Daltonix

Meet Daltonix, a jar of mixed gases where every gas pushes its own share.

In a mixture of gases that do not react, each gas behaves as if it had the whole container to itself. The pressure it would make alone is its partial pressure. John Dalton found in 1801 that the total pressure is simply the sum of the partial pressures, p = p₁ + p₂ + p₃.

Air at sea level has a total pressure of about 101.3 kPa. Oxygen makes up about 21% of the molecules, so its partial pressure is about 0.21 × 101.3 ≈ 21 kPa. Nitrogen, at about 78%, supplies about 79 kPa, and the other gases supply the rest.

Partial pressure is what matters for breathing. High on a mountain the total pressure falls, so the oxygen partial pressure falls with it, even though oxygen is still 21% of the air.

Divers also plan their gas mixtures using partial pressures, because too much or too little of a gas can be harmful.