GCSE Chemistry revision

How carbon dioxide decreased

The composition and evolution of the Earth's atmosphere

AQA 4.9.1.4 Foundation & Higher

Your specification

AQA student objectives

Learning pathway

ALL 🎯

MOST 🎯🎯

SOME 🎯🎯🎯

Revision summary

Read on screen, then print for Cornell-style active revision.

As the early Earth cooled, water vapour in the atmosphere condensed to form the oceans, drastically reducing atmospheric water vapour and allowing large amounts of carbon dioxide to dissolve into the water.
Example: Dissolved carbon dioxide reacted with metal ions in the early oceans to precipitate out as insoluble carbonate compounds, forming ocean bed sediments.

1. Cooling and Ocean Formation AQA

As the early Earth cooled, water vapour in the atmosphere condensed to form the oceans, drastically reducing atmospheric water vapour and allowing large amounts of carbon dioxide to dissolve into the water.
Example: Dissolved carbon dioxide reacted with metal ions in the early oceans to precipitate out as insoluble carbonate compounds, forming ocean bed sediments.

The evolution of early photosynthetic organisms (algae and green plants) rapidly reduced carbon dioxide levels as organisms absorbed CO₂ from the atmosphere and water to synthesize glucose.
Example: Primitive algae began absorbing carbon dioxide roughly 2.7 billion years ago, converting atmospheric CO₂ into solid plant matter and releasing oxygen.

2. Photosynthetic Consumption AQA

The evolution of early photosynthetic organisms (algae and green plants) rapidly reduced carbon dioxide levels as organisms absorbed CO₂ from the atmosphere and water to synthesize glucose.
Example: Primitive algae began absorbing carbon dioxide roughly 2.7 billion years ago, converting atmospheric CO₂ into solid plant matter and releasing oxygen.

Photosynthesis continuously released oxygen gas as a byproduct, building up atmospheric oxygen levels from near zero to the 21% concentration present today.
Example: As oxygen levels built up in the atmosphere, it enabled the evolution of land animals and created the ozone layer (O₃) that shields the surface from ultraviolet light.

3. Oxygen Generation via Photosynthesis AQA

Photosynthesis continuously released oxygen gas as a byproduct, building up atmospheric oxygen levels from near zero to the 21% concentration present today.
Example: As oxygen levels built up in the atmosphere, it enabled the evolution of land animals and created the ozone layer (O₃) that shields the surface from ultraviolet light.

Volcanic outgassing and the chemical breakdown of ammonia released nitrogen into the atmosphere; because nitrogen gas is unreactive, it accumulated over geological time to become the dominant gas (~78%).
Example: Ammonia gas (NH₃) from early volcanoes reacted with oxygen or soil bacteria to form inert nitrogen gas (N₂), which gradually built up without being consumed.

4. Accumulation of Nitrogen AQA

Volcanic outgassing and the chemical breakdown of ammonia released nitrogen into the atmosphere; because nitrogen gas is unreactive, it accumulated over geological time to become the dominant gas (~78%).
Example: Ammonia gas (NH₃) from early volcanoes reacted with oxygen or soil bacteria to form inert nitrogen gas (N₂), which gradually built up without being consumed.

Limestone formed from the compressed shells and skeletons of marine organisms that absorbed dissolved carbonates, whereas coal formed from dense plant material in ancient swamps buried under anaerobic, high-pressure sediment layers.
Example: Ancient trees and ferns in Carboniferous swamps died, sank into mud, and were compressed over millions of years under high heat and pressure to form coal seams.

5. Limestone and Coal Formation AQA

Limestone formed from the compressed shells and skeletons of marine organisms that absorbed dissolved carbonates, whereas coal formed from dense plant material in ancient swamps buried under anaerobic, high-pressure sediment layers.
Example: Ancient trees and ferns in Carboniferous swamps died, sank into mud, and were compressed over millions of years under high heat and pressure to form coal seams.

Crude oil and natural gas formed from tiny marine organisms (such as plankton) that died, settled on the seabed, and were buried beneath mud and silt in oxygen-depleted conditions over millions of years.
Example: Heat and pressure acting on buried ancient plankton trapped under impermeable rock layers converted the organic remains into liquid crude oil and natural gas (methane).

6. Crude Oil and Natural Gas Formation AQA

Crude oil and natural gas formed from tiny marine organisms (such as plankton) that died, settled on the seabed, and were buried beneath mud and silt in oxygen-depleted conditions over millions of years.
Example: Heat and pressure acting on buried ancient plankton trapped under impermeable rock layers converted the organic remains into liquid crude oil and natural gas (methane).