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Chemistry – Key Topic 9
4.9 Chemistry of the atmosphere
4.9.1 The composition and evolution of the earth’s atmosphere
4.9.1.1 The proportions of different gases in the atmosphere
For 200 million years, the proportions of different gases in the atmosphere have been
much the same as they are today:
- About 78% nitrogen
- About 21% oxygen
- Small proportions of various other gases, including carbon dioxide, water vapour
and noble gases
Nitrogen – odourless and colourless – used to make fertilisers
Oxygen – very reactive and colourless – used for breathing
Argon – very unreactive and colourless – used in the production of titanium
Carbon dioxide – denser than air – used in photosynthesis
Water – 100 boiling point and 0 melting point – used for drinking
4.9.1.2 The Earth’s early atmosphere
Theories about what was in the Earth’s early atmosphere and how the atmosphere
was formed have changed and developed over time. Evidence for the early
atmosphere is limited because of the time scale of 4.6 billion years
- Scientist have reconstructed what they think the atmosphere must have been like
based on evidence from gas bubbles trapped in ancient rocks
One theory suggests that during the first billion years of the Earth’s existence there
was intense volcanic activity that released gases (nitrogen, water vapour and small
amount of methane and ammonia) that formed the early atmosphere and water
vapour that condensed to form the oceans. At the start of this period the Earth’s
atmosphere may have been like the atmosphere of Mars and Venus today, consisting
of mainly carbon dioxide with little or no oxygen gas
- Volcanoes also produced nitrogen which gradually built up in the atmosphere and
there may have been small proportions of methane and ammonia
, - When the oceans formed carbon dioxide dissolved in the water and carbonates
were precipitated producing sediments on the seabed, reducing the amount of
carbon dioxide in the atmosphere.
- Later, marine animals evolved, their shells and skeletons contained some of these
carbonates from the oceans
- Green algae evolved and absorbed some of the CO2 so they could carry out
photosynthesis
1. 4.6 billion years ago – the earth was all rock and volcano with no real atmosphere
and the planet was solidified as heat was lost
2. 4 billion years ago – 0% oxygen and volcanoes released nitrogen, carbon dioxide,
hydrogen and water vapour to make a fairly dangerous atmosphere
3. 2.7 billion years ago – 0.001% oxygen and tiny photosynthetic bacteria started to
slowly appear. They gave out oxygen and its concentration in the atmosphere
began to rise slowly
4. 2.7-1.7 billion years ago – plants started to appear and they produced even more
oxygen.
- Some of the older bacteria start to die out as they have evolved in a time with no
oxygen and were damaged by the rise in oxygen levels.
- Animals can now evolve
- skeletons and skulls of marine animals have built up on the sea bed over millions
of years and were put under huge pressure to form limestone
- oceans formed as the atmosphere cooled and water vapour condensed
5. 200 million years ago – today – the proportion of gases in the atmosphere has
been constant for the past 200 million years – this is due to the natural cycle of carbon in
which carbon moves between the oceans, rocks and atmosphere. However there is
evidence that carbon dioxide levels are increasing
Some of the carbon that organisms took in from the atmosphere and oceans became
locked up in rocks and fossil fuels after the organism died.
- When plants, plankton and marine animals die, they fall to the seabed and get
buried in layers of sediment
- Over millions of years, they become compressed and form sedimentary rocks, oil
and gas – trapping the carbon within them (helping to keep it out of the
atmosphere)
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