Air
The mixture everything breathes and the feedstock of an industry — the source of nitrogen, oxygen, argon and the rarer noble gases.
Air is a mixture rather than a compound, and its composition is remarkably constant: roughly 78 per cent nitrogen and 21 per cent oxygen, with argon making up almost all of the remaining one per cent. Carbon dioxide, which does most of the arguing in public, is a few hundredths of one per cent.
Industrially it is a feedstock, and an unusual one in that it is free, unlimited and available anywhere. Air separation plants liquefy it and distil it, and the products — nitrogen for inerting and for fertiliser, oxygen for steelmaking and for hospitals, argon for welding — are among the largest-tonnage chemicals in the world.
How it forms
The present atmosphere is a biological product. The oxygen in it was put there by photosynthesis, over roughly two billion years, and the banded iron formations record the first stage of that process — dissolved iron in the oceans consuming the oxygen as fast as it was made, until the iron ran out and the gas began to accumulate.
Before that the air was mostly nitrogen and carbon dioxide, with no free oxygen at all. Almost nothing alive today could have survived in it, and almost nothing alive then survived the change.
The noble gases arrived differently. Argon is overwhelmingly argon-40, produced by the radioactive decay of potassium in rocks and leaked to the surface over the age of the Earth, which is why it is nearly a hundred times more abundant in air than neon despite being the heavier element.
Economic significance
Air separation is the reason nitrogen and oxygen cost what they do, which is almost nothing per tonne and a great deal in energy. That makes the industry a large electricity consumer and locates it next to whatever is consuming its product — an oxygen plant is generally built beside the steelworks it feeds rather than shipping the gas any distance.
The rarer components are what make the economics work at the margins. Neon, krypton and xenon are present in parts per million and are recovered only as a by-product of large-scale oxygen and nitrogen production, which is why the semiconductor industry's neon supply turned out to depend on a handful of steelworks.
How we know: checked recently · only one source, so there is nothing to cross-check it against · stated directly by the source.
How this connects
Where a connection has been confirmed by an outside reference, that reference is named beside it.
contains
- Nitrogen — element · 78% · roughly seventy-eight per cent by volume, and effectively inert — which is why it can be the bulk of what everything breathes
- Oxygen — element · 20–21% · about twenty-one per cent, and all of it a biological product — photosynthesis put it there over roughly two billion years
- Argon — element · just under one per cent, and almost entirely argon-40 from the decay of potassium in rocks — which is why it is a hundred times commoner than neon despite being heavier
- Carbon — element · as carbon dioxide, at a few hundredths of one per cent — the component that does most of the arguing in public
- Neon — element · eighteen parts per million, recovered only as a by-product of large-scale oxygen production — which is why the semiconductor industry's neon supply depends on a handful of steelworks
- Krypton — element · one part per million, and separated from air for the same reason and by the same plant
- Xenon — element · under a tenth of a part per million, which makes it the rarest thing recovered from air at scale
- Helium — element · five parts per million — genuinely present and never recovered from air, because natural gas carries it at a thousand times the concentration
- Radon — element · in traces, and variable: it is a decay product of uranium in the ground rather than a permanent constituent, and it accumulates indoors, which is where it is a health problem rather than a curiosity
is an input to
- Air separation — process · the feedstock, and an unusual one in being free, unlimited and available anywhere — the cost is entirely the energy to liquefy it
Sources
- Wikimedia Foundation · Creative Commons CC0 1.0 Universal (public domain dedication)
- Material WorldOur own writing