Effusion: the process by which a gas escapes through a tiny hole in its container into an evacuated space.
Thomas Graham studied the effusion rate of a gas, “the number of molecules escaping per unit time”, and found that: effusion rate was inversely proportional to the square root of the gas density.
Graham’s law of effusion:
The rate of effusion of a gas is inversely proportional to the square root of its molar mass,
Diffusion: is the movement of one gas through another
Diffusion also occurs when a gas enters a liquid (and even to a small extent a solid).
In a liquid, the average distances between molecules are so much shorter that collisions are much more frequent; thus, diffusion of a gas through a liquid is much slower than through a gas.
Diffusion of a gas through a liquid is a vital process in biological systems, for example, in the movement of O2 from lungs to blood