Chemistry - Earth's Atmosphere (AQA Syllabus A)

One topic looked at in GCSE Science is the changes which have and do occur on the Earth and in its atmosphere. This is the second of three quizzes on the subject and it focusses specifically on the Earth's atmosphere and the gases found there.

Something that you probably take for granted is the atmosphere around us and the gases that we breathe. But the atmosphere doesn't only supply the oxygen that our bodies need for respiration, it also protects us from space. High level ozone protects us from the effects of the most harmful ultra-violet rays from the Sun and carbon dioxide ensures that the Earth's temperature stays within the limits needed for life. The atmosphere also protects us from most fragments of space debris by causing them to burn up well before they hit the Earth's surface and it allows water and carbon to be recycled through food chains.

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But how did the atmosphere get here and how has it ended up as being perfect to sustain life? These are questions that can't be answered with certainty as it all happened long ago in the distant past. It seems that our atmosphere has been pretty much the same mixture of gases for about 200 million years!

Scientists have many theories as to how the atmosphere was formed and how changes have occurred over time. The most widely accepted is the one that suggests it came from volcanic activity. Molten magma contains a lot of gas dissolved in it which is released when it erupts at the surface. The early atmosphere was probably mostly carbon dioxide with little or no oxygen. There were smaller proportions of water vapour, ammonia and methane. As the Earth cooled down, most of the water vapour condensed and formed the oceans. The atmospheres of Venus and Mars are thought to be like the Earth's early atmosphere but the changes which happened on Earth never took place on those planets.

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  1. As well as the main gases, which of the following are found in the atmosphere in small proportions?
    About 0.9% of the air you breathe is argon
  2. About how much of the atmosphere is nitrogen?
    You are expected to know the proportions of the gases in the atmosphere for the exam
  3. The level of carbon dioxide in the atmosphere has remained almost the same for 200 million years. Since the start of the twentieth century it has increased dramatically. What is thought to have caused this?
    The curve showing the increase of carbon dioxide seems to have followed the development of the motor car. As cars have become more affordable, millions more have appeared on the roads, burning huge amounts of fossil fuels and releasing carbon dioxide that has been locked away for millions of years
  4. The first green algae-like plants evolved about 2,000 million years ago. How would this have affected the early atmosphere?
    Photosynthesis by the earliest plants is thought to have converted a lot of the carbon dioxide into oxygen
  5. Argon is a noble gas and is inert (does not react with other substances). For that reason it is used in light bulbs but how is it obtained from the air?
    Air is a mixture of gases so if you cool it down enough, it will turn into a mixture of liquids that can be separated by fractional distillation
  6. What may have happened to the rest of the carbon dioxide from the early atmosphere?
    Limestone is made under the sea from the mineral calcium carbonate and its formation required carbon dioxide. Carbonate containing rocks form on the sea bed
  7. Why is nitrogen used in food packaging?
    Food goes off because it is exposed to oxygen and microorganisms. If nitrogen is used instead of air, there is no oxygen so the food is less likely to spoil before it reaches you since the bacteria can't thrive
  8. What natural process is thought to have created the early atmosphere and which was the main gas?
    The early Earth was a much more volcanically active place than it is now
  9. About how much oxygen is there in the atmosphere?
    Only about a fifth of the atmosphere is life-sustaining oxygen
  10. One theory is that life on Earth started from chemicals in the early atmosphere. According to that theory, which chemicals are thought to have been involved?
    It has been suggested that lightning provided the energy for the chemical reactions to take place

Author: Kev Woodward

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