Doc Brown's Chemistry - Earth Science & Geology Revision Notes

for KS4 Science, GCSE, IGCSE & O Level Courses

1. The Evolution of the Earth's Atmosphere

The Gases in Air, The Carbon Cycle and the Origin of Life

Where has today's atmosphere come from? How did our atmosphere evolve? What is the percentage % composition of the Earth's atmosphere? What were the constituent gases of the early atmosphere of the Earth? What part does the Carbon Cycle play in the composition of the Earth's atmosphere? Read on ...!

GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets

1. The Evolution of the Earth's atmosphere breathe easy!

The Earth and its atmosphere provide everything we need with a little help from the sun

Minerals from the crust and gases from the air

1(a)(i) Today's atmosphere consists of the elements: 78% nitrogen (about ⁴/₅ths), 21% oxygen (about ¹/₅th), 1% argon (¹/₁₀₀th), plus traces of other Group 0 Noble Gases (He, Ne, Kr, Xe) and the compounds carbon dioxide (0.039%, 390 ppm or parts per million and rising!) with variable amounts of water vapour (depends on humidity).

There are also 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 principally due to the Carbon-Cycle balance and helps provide the conditions to sustain complex life on Earth..

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.

copper + oxygen ==> copper(II) oxide

2Cu_(s) + O_2(g) ==> 2CuO_(s)

Copper oxide 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)

Environmental note: We know the oceans act as a reservoir for carbon dioxide, which is quite soluble in water, but increased amounts of carbon dioxide absorbed by the oceans has an impact on the marine environment. Carbon dioxide is a weakly acidic gas and as more dissolves in the seas and oceans from fossil fuel burning, they are becoming more acidic (a small fall in the pH). However, even a small decrease in pH, can lead to disruption of previously stable ecosystems, so in terms of the flora (plants) and fauna (animals), 'resident' species can suffer and 'foreign' species move in!

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 percentage of oxygen in the atmosphere as well as controlling the carbon dioxide levels.

Note that 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 and may be leading to significant climate change.

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/environmental problems Oil Notes Part 4 Pollution & Extra Global Warming Notes & 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!). As the Earth cooled down sufficiently to form the crust, there would be far more volcanic activity that we see today. The gases from volcanoes would form the initial 'stable' atmosphere and the first seas and oceans.

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 about the history of the Earth's first atmosphere, but study of the atmospheres of other planets are helping our understanding of the Earth's early atmosphere, and all of these gases exist on one or more of the planets. The initial surface of the Earth after its formation would be extremely hot and molten. Therefore much of the hydrogen, helium and other gases 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 years) and the first green algae like plants from about 2000 million years ago (2.0 billion y).

The green algae and more complex plants thrived in the carbon dioxide rich atmosphere and much of the carbon dioxide would dissolve in the seas and oceans.

These green plants were absorbing the carbon dioxide and producing oxygen in the process of photosynthesis.

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.

It should be appreciated that the remains of dead algae, rotting plants, skeletons and shells of marine creatures became buried under layers of sediment to form sedimentary rocks like coal (carbon), limestone (carbon dioxide to carbonate) and in oil and gas deposits (hydrocarbons).

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 as the oxygen levels in the atmosphere increased and this oxygen would absorb and filter out much of the ultraviolet light that is harmful to many organisms.

However, many early organisms could not cope with rising levels of oxygen and many species would become extinct.

BUT the ozone uv filtering and increased oxygen in the atmosphere would then allow a much wider evolutionary development of much more complex plant and animal organisms.

More on ozone

Appendix 1.1 - Obtaining and Using Gases from Air

• Air is not an obvious resource for the chemical industry and commerce, we take it for granted, and just expect it to be there for its oxygen needed for our respiration.

• However, it is a most valuable resource in its own right.

• How do we get useful products from air?

  • The air must be initially filtered to remove dust particles, before liquefaction.

  • It is then compressed and cooled to around -200^oC to form a liquid.

  • The cooling process is performed in such a way that water vapour is condensed out first and then carbon dioxide frozen out before fractional distillation (see Oil Notes).

  • The liquified air is now warmed before entering a fractionating column and fractionally distilled twice to separate the gases nitrogen, oxygen, argon and helium.

    • Nitrogen is used in the synthesis and manufacture of ammonia

    • Oxygen is used in welding, respiratory systems in hospitals etc.

    • Carbon dioxide as used as 'dry ice' for stage effects and as a coolant (-78^oC).

    • Argon is in filament bulbs and to provide an inert atmosphere to stop reactions with air eg in welding and steel making.

    • Helium is used in balloons and liquid helium is used to provide extremely low temperatures in technologies like cryogenics.

Appendix 1.2 - The Origin of Life

• There are several theories of how primitive bacterial life evolved about 3500 million years ago (3.5 billion years).

• We can do experiments which provide tantalising clues as to how the basic molecules of life can be formed.

• However, there is no accepted scientific theory that adequately explains the origin of life, all that we can be sure about is that life has evolved from some kind of primitive cell microorganisms and that through the course of evolution diverse and more complex forms of life increasingly inhabited the Earth's surface.

• The 'primordial soup theory' is based on the chemistry of what can happen to mixtures of gases when exposed to high temperatures, uv light or high voltage electrical discharges (lightning).

  • Some of the first experiments were done by Miller and Urey who used a mixture of water, methane, ammonia, carbon monoxide and hydrogen and applied a high voltage electrical charge to them.

  • Many organic compounds were formed eg amino acids and sugars and so the electrical spark certainly sparked some chemical ideas on the origin of life and its subsequent evolution.

  • These were significant experimental results because organic compounds form the basis of life on Earth as we know it.

  • BUT, how these relatively simple molecules somehow combined to give a self-replicating organism ie the simplest possible living cell, is a complete mystery!

    • basic organic molecules == ???????? ===> living organism

• Since then many other experiments done using different mixtures of gases, increasing the range of organic compounds that could have been formed in the early history of the Earth offering all sorts of chemical pathways to much more complex molecules, BUT ....!!!!

• NOTES Cue, QUIZ and WORKSHEET QUESTION LINKS

• A structured question covering much of the GCSE Earth Science

• Earth Science GCSE study revision notes index (responses to the above Questions)

• foundation-easier multiple choice quiz on Earth Science

• higher-harder multiple choice quiz on Earth Science

•  Five Earth Science word-fill worksheets Q1 * Q2 * Q3 * Q4 * Q5

• Answers to the five GCSE Earth Science worksheets listed above

GCSE/IGCSE/O Level KS4 Earth Science-Geology ANSWER-REVISION-NOTES 1. Evolution of the Earth's atmosphere, Gases in Air, Carbon Cycle, Origin of Life ... 2. Rock Cycle, Types of rock ... 3. Weathering of Rocks ... 4. Igneous Rocks ... 5. Sedimentary Rocks ... 6. Metamorphic Rocks ... 7. The Layered Structure of the Earth ... 8. Tectonic plate theory, Wegener's theory, evidence for continental drift ... 9. More on Plate Tectonics, effects of plate movement, volcanoes, earthquakes, faults etc. ... 10. A few geology and atmosphere notes on the Moon and Planets

Studying Revision for KS4 Earth Science GCSE/IGCSE/O level Chemistry Information Study Notes for revising for AQA GCSE Earth Science, Edexcel GCSE Science/IGCSE Chemistry & OCR 21stC Science, OCR Gateway Science  WJEC gcse science chemistry CCEA/CEA gcse science chemistry (revise courses equal to US grade 8, grade 9 grade 10)

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