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Balloon Permeability
Name: Elizabeth
Status: student
Grade: 6-8
Location: Outside U.S.
Country: USA
Date: N/A
Question:
How would you keep air from leaking out of a balloon? How
would you keep helium for leaking out of a balloon? Which gasses
would leak out fastest?
Replies:
Hi Elizabeth,
It is very difficult to prevent completely the leaking of gases
out of a balloon. The property that gives balloons their elasticity
also makes them porous to very small molecules. Effusion is the term
used to describe how fast a gas particle can pass through tiny holes
in the walls of a container. Most plastics and rubbers have little,
tiny gaps between the molecules, and these gaps are big enough for
gases to pass through - eventually.
The helium atom is very small, has a very low molar mass - and
this is what gives this gas a very low density. This low density is
what makes balloons buoyant in air. However, we also know (from Graham's Law)
that the lower the molar mass of the substance, the faster it will effuse.
So helium, which is second lightest gas and the lightest non-flammable
gas, will effuse faster than most other gases. So the property that
makes helium buoyant in air, is also what allows it to effuse out of a
rubbery container.
Dr. Roberto Ma. Gregorius, Assoc. Prof.
Hi Elizabeth,
The only practical way to reduce air leakage through a normal rubber
balloon, is to make the balloon from a different material that is not as
permeable to air. Balloons made of Mylar (a type of plastic material)
are not very "rubbery", but they are far less permeable to air
molecules, and so they stay inflated longer.
Helium leaks out the fastest of any gas. This is because, first of all, the
helium atom is very small, and second, because helium is an "inert"
gas and exists only as single helium atoms. Air, on the other hand, is
mainly composed of oxygen and nitrogen, and both these gases exist
only as molecules composed of two atoms (O2 or N2) that are "stuck"
together. As a result, the molecules that mainly make up air are much
larger than single helium atoms. This explains helium's much greater
leakage rate.
A hydrogen atom is smaller than a helium atom, so you would expect
hydrogen to have a very high leakage rate. But like nitrogen or oxygen,
hydrogen molecules exist only as a pair of hydrogen atoms stuck
together, and so are much larger than a helium atom.
Regards,
Bob Wilson
Helium leaks out of a rubber balloon much faster than air does. Helium only
remains in the balloon for a day or so (You can tell because the balloon
"sinks". )
If you use aluminized Mylar, the helium stays in the balloon for a much
longer time, because it does not diffuse through the aluminum coating. If you
do a web search on the search topic "diffusion of helium in balloons" you
can find a lot more details.
Vince Calder
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Update: June 2012
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