AQA · GCSE · Chemistry
Course version: 8462 · Version from Sept 2018
Students should be able to: describe the main changes in the atmosphere…
Full specification wording
How carbon dioxide decreased: Students should be able to: describe the main changes in the atmosphere over time and some of the likely causes of these changes describe and explain the formation of deposits of limestone, coal, crude oil and natural gas.
Earth's atmosphere changed from an early mixture rich in carbon dioxide with little or no oxygen to today's nitrogen-rich, oxygen-containing air.
Checked against the AQA 8462 specification (opens in a new tab) on .
- Point 3 of 3
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Earth's atmosphere changed from an early mixture rich in carbon dioxide with little or no oxygen to today's nitrogen-rich, oxygen-containing air. Likely causes include volcanic activity releasing gases, condensation of water vapour to form oceans, photosynthesis by algae and plants adding oxygen while removing carbon dioxide, and removal of carbon dioxide by dissolving in oceans and forming carbonate rocks. Deposits form when carbon-rich remains are buried. Marine shells of calcium carbonate compact into limestone. Buried plant material, heated and compressed without oxygen over millions of years, forms coal. Buried marine organisms form crude oil and natural gas. These deposits store carbon removed from the atmosphere.
What earns the marks
- Describe the early atmosphere as containing little or no oxygen, with carbon dioxide and water vapour as major gases, plus nitrogen and small amounts of methane and ammonia.
- State that volcanic activity released gases including carbon dioxide, water vapour and nitrogen, and that condensing water vapour formed the oceans.
- Explain that photosynthesis by algae and plants removed carbon dioxide and added oxygen, allowing oxygen levels to rise.
- Describe carbon dioxide dissolving in oceans and forming carbonate sediments, later compacted into sedimentary rock such as limestone.
- Explain limestone formation from calcium carbonate shells and skeletons of marine organisms that settle, compact and cement over millions of years.
- Explain coal formation from buried plant material subjected to heat and pressure without oxygen over millions of years.
- Explain crude oil and natural gas formation from buried marine organisms subjected to heat and pressure over millions of years.
Common mistakes and how to put them right
Saying oxygen was always present.
Put it right by stating oxygen increased only after photosynthesis became widespread.
Confusing the formation conditions of coal with those of crude oil and natural gas.
Put it right: Coal comes mainly from land plants, while crude oil and natural gas come mainly from marine organisms.
Describing fossil fuel formation as a quick chemical reaction.
Put it right by emphasising burial, heat, pressure and millions of years.
Exam tips
- Use a sequence when describing atmospheric change: volcanic gases, oceans condense, carbon dioxide dissolves, photosynthesis adds oxygen.
- For each deposit, name the starting material, the burial conditions and the long timescale.
- Keep the two parts of the statement separate in your answer: changes in the atmosphere, then formation of deposits.
Try the lesson
The start of the How carbon dioxide decreased: Students should be able to: describe the main changes in the atmosphere over time and some of the likely causes of these changes describe and explain the formation of deposits of limestone, coal, crude oil and natural gas. lesson, made from checked teaching notes for this specification point.
Have a go first
Which gas made up most of the Earth's early atmosphere before it changed over time?
Listen
Earth's atmosphere changed from an early mixture rich in carbon dioxide with little or no oxygen to today's nitrogen-rich, oxygen-containing air. Likely causes include volcanic activity releasing gases, condensation of water vapour to form oceans, photosynthesis by algae and plants adding oxygen while removing carbon dioxide, and removal of carbon dioxide by dissolving in oceans and forming carbonate rocks. Deposits form when carbon-rich remains are buried. Marine shells of calcium carbonate compact into limestone. Buried plant material, heated and compressed without oxygen over millions of years, forms coal. Buried marine organisms form crude oil and natural gas. These deposits store carbon removed from the atmosphere. A strong answer earns marks for these points. Describe the early atmosphere as containing little or no oxygen, with carbon dioxide and water vapour as major gases, plus nitrogen and small amounts of methane and ammonia. State that volcanic activity released gases including carbon dioxide, water vapour and nitrogen, and that condensing water vapour formed the oceans. Explain that photosynthesis by algae and plants removed carbon dioxide and added oxygen, allowing oxygen levels to rise. Describe carbon dioxide dissolving in oceans and forming carbonate sediments, later compacted into sedimentary rock such as limestone. A common slip: Saying oxygen was always present. Instead, stating oxygen increased only after photosynthesis became widespread. Exam tip: Use a sequence when describing atmospheric change: volcanic gases, oceans condense, carbon dioxide dissolves, photosynthesis adds oxygen.
Key words
Say what each word means before you turn the card.
Evolution of the atmosphere
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Photosynthesis and oxygen accumulation
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Formation of sedimentary rocks
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Formation of fossil fuels
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See every definition
- Evolution of the atmosphere
- The change from an early mixture rich in carbon dioxide with little or no oxygen to today's nitrogen-rich, oxygen-containing air.
- Photosynthesis and oxygen accumulation
- Photosynthesis by algae and plants removed carbon dioxide and added oxygen, allowing oxygen levels to rise.
- Formation of sedimentary rocks
- Carbon dioxide dissolving in oceans formed carbonate sediments, later compacted into sedimentary rock such as limestone.
- Formation of fossil fuels
- Buried carbon-rich remains subjected to heat and pressure over millions of years form deposits such as coal, crude oil and natural gas.
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