Earth and atmosphere · GCSE Chemistry
The early atmosphere
GCSE Chemistry revision on the early atmosphere: volcanic CO₂, condensing oceans, photosynthesis producing oxygen, and how the modern nitrogen–oxygen air formed.
Volcanoes: mostly carbon dioxide, little or no oxygen. Oceans dissolve CO₂. Algae and plants photosynthesise: CO₂ down, O₂ up. For 200 million years the air has been about 80% N₂ and 20% O₂.
The important bits
What you need to know
- 1
Today’s atmosphere is about four-fifths nitrogen (≈78%), one-fifth oxygen (≈21%), plus argon, carbon dioxide (≈0.04%) and variable water vapour.
- 2
The early atmosphere, about 4.6 to 2.5 billion years ago in the GCSE story, was dominated by carbon dioxide from intense volcanic activity, with little or no oxygen, plus water vapour, methane and ammonia.
- 3
As the Earth cooled, water vapour condensed to form the oceans. Some carbon dioxide dissolved in the water and formed carbonate sediments, locking carbon into rocks such as limestone (CaCO₃).
- 4
Algae and then plants photosynthesised: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon was stored in biomass; oxygen was released. Over time oxygen rose to modern levels.
- 5
Some of that stored carbon became fossil fuels (coal, oil, gas). Burning them now returns CO₂ faster than it was locked away.
- 6
Nitrogen built up because it is unreactive (N≡N triple bond) and was produced by volcanoes and by the breakdown of ammonia; it was not removed as quickly as CO₂.
- 7
Evidence is limited: we were not there. Scientists use ancient rocks, the composition of other planets’ volcanic atmospheres, and the appearance of oxidised iron in banded rock as clues. GCSE wants the sequence, not a debate about every date.
- 8
The last 200 million years: composition has been similar to now. The recent rise in CO₂ is tiny as a percentage but large compared with the last few hundred thousand years of ice-core data.
Quotations worth analysing
Short evidence. Real method.
“6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂”
This is the oxygen factory. Volcanoes did not fill the air with O₂. Students who write “oxygen from volcanoes” lose the central mark of the topic.
“Early air: mostly CO₂. Modern air: mostly N₂ and O₂.”
Give two processes that cut CO₂ (dissolve in oceans / form carbonates / photosynthesis / fossil fuels) and one that raised O₂ (photosynthesis).
“Ca²⁺(aq) + CO₃²⁻(aq) → CaCO₃(s)”
Dissolved carbon dioxide ends up in sedimentary carbonates. That carbon is not in the air until weathering and heating (or humans making cement) release it.
Go deeper
Tell it as a sequence of sinks and sources
Source of early CO₂: volcanoes. Sink 1: cooling, rain, oceans, dissolution, carbonate rocks. Sink 2: photosynthesis into biomass and, later, fossil fuels. Source of O₂: photosynthesis, which also is a CO₂ sink. Nitrogen: produced and left behind because it is so unreactive. Methane and ammonia were removed by reaction with the new oxygen and by other processes — you do not need a full redox list, but you should say they decreased as oxygen increased. A timeline sentence scores: volcanoes first, oceans next, photosynthetic life, then the oxygen-rich air we have now. Mixing the order (plants, then oceans, then volcanoes) is how the story unravels.
Go deeper
Limestone and fossil fuels are ancient air in solid form
When you heat CaCO₃ in a lime kiln you are reversing one of the sinks: CaCO₃ → CaO + CO₂. When you burn coal you oxidise carbon that photosynthesis reduced hundreds of millions of years ago. That is why the atmosphere page and the energy/combustion pages are the same carbon atoms on a different clock. The exam likes: “carbon dioxide decreased because it was locked in sedimentary rocks and fossil fuels.” Give both, plus photosynthesis. One process is not enough for a four-mark explain. Cement manufacture also heats carbonates and is another modern return of that ancient carbon.
Go deeper
Percentages you should actually remember
Nitrogen ~78%, oxygen ~21%, argon ~0.9%, carbon dioxide ~0.04% (often still quoted as 0.03% in older books). Water vapour varies. You do not need more decimal places. If a pie chart is shown, read it rather than reciting. The early atmosphere pie would be mostly CO₂ with little O₂. Mars and Venus are sometimes mentioned as CO₂-rich volcanic atmospheres that never grew a photosynthetic oxygen source like Earth’s. That comparison is optional extra unless the paper prints it. Stick to Earth unless asked. Do not swap oxygen and nitrogen percentages; that is a surprisingly common slip.
See the idea in action
Describe how the amounts of carbon dioxide and oxygen in the atmosphere changed, with processes. Early volcanic air was mostly CO₂ with little O₂. CO₂ fell as it dissolved in oceans and formed CaCO₃ sediments, and as algae and plants photosynthesised (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂), storing carbon in biomass and fossil fuels. O₂ rose because of that photosynthesis. Nitrogen became the main gas because it is unreactive and accumulated. Burning those fossil fuels today is the reverse transfer of carbon back to CO₂(g).
Exam technique
Turn knowledge into marks
Describe early atmosphere, then two processes that reduced CO₂ and photosynthesis for O₂. Quote modern percentages if asked. Never say oxygen came from volcanoes. Link limestone and fossil fuels as carbon stores.
Common mistakes
Do not give these marks away
- 01
Writing that oxygen in the early atmosphere came from volcanoes.
- 02
Forgetting oceans and limestone as a CO₂ sink, and only mentioning photosynthesis.
- 03
Giving modern CO₂ as 20%, mixing it up with oxygen.
Which process is mainly responsible for the rise of oxygen in Earth’s atmosphere?
AVolcanic activity
BPhotosynthesis by algae and plants
CComplete combustion
DAcid rain
Show the answer
Photosynthesis by algae and plants. Photosynthesis uses carbon dioxide and water and releases oxygen. Over geological time this produced the oxygen-rich atmosphere we have now.
Quick questions
If this is the bit you searched
What was the early atmosphere like GCSE Chemistry?
It was mostly carbon dioxide from volcanoes, with little or no oxygen, plus water vapour, methane and ammonia. It was similar in idea to volcanic atmospheres, not to today’s air.
How did carbon dioxide in the atmosphere decrease?
It dissolved in the oceans, formed sedimentary rocks such as limestone, was used in photosynthesis, and was locked into fossil fuels.
How did oxygen increase in the atmosphere?
Algae and plants photosynthesised, producing oxygen as a waste product: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
What is the composition of the atmosphere now?
About 78% nitrogen, 21% oxygen, almost 1% argon, and a small amount of carbon dioxide (about 0.04%), plus variable water vapour.