AQA GCSE Chemistry contents

Unit 9 · Chemistry of the Atmosphere

Composition & Evolution of the Atmosphere

2 min read · free to read

The atmosphere today

  • For the last 200 million years the atmosphere has been much the same as now:
    • About four-fifths (80%) nitrogen
    • About one-fifth (20%) oxygen
    • Small proportions of other gases: carbon dioxide (about 0.04%), water vapour and noble gases (mainly argon)

The early atmosphere

  • The Earth is about 4.6 billion years old, so evidence about the early atmosphere is limited and theories have changed over time. You need one theory:
  • During the first billion years there was intense volcanic activity. Volcanoes released the gases that formed the early atmosphere, and water vapour that condensed to form the oceans
  • The early atmosphere was probably like Mars and Venus today: mainly carbon dioxide with little or no oxygen
  • Volcanoes also released nitrogen, which gradually built up because it is unreactive, and perhaps small amounts of methane and ammonia
  • When the oceans formed, carbon dioxide dissolved in the water and carbonates were precipitated, forming sediments. This reduced the carbon dioxide in the atmosphere

How oxygen increased

  • Algae and later plants produced oxygen by photosynthesis:
6CO2+6H2O→C6H12O6+6O2 6\text{CO}_2 + 6\text{H}_2\text{O} \rightarrow \text{C}_6\text{H}_{12}\text{O}_6 + 6\text{O}_2
  • Algae first produced oxygen about 2.7 billion years ago. Over the next billion years plants evolved and oxygen rose gradually to a level that allowed animals to evolve

How carbon dioxide decreased

  • Photosynthesis by algae and plants took carbon dioxide out of the atmosphere
  • Carbon was also locked up in sedimentary rocks and fossil fuels:
Deposit How it formed
Limestone carbon dioxide dissolved in the oceans; carbonates precipitated, and shells and skeletons of sea creatures settled as sediment and were compressed
Coal plants in swamps died and were buried under sediment; with no oxygen they did not decay, and heat and pressure over millions of years turned them into coal
Crude oil and natural gas plankton (tiny sea organisms) died and settled in mud on the sea bed; covered by sediment, in the absence of oxygen, high temperature and pressure over millions of years converted them into oil and gas, which were trapped under rock

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the Earth's atmosphere changed4.6 billionyears ago3.5 billionyears ago2.7 billionyears ago1 billionyears agotodayintense volcanic activityreleased carbon dioxide,water vapour, methane andammonia — almost no oxygenthe water vapour condensedand formed the oceans;carbon dioxide dissolved intothem and formed carbonatesalgae and then plants beganphotosynthesis: carbon dioxidedown, oxygen upabout 80% nitrogen,20% oxygen, and onlya small amount ofcarbon dioxideFossil fuels and limestone locked away much of the carbon that was once in the air. Burningfossil fuels now releases that carbon again, which is why carbon dioxide levels are rising.
Timeline of the atmosphere from 4.6 billion years ago to now: volcanoes and a carbon dioxide atmosphere, oceans forming, algae at 2.7 billion years, oxygen rising and carbon dioxide falling, with the modern 80/20 mix at the end