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For other uses, see effusion (disambiguation).

In chemistry, effusion is the process where individual molecules flow through a hole without collisions between molecules. This will occur if the diameter of the hole is considerably smaller than the mean free path of the molecules[1]. According to Graham's law, the rate at which gases effuse is dependent on their molecular weight; gases with a lower molecular weight will effuse more quickly than gases with a higher molecular weight. For two gases at the same temperature (and having the same specific heat), and thus having the same kinetic energy, the average molecular speed of each gas can be found using the equation E = (1 / 2)mv2. Thus, lighter molecules will have a higher speed. This will result in more molecules passing through the hole per unit time. This is why a balloon filled with low molecular weight hydrogen will deflate, if left alone for a reasonable amount of time, faster than one full of higher molecular weight oxygen.

See also


  1. ^ K.J. Laidler and J.H. Meiser, Physical Chemistry, Benjamin/Cummings 1982, p.18
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Effusion". A list of authors is available in Wikipedia.
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