Seepix

Meet Seepix, a leaky balloon who notices that light gas escapes much faster than heavy gas.

Gas molecules are always moving, and at the same temperature they all have the same average kinetic energy. That means lighter molecules must move faster. When gas leaks out through a tiny hole, a process called effusion, the faster molecules find the hole more often and escape more quickly.

Thomas Graham found that the rate of effusion goes as one over the square root of the molar mass, so r₁/r₂ = √(M₂/M₁). Helium has a molar mass of 4 g/mol and nitrogen 28 g/mol, so helium escapes about √7 ≈ 2.6 times faster than nitrogen.

That is why a helium party balloon goes flat in a day or so while an air-filled balloon lasts much longer.

The same idea was used on a huge scale to separate uranium isotopes, because uranium hexafluoride gas made with the lighter isotope seeps through porous barriers very slightly faster.