Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top

I've been wondering about the porosity of materials, I know that, for example the air comes out of tires/balloons because (besides having huge gaps on the rim contact area/knot) they are made of a porous material, but it has nothing to do with molecules, because the porosity appears in a much larger scale.

Now, let's say we have a flask made of a solid material, and let's say it's "perfect", I mean, the molecules are perfectly aligned, and it has a normal thickness like 1mm or so, if we fill this flask with a gas and put it in a vacuum chamber*, will the gas come out over time? I don't know if all elements have "alignable" molecules, but you have the idea.

*-let's say it's an inert chamber, no gravity, radiation and stuff.

I have a few possibilities I could think of:

  • It depends on the size of the gas molecule and the gaps between the molecules on the solid material. If it fits in, then yes.
  • It isn't possible because the forces keeping the molecules on the solid material together would also keep the gas molecules from transpassing it.
  • If the above is correct, after some time the flask itself would disintegrate (?) depending on how strong is the force keeping the molecules together. I know that in normal conditions, objects are always "losing" molecules, but will it happen in this hypothetical chamber also?
share|cite|improve this question
It depends a lot on the gas, the temperature, and the flask material. When you really need something to be gas tight, helium is an awfully frustrating little atom. Even through inches of stainless steel, trace amounts of helium are detectable. In short, to your proposed answers: Yes (the gas and the flask material matter), not really (forces may hold materials together while still allowing for imperfections, and even without this, gasses can tunnel), and yes (even tungsten has a finite vapor pressure). – KDN Dec 22 '12 at 15:26
Thank you. I think eventually everything will "disappear" in some way. Even a perfect diamond in a completely inert environment. – Nikki Vidanicus Dec 22 '12 at 18:01
up vote 3 down vote accepted

All of the ice core methods of measuring temperature in past millenia from gas trapped in bubbles in ice, and measuring concentrations , depend on the fact that permeability is small even in imperfect containers, as the ice in the glaciers.

So the answer to "how long" would depend on the exact materials and geometry and temperature, and it will be in any case the gas will disappear if you wait long enough, and long is more than a few million years. There is of course the sublimation of the molecules of the flask which will eventually make holes no matter how perfect the material, but also the quantum mechanical "tunneling effect" will come to play a role if time is long enough.

share|cite|improve this answer
It would be interested to see some real-life estimates for tunneling rates in this case (this has surely been done, but I don't know too much about cond-mat). – Vibert Dec 22 '12 at 23:47

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.