Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Effusion of gas from higher to lower concentration under same pressure

  1. Oct 27, 2014 #1
    If we have a hole in a container with pressure equal to the pressure outside it and the concentration of gas inside the container is more than that outside will the gas effuse out?
  2. jcsd
  3. Oct 27, 2014 #2


    User Avatar

    Staff: Mentor

    How do you have pressure inside both equal and greater to the pressure outside at the same time?

    Do you mean total pressure equal, and partial pressure greater?
  4. Oct 27, 2014 #3
    No I mean to say that pressures are same but concentration of gas inside the container is more than that outside .my question is that ,will gas effuse out if pressures are same but concentration different since gas flows from high pressure to low pressure
  5. Oct 27, 2014 #4
    . No my doubt is that if the pressures are same inside and outside the container but the concentration of gas is greater inside the container, will the gas effuse out ? Since gases effuse region of high pressure to region of low pressure
  6. Oct 27, 2014 #5


    User Avatar

    Staff: Mentor

    So you mean total pressure is identical, and the difference is just in the partial pressure.

    This is tricky, but not because of the physics involved, more because I am not sure how the effusion is defined. If the partial pressures (concentrations) on both sides of the membrane are different, there is no doubt gas will leave the side with the higher concentration. Whether it qualifies as effusion or diffusion (and when the switch between the processes occur) is not clear to me.
  7. Oct 27, 2014 #6
    Well I'm new to this topic I didn't thought that by defining concentrations we also have to account the partial pressure of the gas .yeah then I understand your answer but my actual doubt is not solved.I came across this doubt while solving a problem in which they have collected a gas over mercury in a tube with a porous plug and after some time standing in air the volume increases. So i think due pressure difference air outside enters the tube and volume increases but in the solution they have given that the gas inside diffused out and air enters the tube. The thing which contradicts is if the pressure outside is less than air will enter and if it's low gas will diffuse out till the pressure becomes same on the inside and outside .but it's given that the whole gas diffuses out and air enters .can you explain where I'm going wrong?
  8. Nov 2, 2015 #7
    Diffusion is a function of partial pressures - a gas at high concentration will diffuse into a container where its concentration is low, even if it's "against" a gradient in total pressure. That's a bit counter-intuitive, and it might be where your thinking is off. "Isobaric Counterdiffusion" is a real-world medical consequence of diffusion against a total pressure gradient.

    If you have a hole or a tube, through which gas can flow in bulk, you'll get rapid equalization of the total pressures (effusion.) After that, diffusion down the partial pressure gradients (in both directions) will take place, at a rate that depends on how long and narrow the tube is (analogous to the permeability of the walls of the container.) The tube can gradually be narrowed until it's of molecular dimensions, so where bulk flow becomes diffusion may not be sharply defined. (So long as at least one gas is moving against it's partial pressure gradient, I'd say you're dealing with bulk flow, but that's just an opinion.)
  9. Nov 2, 2015 #8

    James Pelezo

    User Avatar
    Gold Member

    You question is obscured by an opposing question ... "How can a gas at higher concentration in a closed container have the same pressure of the same gas on the outside of the container at lower concentrations?" The inherent fact that the gas is in a closed container is at a higher concentration would by simple physics produce a higher pressure and effusion from higher pressure to lower pressure would be like a ball rolling down an incline. So, yes the gas would effuse out of the container if punctured b/c you can't have equal pressures under the conditions specified in you original question.
  10. Nov 2, 2015 #9
    "No I mean to say that pressures are same but concentration of gas inside the container is more than that outside .my question is that ,will gas effuse out if pressures are same but concentration different since gas flows from high pressure to low pressure"

    If I understand the situation, TOTAL pressure is the same inside and outside of the container. The PARTIAL pressure of the gas in question, however, remains higher inside the container. Think of a propane tank with a pinhole leak, that's just reached atmospheric pressure. The partial pressure of propane inside the tank is one atmosphere, and it's pretty much zero outside of the tank. The propane, at this point, will diffuse out (not effuse), with every molecule that diffuses out being replaced by a molecule from the outside air, diffusing inward (so the total pressures remain equal.) This continues (for a long time) until the composition inside the tank matches the composition of the air outside the tank.

    It makes sense: if molecules are bouncing around at random, it's more likely that a propane molecule will find its way out than find its way in, as long as there are more of them in the tank than there are outside. The process is described mathematically by Fick's[/PLAIN] [Broken] Laws of diffusion.
    Last edited by a moderator: May 7, 2017
  11. Nov 13, 2015 #10


    User Avatar
    Science Advisor

    There will be a diffusion of mercury in one direction and of air in the other, but the rates will be generically different . To keep pressure constant, there will be a also a flow of gas.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook