Effusion
Definition
Gas escaping through a tiny hole into an empty space. Lighter gases leak out faster, which is how uranium isotopes were once separated.
Technical definition
Effusion is the passage of gas molecules through a pinhole into a vacuum without collisions, with rates governed by Graham's law: the rate is inversely proportional to the square root of molar mass.
Example
Hydrogen (molar mass 2) effuses 4 times faster than oxygen (molar mass 32), since the square root of 32/2 is 4. The same principle was once used to enrich uranium-235.
Common confusion
Graham's law compares rates of effusion, not amounts, and it assumes the gases are at the same temperature, since molecular speed depends on temperature.