Ethanol is a relatively light molecule and diffuses rapidly in a natural environment.

Perceived immediately upon application of a perfume, top notes consist of small, light molecules that evaporate quickly.

That is, lighter molecules tend to boil off in larger quantities at the processing temperatures.

This diffusion results in the heavier and lighter molecules separating.

This means that light molecules will move a lot faster, on average, than heavy molecules at the same temperature.

H2 and He are very light molecules, and they move very fast.

Thus, heavy molecules are pumped much more efficiently than light molecules.

Thus, lighter molecules have a higher speed.

Using lighter molecules, as occurs when helium is used in a pneumatic.

Diffusion is also faster in smaller, lighter molecules of which helium is the extreme example.