Could you please explain for me why a balloon filled with
H2 or He has very little gas in it after 24 hours?
The rubber walls of a balloon are porous on a very microscopic scale. H2
and He are very light molecules, and they move very fast. They can pass
through the walls of a balloon faster than other gases. A rubber balloon
filled with air will also go flat, but more slowly.
Mylar balloons are less porous; a Mylar balloon, even when filled with
helium, will stay inflated for a much longer time.
Richard E. Barrans Jr., Ph.D.
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Update: June 2012