flight. You’ve got to consider that the system in question isn’t just He, but He + balloon. The balloon totally filled with He is lighter than air and floats; with just a little He, and relatively more of the system consisting of balloon, it ceases to float.
If you think about it backwards, suppose you started filling the balloon. Would it float when there was one molecule of pure He in the balloon? Two? How about a whole mole of He atoms/molecules? Eventually it will have enough and start floating, until then, it’s still to heavy.
As atoms go the two most common lighter than air gasses, Helium and Hydrogen, are pretty small. Small enough to easily slip through a permeable membrane like a toy balloon more easily than Nitrogen and Oxygen. As far as not floating Pantellerite is right on the money.
Also, helium is not the only gas that can “diffuse” through the latex. Blow up a balloon and let it sit for a few days. It’ll still shrink, (presumably until the air pressure is equalized(?)). So, it’s entirely possible that, as the helium is flowing out, other gases (oxygen, nitrogen, carbon dioxide, etc.) will also flow in, thus preventing the balloon from completely deflating.
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Hey, I like that hat, man. They sell men’s clothes where you got that?
If the membrane is thin enough to pass helium perhaps it’s a two way street and other gasses also contaminate the helium in accordance with Entropy (God, I love that word,) until the balloon reaches (drumroll please,) equilibrium with the surrounding air’s latex permeable gases.
As long as the balloon maintains positive interior pressure (greater than 1 atmosphere caused by the resistance of the stretchy material compressing the gas inside) other gases won’t flow in. That would be like ocean flowing up a river (never mind tides–that’s something else). If the inside of the balloon and the outside of the balloon are in equilibrium, the balloon would be flat.
Interesting theory, but I can’t see how gas (air) at normal atmospheric pressure can flow INTO a balloon that contains gas at higher pressure, even if the balloon were 100% gas permeable. Once you reach a pressure equilibrium, I would tend to think the flow would mostly come to a stop.
Since I’ve never seen a balloon INflate due to spontaneous gas penetration (blow up a balloon 1/3 of the way and see if it grows…), I’d venture a guess that there is a one-way flow outward of helium and virtually no inflow of other gases. Especially since the balloon is still slightly pressurized from the elasticity of the skin. It seems that there is a minimum pressure threshhold for gas to leak.
Experimentation is in order…somebody needs to take a bunch of old “stale” He2 balloons, cut the stems, and HUFFFFFFFFF. If you talk like a munchkin, then they are filled with He2 and you know that it leaks out until it reaches a near pressure equilibrium. If your voice sounds normal, then you know that the gas flow is 2-way, and the balloon refills with air as it empties itself.
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To answer the OP, the gas contained in floating balloons is, of course, not pure Helium if you want to get technical. I would guess that it’s probably around 95% Helium when it was filled, although I wouldn’t be surprised if it were 99% pure; Helium is a relatively cheap gas.
As has been mentioned, in order for a balloon to float, it’s overall density must be less than that of the air surrounding it. Balloons are not impermeable to the effusion (geek term) of small molecules however. When you fill a balloon up with Helium and put it in air, the Helium inside will tend to escape outwards, trying to balance the concentration of Helium on both sides of the balloon. Similarly, the air outside will tend to leak into the balloon, as it wants to balance the concentration of air on both sides as well. Since Helium is a very light molecule, it moves around space at a higher average speed than air molecules. That means Helium molecules can penetrate the balloon more easily than air (just imagine them bumping into the balloon randomly), and you end up with a net movement of gas molecules in the outward direction. That’s why your balloon will shrink as time passes; more gas is coming out of it then going in.
While all this is going on, your balloon is gaining weight because the small amount of air entering it weighs a lot more than the large amount of Helium leaving it. As the balloon shrinks and gets heavier at the same time, its density eventually surpasses that of the air surrounding it and it will just sink to the ground. At this stage, your balloon might still look pretty buff, but its density is just too high to float anymore.
Having a pressure gradient across the balloon surface will change things a little, but it can’t stop effusion altogether. If the balloon was overfilled a little bit, you’ll just start with more Helium leaking out, and less air getting in. The little positive pressure you had will soon be gone, and you’ll end up in equilibrium as far as pressure is concerned.
So, in the end, the concentration of Helium and air will be balanced on both sides of the balloon. No more net movement of molecules will occur, and your balloon should stop shrinking as well. Then again, if your balloon was made of some elastic material, it will continue to exert a small pressure on the air within it until it just shrivels…
Zor’s right. A gas will have a net movement from a low pressure to a high pressure if the concentration of said gas is higher in the low pressure area. I work in a chem. lab, and see this happen all the time. Our high pressure gas lines or high pressure reactors sometimes get (slow, inaudible) leaks, resulting in air getting into the gas lines or reactors even though the gas lines or reactors can be as high as 2500 psi (that’s over 150X atmospheric pressure).
Things are random only insofar as we don’t understand them.
Actually, heliums is a very expensive gas. There are only a few natural gas mines in the world which contain any usable amount of helium. The “helium” sold for inflating balloons contains far less than 95% helium. More like 50%, if I remember correctly - just enough to make the balloon float. I’d guess the rest is air. In fact, they probably contain a fair amount of oxygen, so people who inhale them don’t suffocate. At least that’s what I’d do if I were selling balloons.
OK, as the author of the OP, I judge scr4 to be the winner.
(1) It confirms what I suspected, that pure helium was expensive, and that balloon-grade helium contains a lot of plain old air. That alone explains why the balloon is still inflated.
Questions for Zor and Steve-o: if you have a semi-permiable barrier between two different gasses at the same pressure/temperature, is Brownian motion the only force that causes the two gasses to mix? I can’t imagine that the gas “knows” that is hasn’t spread to the other side of the barrier - it just bumps around, and gets to the other side that way.
In the case of unequal pressure between the two gasses (a balloon), more gas leaks out out than leaks in because there more atoms, right? So not only does helium leak out, but it becomes less pure over time. This might explain the still-inflated non-floating balloon, but not some other behaviors I’ve noticed.
The balloon loses perhaps third of its volume when it gets to the stage that it stops floating (over the space of very few days), but once it gets to this state, its volume seems to stabilize. It can be weeks before it completely deflates. So that behavior lends additional credence to the theory that there are two gasses in the balloon.
The gas doesn’t have to ‘know’ whether it has spread to the other side of the barrier. What happens is that since there is a higher concentration of helium atoms in the balloon, there are more helium atoms near the surface of the balloon inside of it than outside of it. Only those atoms that hit the balloon can go through, and since there are more helium atoms inside of the balloon than out, more helium atoms will leave the balloon than enter it. When the concentration of helium is the same inside and out, then the number of atoms hitting the outside and coming in will be equal to the number of atoms hitting the inside and coming out.
(You can also see that the temperature will affect the rate of diffusion, since there will be more helium atoms hitting the ballon when the temp is higher).
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I believe that those half-filled, semi-limp balloons tend to stay that way because they’ve been stretched out. They’re not as “elastically tight” as they used to be. You could poke a hole in one, and it wouldn’t regain its original shape because it’s been deformed. As a result of that deformation, it’s no longer exerting as much pressure on the gas inside, and the leakage slows thereby allowing the balloon to remain in it’s saggy, semi-filled state for a long time.