Science Buddies: "Ask an Expert"

My son has run trials comparing Air[exhaled], He and CO2 diffusion through latex balloons. He inflated 4-5 each to the same size and measured the changes at 3-hour intervals for a few days. He expected that the He would be the fastest to escape because it has the lowest atomic number/mass but the CO2 was fastest by far. Now he figures that maybe the atomic numbe does not directly equate to the size of the atom/molecue in question but he can not find any data on the relative/actual size of the various atoms/molecules. Can you help him find such data. Google searches have been less than helpful thus far.

Balloon Boy wrote:My son has run trials comparing Air[exhaled], He and CO2 diffusion through latex balloons. He inflated 4-5 each to the same size and measured the changes at 3-hour intervals for a few days. He expected that the He would be the fastest to escape because it has the lowest atomic number/mass but the CO2 was fastest by far. Now he figures that maybe the atomic numbe does not directly equate to the size of the atom/molecue in question but he can not find any data on the relative/actual size of the various atoms/molecules. Can you help him find such data. Google searches have been less than helpful thus far.

Sounds like a wonderful project with good experiment processes. Be sure record temperature too.

Here are four sites I found using "Helium balloons diffusion" on dogpile.com that might be useful:
http://www.madsci.org/posts/archives/fe ... .Ph.r.html
(emphasizes that temperature is vital variable)

http://wine1.sb.fsu.edu/chm1045/notes/G ... ases09.htm
(this one seems to confirm what you found

http://www.balloonhq.com/faq/deco260.html
(some practical tips)

http://mimp.mems.cmu.edu/~ordofmag/gassize.htm
(Molecule size vs. atom size, etc.)

Another good site is scholars.google.com (a little different results than straight google).

Hope these help; this is not my field exactly.

Ceal Craig
Ask An Expert
Science Buddies

It is an excellent project but you could make a few modifications to your experimental procedure to make sure that your results are correct. To make sure that it really was the CO2 that was escaping rather than a deffective balloon, it would be a good idea to make up several balloons filled with each gas so that you could see if all of the balloons of each type deflated at the same rate. If one of the balloons deflates much faster than the others, then the balloon could be defective rather than the gas diffusing through the balloon.

You would definitely expect the lighter gas to diffuse more quickly, but there may be other factors influencing diffusion including interactions between the gas and the material making up the balloon. It might be too involved, but it could be interesting to learn more about the polymer that makes up the balloon to see if you can learn anything about why certain gases might diffuse through it faster than others.