Helium and Air Behavior
Why does a balloon filled with helium shrink faster than a
balloon that is blown up with your mouth?
Any balloon shrinks because the gas molecules within the balloon can slowly
pass through the thin material of which the balloon is made. A helium
molecule may be the smallest molecule in existence, composed of an
individual helium atom. A carbon dioxide molecule (what you breath into a
balloon) is much larger, composed of one carbon atom and two oxygen atoms.
Also, helium molecules don't interact. They can be right next to another
molecule and hardly be affected. Carbon dioxide interacts quite a bit with
many other molecules. The balloon has much more trouble holding the small,
non-interactive helium molecules than the large, interactive carbon dioxide.
Helium molecules are smaller than the molecules in the air that comes out of
your mouth. Balloons have small holes in them.
Helium atoms are smaller and move faster than air molecules. They pass
through the tiny pores in the rubber of the balloon faster than the air
Helium atoms are much smaller that nitrogen atoms. Air is mostly
nitrogen. Thus, it is far easier for helium than air to escape an
Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory
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Update: June 2012