The Earth and its atmosphere · Model
What's in the air
The air you are breathing would have killed almost everything alive two and a half billion years ago — and the things that made it were the ones it killed.
Start here
Take a deep breath. Most of what just went in did nothing at all.
Your lungs took in about half a litre of air. Some of it was absorbed and used; the great majority went straight back out unchanged, exactly as it came in.
Roughly what fraction of the air is oxygen?
About a fifth. Nearly four fifths of every breath is nitrogen, which your body cannot use for anything and which comes straight back out again. Oxygen is the minority gas, and you only actually absorb about a quarter of the oxygen you take in — which is precisely why breathing into someone during resuscitation works.
The atmosphere is a mixture of gases — not a compound, and not a single substance called air — and it has been remarkably steady for about the last 200 million years. Almost all of it is just a few gases, and they are not present in the proportions most people assume.
A mixture is not a compound. Nothing in air has reacted with anything else: the gases are simply mingled, each one keeps its own properties, and the proportions are not fixed. That last part matters more than it sounds — the amount of water vapour in air can be four parts in a hundred over a warm sea and almost nothing over a desert.
Key fact
Air is a mixture, not a compound. The gases in it are not chemically joined, each one keeps its own properties, and the proportions are not fixed — which is why the composition is always quoted for dry air, with the variable water vapour taken out.
Your turn · what is actually in it
The bar is drawn to scale. Tap a gas.
1 of 4 opened
Two of these four are gases your body does not touch at all: they go in and come straight back out unchanged, and together they are 78.9% of every breath.
Nitrogen
78% of the air
Almost four fifths of the air, and it does nothing in your lungs — you breathe it straight back out. It is very unreactive as a gas, which is why it built up and stayed. It is also essential to every protein in your body, but you get it from food, not from breathing.
Oxygen
21% of the air
A fifth of the air. Every animal needs it for respiration and nothing burns without it — and all of it was put there by photosynthesis. Before there was life, there was essentially none.
Argon
0.9% of the air
Completely unreactive, and with no biological role at all. It is simply there: made over billions of years by radioactive decay in the rocks, and too inert for anything to take it out again. This is the gas that had no square in Mendeleev's table.
Carbon dioxide
0.04% of the air
A sliver — and the reason the bar above cannot be drawn honestly at both ends. Every plant, every tree and ultimately every animal is built out of carbon taken from this fraction, and it is the fraction that is rising now because of what people burn.
Argon and carbon dioxide are drawn wider on the bar than they really are. Together they are 0.94% of the air, and a strip that thin is not something anybody could tap. Every other width above is the real proportion.
These are the figures for dry air. Real air always carries some water vapour as well, and how much changes from day to day and place to place — which is exactly why the composition is quoted with the water taken out.
Watched first · four and a half billion years
How the air got like this
0 of 5 revealed
It started as something you could not have survived for a minute. Reveal the stages one at a time.
- 4.6 bya
Volcanoes everywhere. The air is mostly carbon dioxide and water vapour.
The young Earth was violently volcanic, and the leading idea is that everything the volcanoes released became the atmosphere: mainly carbon dioxide and steam, with some nitrogen and smaller amounts of methane and ammonia. Nobody has a sample of air that old, so this is a model built from what volcanoes give off today and from what the oldest rocks record. What every version of it agrees on is that there was no oxygen worth measuring, and nothing alive could have breathed it.
- 4.4 bya
The Earth cools enough for the water vapour to condense. It rains for a very long time.
As the surface cooled, the steam in the atmosphere condensed and fell, filling the low ground and forming the oceans. This one change moved an enormous quantity of water out of the air and onto the surface, and it is the reason there is a planet with liquid water on it at all.
- 4.0 bya
Carbon dioxide dissolves into the new oceans.
Carbon dioxide is soluble in water, so a planet that suddenly has oceans loses a great deal of it from the air. Dissolved in the sea it reacted with other substances and began to settle on the sea floor as carbonate — the beginning of limestone, and the beginning of carbon being taken out of the atmosphere and put into rock.
