Diffusion of Gases:
It is a common experience of everyone that when a bottle of perfume is opened in a room, its smell is felt throughout the room. The cause of this is the intermixing of the vapours of perfume with air. Such a spontaneous intermixing of gases is called diffusion of gases or gaseous diffusion. Gases show the phenomenon of diffusion irrespective of their densities. But the rate of diffusion depends on the density of the gas. The lighter gases diffuse more quickly than the heavier gases. This can be demonstrated by the following experiment.
Experiment:
Fit up a porous pot with a bent bulb tube having one end drawn into a jet and containing coloured water through a rubber stopper. Now invert a jar containing hydrogen over the porous pot as shown in the figure. Hydrogen being the lighter diffuses into the porous pot more quickly than air can get out. This cause an increase in the pressure inside the pot and, consequently, the water is forced out in the form of a fountain. If now we remove the jar, hydrogen within the pot diffuses out more rapidly than air can get in. As a result, there occurs a decrease of pressure inside the pot and the water stops coming out of the jet.
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