This animation shows the diffusion of Helium (He) into air (a mix of O2 and N2) held at two different pressures. The top panel is held at a lower pressure than the bottom.  Molecules in the lower pressure system will diffuse faster than those in the higher pressure system because the mean free path – the distance a molecule can travel before colliding with another molecule or barrier – is greater under lower pressure. The effect of pressure on diffusion in a gas is similar to that seen when comparing two different states of matter. This animation assumes that all other variables are the same between the two systems.

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Oxygen (O2)

Nitrogen (N2)

Helium (He)

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The Effect of Pressure  on Diffusion