- 2.7 bya
Photosynthetic bacteria appear and start releasing oxygen.
The first organisms to photosynthesise took in carbon dioxide and water, used sunlight to build sugars, and released oxygen as a waste product. It did not reach the air for a long time: the oxygen went into the oceans first, rusting the iron that was dissolved in them out of solution, and only once enough had been made to overwhelm that iron did it begin to build up in the air — around two and a half billion years ago.
- 400 mya
Carbon is buried in rock as limestone, coal, oil and gas.
Limestone had been forming since the oceans appeared, from the shells and skeletons of sea creatures. What is new here is the land: the remains of vast swamp forests, buried and compressed, became coal, and buried marine organisms became oil and natural gas. Every one of those is carbon taken out of the atmosphere and locked away, which is why the modern air has so little of it left.
Two questions answered at once.
Where the oxygen came from: living things made it. Where the carbon dioxide went: into the oceans, then into limestone, coal, oil and gas — locked into rock, where most of it still is.
Which is why burning fossil fuels matters. It takes carbon that took hundreds of millions of years to bury and returns it to the air in a couple of centuries.
Key fact
Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide. The oxygen was made by photosynthesis, and the carbon dioxide that used to dominate is now locked in rocks and fossil fuels.
Five words
Say your answer out loud before you turn each card over. If you cannot say it, you do not know it yet.
Think again
“Air is mostly oxygen — that is the point of it.”
Oxygen is the gas you need. Commit before you read on.
Air is roughly four parts nitrogen to one part oxygen, and the nitrogen is not filler. It is the reason a struck match lights the candle and not the room: in pure oxygen, everything that can burn burns ferociously, steel wool included. The nitrogen dilutes the oxygen down to a level at which fire is possible but not automatic.
Being the gas you need does not make it the gas there is most of. Carbon dioxide makes the point at the other extreme: 0.04 per cent of the air, and every plant on Earth is built out of it.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Recall
What is the approximate composition of dry air today?
Rung 2 · The one that catches people
Where did the oxygen in today's atmosphere come from?
Rung 3 · Explain
Explain how the atmosphere changed from mostly carbon dioxide to mostly nitrogen and oxygen, naming the two processes that removed the carbon dioxide.
Rung 4 · Take it somewhere new
Venus has an atmosphere that is 96 per cent carbon dioxide and a surface hot enough to melt lead. Using what you know about the Earth's history, suggest why the two planets ended up so differently.
Key note
Dry air today is about 78 per cent nitrogen, 21 per cent oxygen, 0.9 per cent argon and 0.04 per cent carbon dioxide. The best account we have of the early atmosphere is that it came from volcanoes and was mostly carbon dioxide and water vapour, with no oxygen. The water vapour condensed to form the oceans, photosynthesis by early life released oxygen, and carbon dioxide became locked into sedimentary rocks and fossil fuels.
Going further
The arrival of oxygen was a catastrophe for almost everything then alive. To organisms that had evolved in a world without it, oxygen was a corrosive poison, and most of them died — killed by the waste product of the ones that had learned to photosynthesise. The survivors were the few that could tolerate it, and eventually the ones that learned to use it. Respiration with oxygen releases far more energy than the chemistry that came before, and every complex living thing depends on it.
The evidence for all this is written in rock. Iron dissolved in the early oceans could only stay dissolved while there was no oxygen; when oxygen appeared it reacted with that iron, which came out of solution and settled on the sea floor as bands of rust. Those banded iron formations are laid down in enormous quantities from about two and a half billion years ago and are largely finished several hundred million years later — the record of the oceans being swept clear of iron before oxygen could begin to accumulate in the air above them. Most of the world's iron ore was put there by that one change.
Before this lesson
Connects to
At GCSE this becomes
- The evidence for the early atmosphere, the greenhouse effect in detail, and the chemistry of the Great Oxidation Event.
Where to next
Ask Mr Badmus AI
Still not sure where all the carbon dioxide went?
Lesson content © MrBadmusAI.