How carbon dioxide decreased — AQA GCSE Chemistry
Test yourself on How carbon dioxide decreased with AQA GCSE practice questions.
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How carbon dioxide decreased explained
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.
Read the full explanation
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.
Your focus
- Describe the main changes in Earth's atmosphere over time and give likely causes.
- Describe and explain how limestone, coal, crude oil and natural gas deposits form.
- Link the formation of carbon-containing deposits to the decrease in atmospheric carbon dioxide.
How carbon dioxide decreased exam tips
Marking 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.
- 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.
Examiner 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.
Common Mistakes
- Saying oxygen was always present; correct this by stating oxygen increased only after photosynthesis became widespread.
- Confusing the formation conditions of coal with those of crude oil and natural gas; 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; correct this by emphasising burial, heat, pressure and millions of years.