How carbon dioxide decreased — AQA GCSE Chemistry
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How carbon dioxide decreased explained
How carbon dioxide decreased covers 3 specification points, including Algae and plants decreased the percentage of… and Carbon dioxide was also decreased by the….
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Algae and plants decreased the percentage of carbon dioxide in the atmosphere by photosynthesis.
This statement explains that algae and plants reduced the percentage of carbon dioxide in the atmosphere through the process of photosynthesis. During photosynthesis, carbon dioxide is taken in from the atmosphere and used to produce glucose, while oxygen is released. Over billions of years, this process significantly decreased the amount of carbon dioxide in the atmosphere. You should be able to explain the link between photosynthesis and the decrease in carbon dioxide, and relate this to the increase in oxygen. For example, as algae and plants photosynthesised, they removed carbon dioxide and added oxygen, leading to the composition of the atmosphere we have today. This is assessed by your ability to describe and explain this process.
Carbon dioxide was also decreased by the formation of sedimentary rocks and fossil fuels that contain carbon.
Early Earth's atmosphere had little oxygen and much more carbon dioxide than today. Two linked processes removed carbon dioxide. When marine organisms made shells of calcium carbonate, the shells sank and compacted into sedimentary rock such as limestone, locking carbon in carbonate minerals. When dead plants and marine organisms were buried without oxygen, heat and pressure over millions of years converted their carbon into coal, crude oil and natural gas. So carbon that was once atmospheric carbon dioxide became stored in the geosphere as sedimentary rock and fossil fuels. This removal lowered carbon dioxide concentration, helping conditions for oxygen to rise later.
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. 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
- Explain how photosynthesis by algae and plants removes carbon dioxide from the atmosphere.
- Describe the effect of photosynthesis on the percentage of carbon dioxide in the atmosphere over time.
- Relate the decrease in carbon dioxide to the increase in oxygen in the atmosphere.
Show all 9 objectives
- State that sedimentary rocks and fossil fuels contain carbon that was removed from the atmosphere.
- Describe how limestone forms from compacted calcium carbonate remains.
- Explain how buried plant and marine material forms coal, crude oil and natural gas over millions of years.
- 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 flashcards
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Photosynthesis
How carbon dioxide decreased exam tips
Marking Points
- State that algae and plants carry out photosynthesis.
- Explain that photosynthesis removes carbon dioxide from the atmosphere.
- Describe how the removal of carbon dioxide by photosynthesis decreased its percentage in the atmosphere.
- Link the decrease in carbon dioxide to the increase in oxygen produced by photosynthesis.
- Carbon dioxide is removed from the atmosphere when carbon becomes locked into solid or buried stores rather than remaining as a gas.
- Shells and skeletons of marine organisms contain calcium carbonate; when these settle and are compacted, sedimentary rock such as limestone forms and stores carbon.
- Buried plant and marine material, heated and compressed over millions of years without oxygen, forms fossil fuels: coal, crude oil and natural gas.
- Both processes are slow and geological, so they reduced carbon dioxide gradually over long time periods.
- The carbon in limestone and fossil fuels originated from carbon dioxide in the atmosphere or dissolved in oceans, so these stores represent removed atmospheric carbon.
- Photosynthesis by early organisms also removed carbon dioxide, and some of that carbon later became buried and fossilised.
- 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
- 💡Remember that photosynthesis uses carbon dioxide and produces oxygen, so it decreases carbon dioxide and increases oxygen.
- 💡Use the equation for photosynthesis to support your explanation of how carbon dioxide is removed.
- 💡When asked about changes in atmospheric composition, always link them to the processes of photosynthesis and respiration.
- 💡Link each carbon store to the process that formed it: limestone to marine shells and compaction, coal to buried plant material, crude oil and natural gas to buried marine organisms.
- 💡Use the phrase 'locked up' or 'stored' to show carbon dioxide was removed from the atmosphere, not destroyed.
- 💡When asked to explain a decrease, name the store and state that the process is geological and slow, so it acted over millions of years.
- 💡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
- Thinking that algae and plants decreased carbon dioxide by respiration. Correction: respiration releases carbon dioxide; photosynthesis removes it.
- Confusing the decrease in carbon dioxide with the increase in oxygen, stating that carbon dioxide increased. Correction: carbon dioxide decreased while oxygen increased.
- Believing that the decrease in carbon dioxide was immediate. Correction: it occurred gradually over billions of years as photosynthesis by algae and plants continued.
- Thinking fossil fuels form quickly from living plants; correct this by stressing burial, heat and pressure acting over millions of years.
- Confusing sedimentary rock formation with igneous or metamorphic processes; limestone is a sedimentary rock formed from compacted calcium carbonate remains.
- Believing carbon dioxide simply disappeared; it was not destroyed but transferred into carbonate rock and fossil fuel stores, so carbon is conserved.
- 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.