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Name:  C
Status:  student
Age:  17
Location: N/A
Country: N/A
Date: 2000-2001

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.

Tim Mooney

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 molecules can.



Helium atoms are much smaller that nitrogen atoms. Air is mostly nitrogen. Thus, it is far easier for helium than air to escape an inflated balloon.


Ali Khounsary, Ph.D.
Advanced Photon Source
Argonne National Laboratory

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