* GCSE Earth Science: 1. The Evolution of the Earth's atmosphere at Doc Brown's
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1. The Evolution of the Earth's atmosphere Revision KS4 Science IGCSE/O level/GCSE Chemistry Information Study Notes for revising for AQA GCSE Science, Edexcel 360Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science (revise courses equal to US grades 9-10) based on a GCSE Geological & Earth Science TASK SHEET * Earth Science Homepage * 5 multi-word fill GCSE worksheets + answers * GCSE Earth Science Quiz: Foundation-easier m/c Quiz and Higher-harder level m/c Quiz ANSWERS-NOTES 1. The Evolution of the Earth's atmosphere and Carbon Cycle ... 2. The Rock Cycle and types of rock (details 'evolve' through sections 3. to 9.) ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Structure of the Earth ... 8. Plates and their movement ... 9. Plate Tectonics ... 10. The Moon and Planets 1. The Evolution of the Earth's atmosphere breathe easy! 1(a)(i) Today's atmosphere consists of the elements: 78% nitrogen (about 4/5ths), 21% oxygen (about 1/5th), 1% argon (1/100th), traces of other Group 0 Noble Gases (He, Ne, Kr, Xe), plus the compounds 0.036% carbon dioxide (360 ppm or parts per million) and variable amounts of water vapour (depends on humidity) and traces of many other gases from natural or man-made pollution sources (e.g. sulphur dioxide, nitrogen dioxide and carbon monoxide from fossil fuel combustion (see Oil Products notes) and methane (greenhouse gas) from cows and decomposing plant material). The composition of our atmosphere is thought to be relatively unchanged for about 200 million years due to the Carbon-Cycle balance. 1(a)(ii) One method to determine the % of oxygen in air is to use two 100cm3 glass gas syringes are connected either side of a piece of silica tubing containing copper powder or fine granules. One syringe is empty and the other filled with 100cm3 of air. The silica tube/copper is strongly heated and the gas syringes moved to and fro to pass the air over the hot copper. The oxygen in the air reacts with the copper to form copper(II) oxide. 2Cu(s) + O2(g) ==> 2CuO(s) This is a black solid of little volume. Eventually the total volume reading reaches a minimum value when all the oxygen in the air has reacted. 100 - final reading gives the % oxygen in air. 1(a) water + carbon dioxide + sunlight energy ==> glucose sugar + oxygen 6H2O(l) + 6CO2(g) ==> C6H12O6(aq) + 6O2(g) 1(a)(iv) Production of carbon dioxide: Natural burning of biomass like forests, plant and animal respiration**, biological decay of plant and animal material, 'mans' burning/combustion of fossil fuel, volcanic activity e.g. the thermal decomposition of minerals like carbonates in magma/lava. See Oil notes for more on fossil fuels. ** Respiration: glucose sugar + oxygen ==> water + carbon dioxide + energy (exothermic, energy given out) C6H12O6(aq) + 6O2(g) ==> 6H2O(l) + 6CO2(g) 1(a)(v) Oxygen balance: The main process of CO2 removal by photosynthesis also produces oxygen. Respiration and combustion (natural/man) mainly remove the oxygen from our atmosphere. So this means the Carbon-Cycle effectively maintains a constant % of oxygen in the atmosphere as well as controlling the carbon dioxide levels. (Note: as far as we know the Greenhouse Effect will not significantly change the oxygen level in the Earth's atmosphere). There is no evidence to suggest that the increase in the world's population (respiration!) or the burning of forests (deforestation by combustion) is having any effect on the oxygen level BUT increase in 'man's' industrial and domestic activity by burning fossil fuels is causing the carbon dioxide concentration to rise.
For more on pollution and environmental problems see Oil Part 4 and Global Warming Graphs
1(c)(i) Seas and oceans would form from condensed water vapour as the early Earth surface cooled down. Carbon dioxide and ammonia would dissolve in this water. 1(c)(ii) The carbon dioxide could form soluble sodium carbonate, sodium hydrogencarbonate or calcium hydrogencarbonate and insoluble calcium carbonate.
1(d) Primitive bacterial life evolved about 3500 million years ago (3.5 billion y) and the first green algae like plants from about 2000 million years ago (2.0 billion y). The increasingly successful evolution of green photosynthesizing plants colonising both land and water, produced an increasingly oxygen richer atmosphere and in so doing removing most of the carbon dioxide from the original early atmosphere. This 'oxygenated' atmosphere would be 'polluting' and 'toxic' to many microorganisms which could not tolerate oxygen, having evolved in a non-oxygen environment. However, by 1000 million (1 billion years) years ago, there was sufficient oxygen to allow the evolution of respiring animal life. 1(e)(i) Ammonia would be converted to soluble nitrates mainly by nitrifying bacteria or, to a small extent, ammonia would be directly oxidised to nitrogen gas by the newly formed oxygen. The nitrates are absorbed by plants to form proteins or converted to atmospheric nitrogen by denitrifying bacteria. 1(e)(ii) Methane would be oxidised to carbon dioxide and water by the new 'oxygenated' atmosphere. 1(e)(iii) Ozone (O3) would now be formed and this would absorb and filter out much of the ultraviolet light that is harmful to many organisms. This uv filtering would then allow the much wider evolutionary development of plant and animal organisms.
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