What you'll learn
This topic explores how Earth's atmosphere has changed dramatically since the planet formed approximately 4.6 billion years ago. You'll examine the early atmosphere's composition, understand the processes that increased oxygen levels, and learn the precise composition of today's atmosphere—all essential knowledge for AQA GCSE Chemistry Paper 2.
Key terms and definitions
Atmosphere — the layer of gases surrounding Earth, held in place by gravity
Photosynthesis — the process by which plants and algae use light energy to convert carbon dioxide and water into glucose and oxygen
Sedimentary rock — rock formed from layers of sediment, often containing fossils and other evidence of past conditions
Algae — simple aquatic organisms that photosynthesise; played a crucial role in producing atmospheric oxygen
Fossil fuels — natural fuels formed from the remains of ancient organisms, including coal, crude oil and natural gas
Fractional composition — the proportion of different gases in a mixture, typically expressed as a percentage
Greenhouse gas — a gas that absorbs infrared radiation and contributes to warming the atmosphere, such as carbon dioxide or methane
Volcanic outgassing — the release of gases from molten rock during volcanic eruptions
Core concepts
Phase 1: The early atmosphere (4.6 to 3.8 billion years ago)
When Earth formed, its surface was molten with no atmosphere. As the planet cooled, intense volcanic outgassing released gases that formed the early atmosphere.
Composition of the early atmosphere:
- Primarily carbon dioxide (similar to Mars and Venus today)
- Water vapour (which later condensed to form oceans)
- Small amounts of methane and ammonia
- Very little or no oxygen
Key point: There is no definitive scientific proof about the exact composition of Earth's early atmosphere, but evidence from volcanic gases and other planets supports this model.
When the Earth cooled sufficiently (below 100°C), water vapour condensed to form the oceans. This was critical because:
- It removed large quantities of water vapour from the atmosphere
- It provided an environment where early life could develop
- Carbon dioxide began dissolving in the oceans
Phase 2: Development of oxygen (3.8 to 1 billion years ago)
The appearance of primitive life forms transformed atmospheric composition fundamentally.
The role of algae and early plants:
- Approximately 2.7 billion years ago, algae first appeared in the oceans
- These organisms carried out photosynthesis, using sunlight to convert carbon dioxide and water into glucose and oxygen
- The word equation: carbon dioxide + water → glucose + oxygen
- Oxygen gradually accumulated in the atmosphere over hundreds of millions of years
Evidence for oxygen increase:
- Formation of banded iron formations (oxidised iron in sedimentary rocks)
- Appearance of red beds (iron oxide-rich sediments)
- Development of the ozone layer, which absorbs harmful UV radiation
The increase in oxygen was initially slow because oxygen reacted with iron dissolved in the oceans, forming insoluble iron oxide that settled as sediment. Only after most oceanic iron had oxidised did atmospheric oxygen levels rise significantly.
Plants colonising land:
- Around 500 million years ago, plants began spreading across land
- This accelerated oxygen production through photosynthesis
- By approximately 200 million years ago, oxygen levels reached near-modern proportions
Phase 3: Decrease in carbon dioxide
As oxygen increased, carbon dioxide decreased through several mechanisms:
Photosynthesis:
- Algae and plants continuously removed carbon dioxide from the atmosphere
- This carbon became locked in plant biomass
Ocean dissolution:
- Carbon dioxide is soluble in water, especially cold seawater
- Dissolved CO₂ reacted with minerals to form carbonate precipitates
Carbonate rock formation:
- Marine organisms (shellfish, corals) used dissolved carbon dioxide to make calcium carbonate shells
- When these organisms died, their shells formed layers of carbonate sediment
- Over millions of years, compression formed sedimentary rock (limestone, chalk)
- This locked away vast quantities of carbon
Fossil fuel formation:
- Dead plant and animal matter, particularly in swamps and ocean floors, became buried under sediment
- High pressure and temperature, over millions of years, converted this organic matter into fossil fuels
- Coal formed from land plants in swamps
- Crude oil and natural gas formed from marine organisms
- This process trapped carbon that would otherwise have returned to the atmosphere through decomposition
The modern atmosphere: composition and percentages
For the past 200 million years, atmospheric composition has remained relatively stable.
