First of all , how can one prove that gases have no fixed volume ? And if (though i know that it have) lets take a case . Take small amount of gas and fill it into a large box , about size of earth , consider that box has vacuum in it . The gas should fill all the earth but if does that , then the intermolecular attraction force should be inconsiderable or negligible at such large distances . What happen then ? There should be a limit to the volume ...

Another question is why molecule let H2 not attract another gas particle let He as there is same van der Waals force ........

  • $\begingroup$ Even vacuum can occupy the whole universe... $\endgroup$
    – Greg
    Aug 13, 2016 at 17:57
  • 1
    $\begingroup$ Of course this isn't entirely true. The "normal" gas laws neglect gravity. Fortunately for us a sufficiently large amount of hydrogen gas in outer space can collapse into a star. $\endgroup$
    – MaxW
    Aug 13, 2016 at 18:04

1 Answer 1


Gases have no fixed volume, if you put it in an infinite vacuum, a gas will at equilibrium have an infinite volume. You'll be waiting a while for it to reach equilibrium though. H2 will have some interaction with He, but it's very weak, so for most purposes can be neglected.

  • $\begingroup$ Why b/w he and h2 is weak force there but not b/w h2 and h2 $\endgroup$
    – Prabhat
    Aug 13, 2016 at 15:19
  • $\begingroup$ @dr.honey Yes there is also an attractive force between individual H2 molecules. $\endgroup$
    – Dan Burden
    Aug 13, 2016 at 15:49
  • $\begingroup$ Ya i know .....i want to know why only He-He has attraction but not in H2-He? $\endgroup$
    – Prabhat
    Aug 13, 2016 at 16:12
  • $\begingroup$ @dr.honey All gas molecules interact, but He-He interaction is weakest known. See comments here $\endgroup$
    – Mithoron
    Aug 13, 2016 at 20:47

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