Define — 4 marks
Scientists studying ice cores from Antarctica have discovered trapped air bubbles from millions of years ago. By analysing the composition of these ancient atmospheric samples, researchers can understand how Earth's atmosphere has changed over geological time. The earliest samples show very different gas compositions compared to today's atmosphere.
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(a) Define the term 'outgassing' in the context of the early atmosphere.
[1 mark]
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(b) Define what is meant by the 'ozone layer' and explain why its formation was significant for the evolution of life on Earth.
[2 marks]
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(c) Define 'atmospheric composition' and describe how the presence of photosynthetic organisms changed the composition of Earth's atmosphere during the Proterozoic Eon.
[1 mark]
Show mark scheme
- (a) The release of gases (such as water vapour, carbon dioxide and nitrogen) from within the Earth's interior/crust (1)
- (b) A layer of ozone (O₃) molecules in the stratosphere (1)
- (b) It absorbs ultraviolet radiation / protects organisms from harmful UV radiation, allowing life to develop/evolve on land (1)
- (c) The mixture/proportions of different gases present in the atmosphere; photosynthetic organisms produced oxygen as a byproduct, increasing atmospheric oxygen levels and decreasing carbon dioxide levels (1)
State — 2 marks
Scientists studying rock layers in different parts of the world have found evidence about how Earth's atmosphere has changed over billions of years. Early in Earth's history, the atmosphere was very different from today. Understanding these changes helps us learn about how life developed on our planet.
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State two gases that were present in Earth's early atmosphere.
[2 marks]
Show mark scheme
- {'mark': 1, 'description': 'Correctly states one gas from: water vapour, carbon dioxide, methane, ammonia, nitrogen (or any other appropriate early atmosphere gas)'}
- {'mark': 1, 'description': 'Correctly states a second different gas from the acceptable list'}
Calculate — 2 marks
Geologists study gas bubbles trapped in ancient volcanic rocks to understand how Earth's early atmosphere was different from today. A sample of gas extracted from 3.5 billion-year-old rock was found to contain 4.5 dm³ of carbon dioxide and 0.5 dm³ of nitrogen.
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(a) Calculate the total volume of gas in the sample.
[1 mark]
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(b) Calculate the percentage of carbon dioxide in this ancient atmosphere sample.
[1 mark]
Show mark scheme
- (a) 5.0 dm³ (accept 5 dm³)
- (b) 90% (accept 90)
Explain — 3 marks
Geologists study ancient rock formations to understand how Earth's atmosphere has changed over billions of years. Volcanic rocks from 4 billion years ago contain trapped gases that reveal information about the composition of the early atmosphere.
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(a) Explain what was the main source of the gases that formed Earth's early atmosphere.
[1 mark]
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(b) Explain two ways in which the amount of carbon dioxide in the atmosphere decreased over time.
[2 marks]
Show mark scheme
- (a) volcanic activity / volcanoes / eruptions (released the gases)
- (b) carbon dioxide dissolved in the oceans
- (b) carbonates formed / carbon dioxide reacted with minerals to form rocks
- (b) photosynthesis by plants / algae (absorbed carbon dioxide)
Show — 4 marks
A school chemistry class is investigating how carbon dioxide was removed from Earth's early atmosphere. They bubble a sample of carbon dioxide gas through limewater (calcium hydroxide solution) to model the process that occurred billions of years ago when volcanic gases dissolved in the ancient oceans.
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(a) Show that the gas sample contains carbon dioxide.
[1 mark]
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(b) Explain how this experiment helps to show why the percentage of carbon dioxide decreased in Earth's early atmosphere.
[3 marks]
Show mark scheme
- (a) white precipitate forms / limewater turns cloudy/milky
- (b) carbon dioxide dissolved in the ancient oceans
- (b) reacted with calcium (ions) to form calcium carbonate
- (b) carbonate rocks / sedimentary rocks formed
- (b) locking away/removing carbon dioxide from the atmosphere
GCSE Perfect