AQA GCSE Combined Science Foundation (8464), Foundation tier · Chemistry › Chemistry of the atmosphere › The composition and evolution of the Earth's atmosphere
Practise How carbon dioxide decreased. 7 exam-style questions on this subtopic, at up to four difficulty levels, with full mark schemes and a progress tracker. Free, no account needed.
How the carbon dioxide that made up most of the early atmosphere fell to the tiny proportion in air today: dissolving in the oceans, photosynthesis, and carbon being locked up in sedimentary rocks and fossil fuels. This subtopic is also where 6-mark questions on how the atmosphere has changed, and on how limestone, coal, crude oil and natural gas formed, come from.
Key facts
CO2 removed by: dissolving in the oceans (carbonate sediments), photosynthesis, sedimentary rocks, fossil fuels
Limestone: mainly calcium carbonate, CaCO3, from the shells and skeletons of sea creatures
Coal: remains of plants (mainly trees) buried and compressed
Crude oil and natural gas: remains of plankton buried in mud on the sea bed
Formation of fossil fuels: heat and pressure over millions of years
Limestone and fossil fuels lock up carbon for millions of years
Early atmosphere mainly CO2 → about 0.04% CO2 today
Notes
How carbon dioxide was removed
The early atmosphere was mainly carbon dioxide. Today carbon dioxide is only about 0.04% of the air.
Dissolving in the oceans: carbon dioxide dissolved and carbonates were precipitated, forming sediments.
Photosynthesis: algae and plants took in carbon dioxide and gave out oxygen.
Sedimentary rocks: carbon was locked up in rocks such as limestone (mainly calcium carbonate, CaCO3).
Fossil fuels: carbon was locked up in coal, crude oil and natural gas.
How the deposits formed
diagram
Limestone: many sea creatures make shells and skeletons of calcium carbonate. When they died, these settled on the sea bed, were buried under more sediment and were compressed into limestone over millions of years.
Coal: formed from the remains of plants, mainly trees in ancient swampy forests. They were buried before they could fully decay, then compressed over millions of years.
Crude oil and natural gas: formed from the remains of plankton (tiny sea organisms) that were buried in mud on the sea bed. Heat and pressure over millions of years changed them into crude oil and natural gas.
Buried and compressed over millions of years, the carbon was locked away in rocks and fossil fuels.
The main changes over time
diagram
Carbon dioxide: from most of the atmosphere to about 0.04%, because of dissolving in the oceans, photosynthesis, and the formation of sedimentary rocks and fossil fuels.
Oxygen: from little or none to about 20%, because of photosynthesis by algae and then plants.
Nitrogen: released by volcanoes and gradually built up to about 80%. Nitrogen is very unreactive, so it stayed in the atmosphere.
Water vapour: most of it condensed to form the oceans.
Sketch of one theory, not to scale.
How to answer each type of question
Give ways that carbon dioxide decreased
2 marksGrade 4
Choose two DIFFERENT processes from: dissolving in the oceans, photosynthesis, sedimentary rocks, fossil fuels.
Name the organisms or rock where you can, e.g. 'photosynthesis by algae and plants'.
Example. Give two processes that reduced the percentage of carbon dioxide in the Earth's early atmosphere.
Show the model answerHide the model answer
Any two from: photosynthesis by algae and plants (1) carbon dioxide dissolving in the oceans / carbonates precipitating as sediments (1) formation of sedimentary rocks such as limestone (1) formation of fossil fuels such as coal (1)
Don’t lose marks
Saying crude oil formed from dinosaurs or other large animals. It formed from plankton.
Saying coal formed from sea creatures. Coal formed from plants.
Saying carbon dioxide 'turned into' oxygen. Photosynthesis uses carbon dioxide and water to make glucose and oxygen.
Leaving out the time scale and conditions: 'millions of years' and 'heat and pressure' are often marking points.
In 6-mark answers, listing changes without their causes, or writing them out of order.
More tips
Memory tricks
Where did the CO2 go? 'Dissolved, Photosynthesised, locked in Rocks and Fuels'.
Coal from trees (on land); oil and gas from plankton (in the sea); limestone from shells (in the sea).
Every lump of coal and piece of limestone is stored carbon from the ancient air. Burning fossil fuels sends it back as CO2.
Exam technique
For the 6-mark 'changes over time' question, jot down a quick timeline before you write, then follow it.
For each deposit, say what the organisms were, where they were buried, and what happened over millions of years.
Use numbers when you can: about 2.7 billion years, 200 million years, 80% nitrogen, 20% oxygen, 0.04% carbon dioxide.
What each grade needs
What you need to be able to do, from the first marks up to the top grade.
Grade 3
Name a process that removed carbon dioxidePhotosynthesis by algae and plants took carbon dioxide out of the atmosphere.
Grade 4
Name rocks and fuels that lock up carbonSedimentary rocks such as limestone, and the fossil fuels coal, crude oil and natural gas.
Grade 5
Describe how limestone formedShells and skeletons of sea creatures, made of calcium carbonate, settled, were buried and were compressed into rock.
Grade 5
Describe how coal formedRemains of plants, mainly trees, were buried and compressed over millions of years.
Quick recall
Cover the answers and test yourself. The app has these as flashcards that come back just before you'd forget them.
The shells and skeletons of marine organisms are made mainly of one compound. Name this compound.
calcium carbonate
Name the process by which plants and algae remove carbon dioxide from the atmosphere.
photosynthesis
Calculate how many times greater the mass of carbon in the oceans is than the mass of carbon in fossil fuels.
9.5
Sample questions
Written for this site in the style of AQA exam questions. They are not taken from real past papers.
Question 1Easy4 marks
(a) Give two ways in which the amount of carbon dioxide in the atmosphere was decreased.[2]
(b) Which rock was formed from the shells and skeletons of marine organisms? Tick (✓) one box.[1]
Basalt
Coal
Granite
Limestone
(c) The shells and skeletons of marine organisms are made mainly of one compound. Name this compound.[1]
Show the answer and mark scheme
(a)
photosynthesis by algae and plants
dissolving in the oceans
formation of sedimentary rocks / carbonates
formation of fossil fuels
(b)Answer: Limestone
(c)Answer: calcium carbonate
calcium carbonate
Question 2Medium6 marks
Coal, crude oil and natural gas are fossil fuels. They contain carbon that was once in the atmosphere as carbon dioxide.
(a) Describe how coal was formed.[2]
(b) Describe how crude oil and natural gas were formed.[2]
(c) Explain how the formation of coal reduced the amount of carbon dioxide in the atmosphere.[2]
Show the answer and mark scheme
(a)
from the remains of plants (that lived millions of years ago)
buried and compressed over millions of years (without oxygen)
(b)
from the remains of plankton / tiny sea organisms buried in mud
heat and pressure over millions of years (without oxygen)
(c)
plants took in carbon dioxide for photosynthesis
the carbon was locked up in the coal instead of returning to the atmosphere (when the plants decayed)