Current atmospheric composition:
- Nitrogen (N₂): approximately 78%
- Oxygen (O₂): approximately 21%
- Argon (Ar): approximately 0.9%
- Carbon dioxide (CO₂): approximately 0.04%
- Water vapour: variable amounts
- Trace amounts of other noble gases (neon, helium, krypton)
Important exam points:
- Nitrogen is the most abundant gas (not oxygen)
- The percentages must add up to 100%
- Carbon dioxide is a very small proportion despite its environmental significance
- Water vapour content varies with location and weather conditions
Sources of atmospheric nitrogen:
- Released by volcanic activity in the early atmosphere
- Also produced by bacteria breaking down nitrogen compounds
- Nitrogen is unreactive, so it accumulated over billions of years
How scientists study the ancient atmosphere
AQA GCSE requires you to understand the evidence used to reconstruct atmospheric history:
Ice core analysis:
- Scientists drill deep into Antarctic or Greenland ice sheets
- Air bubbles trapped in ice layers preserve ancient atmospheric samples
- Analysis reveals gas composition from hundreds of thousands of years ago
- Evidence shows historical carbon dioxide and temperature correlations
Sedimentary rock analysis:
- Carbonate rocks indicate carbon dioxide removal from the atmosphere
- Banded iron formations show when oxygen first appeared
- Fossil evidence in rocks reveals what organisms existed at different times
Limitations of evidence:
- Direct measurements only available for recent centuries
- Ancient atmosphere composition involves interpretation and inference
- Volcanic gas analysis assumes ancient volcanoes released similar gases to modern ones
- Models must integrate multiple evidence sources
Worked examples
Example 1: Describing atmospheric evolution (4 marks)
Question: Describe how the composition of Earth's atmosphere has changed from its formation to the present day. (4 marks)
Mark scheme answer:
- Early atmosphere was mainly carbon dioxide with little or no oxygen (1 mark)
- Water vapour condensed to form oceans as Earth cooled (1 mark)
- Algae/plants evolved and photosynthesised, producing oxygen (1 mark)
- Carbon dioxide decreased through photosynthesis, dissolving in oceans, and forming carbonate rocks/fossil fuels (1 mark)
Examiner note: Each distinct process earns one mark. Vague statements like "it changed over time" score zero.
Example 2: Explaining oxygen increase (3 marks)
Question: Explain why oxygen levels in the atmosphere increased over billions of years. (3 marks)
Mark scheme answer:
- Algae/photosynthetic organisms evolved in the oceans (1 mark)
- These organisms carried out photosynthesis, which produces oxygen (1 mark)
- Plants colonised land, increasing the rate of oxygen production (1 mark)
Alternative acceptable points:
- Initially oxygen reacted with iron in oceans, so atmospheric levels rose slowly (1 mark)
Example 3: Current atmosphere composition (2 marks)
Question: State the two most abundant gases in today's atmosphere and give their approximate percentages. (2 marks)
Mark scheme answer:
- Nitrogen, 78% (1 mark)
- Oxygen, 21% (1 mark)
Common error: Students often state oxygen first or give incorrect percentages. Learn the order: nitrogen > oxygen > argon > carbon dioxide.
Common mistakes and how to avoid them
Confusing the order of gases: Remember nitrogen (78%) is MORE abundant than oxygen (21%), not less. Use the mnemonic "NO" for Nitrogen then Oxygen in decreasing order.
Stating oxygen was present in the early atmosphere: The early atmosphere contained virtually no oxygen—it only appeared after photosynthetic organisms evolved. Always specify "little or no oxygen" for the early atmosphere.
Forgetting multiple processes that reduced carbon dioxide: Don't just mention photosynthesis. Carbon dioxide also decreased through ocean dissolution, carbonate rock formation, and fossil fuel formation. In 4+ mark questions, you need multiple processes.
Vague timescales: Avoid phrases like "a long time ago." Use specific terms: "billions of years," "approximately 2.7 billion years ago," or "over the past 200 million years."
Claiming we know the exact composition of the early atmosphere: Scientists have theoretical models based on evidence, but there's no certainty. Acknowledge this limitation when questions ask about evidence.
Mixing up how different rocks formed: Carbonate rocks (limestone) formed from shells of marine organisms. Coal formed from land plants. Crude oil formed from marine organisms. Keep these separate.
Exam technique for "Chemistry of the atmosphere: evolution and composition of the Earth's atmosphere"
Command word "describe" requires you to state features or processes without explanation. For "explain," you must give reasons WHY something happens. A description of oxygen increase states "algae photosynthesised"; an explanation adds "which converts carbon dioxide and water into glucose and oxygen."
Sequencing answers chronologically helps in extended responses. Structure answers: early atmosphere → ocean formation → algae evolution → oxygen increase → plant colonisation → modern composition. This logical flow prevents omissions.
Mark-per-point allocation: Most AQA questions award one mark per distinct scientific point. A 4-mark question needs four separate points. If you've only written two sentences for a 4-mark question, you're likely missing marks.
Use data effectively: When questions provide gas percentages or ice core data, reference specific numbers in your answer. "The ice core shows CO₂ was 280 ppm in 1800" scores better than "CO₂ was lower in the past."
Quick revision summary
Earth's early atmosphere, formed by volcanic activity, contained mainly carbon dioxide with little oxygen. As oceans formed, algae evolved and photosynthesised, gradually increasing oxygen levels over billions of years. Carbon dioxide decreased through photosynthesis, ocean dissolution, carbonate rock formation, and fossil fuel creation. Today's atmosphere is 78% nitrogen, 21% oxygen, 0.9% argon, and 0.04% carbon dioxide. This composition has remained stable for 200 million years. Scientists use ice cores and sedimentary rocks as evidence for atmospheric evolution.