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Pressure in soda can

  1. May 18, 2006 #1
    If you shake an (unopened) soda can or bottle, will the pressure inside increase or stay the same? If it does increase, what is the mechanism? Assumption: the soda has been at constant temperature for a long time.

    (please don't answer "of course it increases" just because the soda explodes when you open it after shaking. That's a different issue)

  2. jcsd
  3. May 18, 2006 #2


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    My opinion only; I've never really looked into it. Since there's no chemical or thermal change in the contents, the pressure should remain constant. I suppose that the agitation merely disturbs the CO2 out of solution more rapidly than it normally escapes.
  4. May 18, 2006 #3
    I dont think you can disregard that statement. The point here, I think, is to explain why the can would explode after shaking, but not if it isnt shaken. obviously the pressure increases when it is shaken, so what causes it? I don't think the CO2 in solution exerts any pressure, but when it is removed from solution, and becomes gaseous, it does exert a pressure.
  5. May 18, 2006 #4
    The explanation for why it explodes is that shaking will cause a lot of microscopic bubbles in the liquid. When you open it these bubbles will want to expand. There is no need for an increase in pressure in this picture which is why it is not "obvious" at all that the pressure would increase.

  6. May 18, 2006 #5
    When you shake the soda you release CO2 gas that was dissolved in the soda and the pressure increases. All you have to do is feel the can when you shake it. It becomes very rigid.
  7. May 18, 2006 #6


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    Can you please clarify your use of the term "release" in this context?
  8. May 18, 2006 #7
    I thought they dissolve C02 into the soda, and it gets released from the liquid when you agitate the soda, and builds up pressure in the can.

    When the C02 changes into a gas state, it has a large volume increase, and so the pressure builds up.

    Last edited: May 18, 2006
  9. May 18, 2006 #8


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    The question here is, how does the CO2 change from dissolved to gaseous merely from shaking, without any other change.
  10. May 18, 2006 #9
    Some chemical substances are sensitive to shock and vibrations, maybe this is what is happening?

    Help, we are in need of a chemist!
  11. May 18, 2006 #10
    The first question is, does it really change from dissolved to gaseous? The first thing we have to establish is whether it really does and whether the pressure actually increases.

    I just made a little experiment using 4 cans of beer. I shook two of them. Then I tried to feel if there was any difference in how rigid they felt before and after and also comparing with the two cans I didn't shake. I couldn't feel any difference.

  12. May 18, 2006 #11


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    Evidence for the pressure increasing; the old designs used to burst if you shook them up enough.
  13. May 18, 2006 #12
    I thought the shaking primarily created and distributed nucleation sites rather than doing all the pressure-increasing there and then. Then when you open it and release a bit of that pressure, all the nucleation sites grow bubbles and push the pressure up enough to expand the contents all over your face.
  14. May 18, 2006 #13
    Ok, now try this: take a plastic bottle of soda, preferably half full or so. close the cap tightly and squeeze the bottle, it should give fairly easily. now shake it and try to squeeze.
  15. May 18, 2006 #14


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    Beer might not be carbonated enough for you to feel the difference.
  16. May 18, 2006 #15


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    I've stopped drinking soda, but this does suggest an interesting test. Have someone else shake up a plastic bottle. Without knowing which bottle was shaken, see if one can determine which was disturbed by manually testing the rigidity which one was shaken up. (I suppose a purist would insist on a double-blind experiment, but that level of care would probably only be needed if initial results look promising).

    This could actually be handy if it works - sometimes a single bottle in a bunch gets disturbed (as in - rolls down the driveway, for instance) and it would be handy to be able to test to identify the disturbed bottle (if one gets it confused witht the other bottles), or to see how safe it is to open yet.
  17. May 18, 2006 #16
    Initially of course the pressure will increase, until a new equilibrium is reached, that's trivial. My question was, does the pressure increase after equilibrium has been reached if you shake the bottle?

  18. May 18, 2006 #17


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    Well, then get two bottles, empty half the soda out of both, let them sit for a few hours to get back to equilibrium, then shake one and see if you can feel the difference.

    If you really want to get scientific, you could drill a hole in the cap of each and feed a bicycle tire valve stem through it to actually measure the pressure.
  19. May 18, 2006 #18
    I think no one is disputing how the experiment would be done. I have done it on unopened cans and I can't feel any difference. I don't see what using half-emptied bottles would add. Since nobody has produced any experimental or theoretical evidence otherwise the preliminary conclusion must be no increase in pressure by shaking, which also theoretically seems plausible.

  20. May 19, 2006 #19
    But it does gain pressure. Shake up a coke bottle and see for yourself.
  21. May 19, 2006 #20
    Drop a 2-liter on hard ground from about 3 foot or so. This will surely make the bottle expand, and you will be able to feel a difference.
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