Doc Brown's Chemistry - Earth Science Revision Notes

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) + O_2(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)(iii) Removal of carbon dioxide (CO₂): Photosynthesis* in green land plants absorbing carbon dioxide to form biomass (and oxygen), then some plant biomass is converted to animal biomass. Some of the CO₂ will dissolve in the seas/oceans => where it may be further changed in photosynthesising marine organisms (plankton) to produce biomass, forming soluble carbonates and insoluble minerals e.g. calcium carbonate (sedimentary limestone rock) as the shelly remains of creatures and coral etc., decay (without oxygen) of any organic material from dead plant and animal remains to form the fossil fuels coal, oil and gas over millions of years. * Photosynthesis:

water + carbon dioxide + sunlight energy ==> glucose sugar + oxygen 

6H₂O_(l) + 6CO_2(g) ==> C₆H₁₂O_6(aq) + 6O_2(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)

C₆H₁₂O_6(aq) + 6O_2(g) ==> 6H₂O_(l) + 6CO_2(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.

1(a)(vi) Global warming, temperature and CO₂ imbalance: The average temperature of the Earth depends on the net effect of the Sun's input and the Earth's output of energy [mainly by heat/infrared (ir) radiation]. However, the relatively rapid rate of burning massive amounts of fossil fuels over the last few hundred years is threatening this and the CO₂ balance. The CO₂ in the atmosphere absorbs some of the re-radiated ir to keep the Earth warm and a constant CO₂ concentration, also means a steady temperature. The increasing CO₂ levels means more ir is absorbed and the global temperatures are rising - the Greenhouse Effect. This global warming is predicted (maybe happening?) to: affect sea levels by melting glaciers, change in weather patters e.g. more drought in Africa, more rain and storms in other parts of the world, forcing change in agriculture management with weather/temperature changes etc. etc. but its all a bit uncertain!

For more on pollution and environmental problems see Oil Part 4  and Global Warming Graphs

1(b) The  early Earth atmosphere consisted of mainly carbon dioxide, water vapour and small amounts of ammonia and methane from intense volcanic activity (mainly in the first billion years!). There would be little or no oxygen (rather like on Mars or Venus today). Some texts also refer to small amounts of hydrogen, nitrogen, carbon monoxide and sulphur dioxide. Note: We can't be absolutely sure, but study of the atmospheres of other planets are helping in understanding of the Earth's early atmosphere, and all of these gases exist on one or more of the planets. Much of the hydrogen and helium originally present would have been 'boiled' off as the Earth's gravity would not be strong enough to hold these fast moving molecules at high temperatures!

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(c)(iii) The seas and oceans contain large quantities of dissolved salts which were once part of rock formations, now weathered, eroded and washed away in rivers. These salts do NOT evaporate, unlike the water, so the oceans have gradually  become more concentrated in salts. Much later some salts are removed as shells of marine organisms, some chemical reactions produce precipitates which form part of the sea-floor sediments, and crystallisation to form salt deposits e.g. in high concentration warm parts of the world like the Dead Sea and enclosed seas/lakes may completely dry up to give 'rock salt' and 'potash' sedimentary rock formations.

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.

More on ozone

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