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13.1 History of Our Atmosphere

GCSE Chemistry · AQA · Chapter 13: The Earth's Atmosphere

A free GCSE Chemistry lesson for AQA (8462) from OBA School: what air is made of today, and how volcanoes and the first oceans shaped Earth’s early atmosphere. Watch the video, read the explanation and worked examples, then try the revision questions. You can work through the full lesson, with every video, practice quizzes and printable worksheets with mark schemes, in our free online course.

What you will learn

  • State the approximate proportions of the main gases in dry air today
  • Describe one theory for how Earth’s early atmosphere formed from volcanic activity
  • Explain how the oceans formed and how they reduced the amount of carbon dioxide
  • Explain why evidence about the early atmosphere is limited, and evaluate different theories (volcanoes, comets)
  • Use simple percentage calculations with the composition of air

Key words

  • Atmosphere: The layer of gases surrounding a planet.
  • Volcanic activity: Eruptions that release molten rock and gases (mainly carbon dioxide and water vapour) from inside a planet.
  • Condense: Change from a gas to a liquid when cooled.
  • Precipitate: An insoluble solid formed from a reaction in solution. Carbonates precipitated out of the early oceans.
  • Sediment: Solid particles that settle at the bottom of water and can later form sedimentary rock.
  • Theory: An explanation supported by evidence. It can change when new evidence is found.
  • Comet: A lump of ice, dust and rock orbiting the Sun. One theory says comets brought some of Earth’s water.

The air today

For the last 200 million years or so, the proportions of gases in the atmosphere have stayed much the same. Dry air is about four-fifths (80%) nitrogen and about one-fifth (20%) oxygen. The rest is small amounts of other gases: carbon dioxide (about 0.04%), water vapour and noble gases such as argon.

Worked example: using percentages of air

A classroom holds 150 m³ of air. What volume of this is oxygen?
Oxygen is about 21% of air (use 20% if the question says “about one-fifth”).
volume of oxygen = 21 ÷ 100 × 150 = 31.5 m³

Answer: 31.5 m³ (about 30 m³ using 20%)

Key fact: In AQA questions, “approximately four-fifths nitrogen and one-fifth oxygen” earns the marks. More precise values (78% and 21%) are also correct.

Measuring the oxygen in air

You can find the percentage of oxygen in air by passing a known volume of air over hot copper. The copper reacts with the oxygen to form black copper oxide: 2Cu(s) + O₂(g) → 2CuO(s). The gas left over is mostly nitrogen.

Worked example: percentage of oxygen

Volume of air at the start = 100 cm³. Volume after cooling = 79 cm³.
volume of oxygen used = 100 − 79 = 21 cm³
percentage of oxygen = 21 ÷ 100 × 100 = 21%

Answer: 21% oxygen

Exam tip: Let the apparatus cool to room temperature before reading the final volume. Hot gas takes up more space, so the reading would be too big.

Earth’s early atmosphere

Earth formed about 4.6 billion years ago. Nobody can sample that air, so scientists piece together indirect evidence: tiny bubbles of gas trapped inside very old rocks and minerals, the gases that volcanoes give out today, the atmospheres of other planets and moons in the Solar System, and computer models. One widely accepted theory is:

  • During the first billion years there was intense volcanic activity. Volcanoes released gases that formed the early atmosphere.
  • The early atmosphere was probably mainly carbon dioxide, with little or no oxygen. It was a bit like the atmospheres of Mars and Venus today.
  • Volcanoes also released water vapour and nitrogen, which gradually built up. There may also have been small amounts of methane and ammonia.
  • Venus; Main gases in the atmosphere today about 96% carbon dioxide, about 3.5% nitrogen
  • Mars; Main gases in the atmosphere today about 95% carbon dioxide, small amounts of nitrogen and argon
  • Earth; Main gases in the atmosphere today about 78% nitrogen, 21% oxygen, 0.04% carbon dioxide

How the oceans formed and removed carbon dioxide

As Earth cooled, the water vapour condensed to form liquid water, which collected as the oceans. Carbon dioxide is fairly soluble in water, so a lot of it dissolved in the oceans:

CO₂(g) + H₂O(l) ⇌ H₂CO₃(aq) (carbonic acid)

Dissolved carbon dioxide reacted to form carbonate compounds, such as calcium carbonate. These are insoluble, so they precipitated out and settled as sediments on the sea bed. This removed even more carbon dioxide from the atmosphere.

Evidence: Evidence about the early atmosphere is limited because the events happened about 4.6 billion years ago. There were no people to record them, and most of the oldest rocks have been worn away or changed. That is why scientists talk about theories.

Weighing up different theories

Because the evidence is indirect, scientists have suggested more than one explanation. Each one has to fit the evidence we have.

  • Volcanic gases; What it says Volcanoes released the gases (mainly CO₂, plus water vapour and nitrogen) that made the early atmosphere; the water vapour condensed to make the oceans.; Evidence that fits / problems Volcanoes today give out these gases; Mars and Venus (lots of past volcanic activity) have CO₂-rich air. The most widely accepted idea.
  • Comets; What it says Icy comets crashing into the young Earth melted and added water (and some gases).; Evidence that fits / problems Comets are known to contain ice, and the young Earth was hit by many objects. But scientists still debate how much of the water they brought.
  • Space debris; What it says Gases came partly from rocky and icy debris that vaporised when it struck Earth.; Evidence that fits / problems Fits the evidence that the young Earth was struck by a great deal of material from space. Harder to test; a minority view.

Exam tip: To evaluate a theory: say what evidence supports it, what it cannot explain, and whether the evidence is direct or indirect. Theories can be combined (e.g. volcanoes and comets supplied water) and may change when new evidence is found.

Looking ahead: Oxygen was not part of the early atmosphere. Simple life appeared roughly 3.4 billion years ago, and oxygen only started to build up after algae began photosynthesis (about 2.7 billion years ago). You will meet this in 13.2.

Common mistakes

  • Saying the early atmosphere was mainly nitrogen. The theory says it was mainly carbon dioxide; nitrogen built up gradually.
  • Saying oxygen came from volcanoes. There was little or no oxygen at first; it came later from photosynthesis (see 13.2).
  • Writing that the oceans formed when water vapour “evaporated”. It condensed as Earth cooled.
  • Giving air as “80% oxygen”. It is about 80% nitrogen and 20% oxygen.
  • Stating theories about the early atmosphere as certain facts. Say “scientists think” and explain why evidence is limited.

Revision questions

Try these, then check your answers below each one.

Question 1. Which row gives the approximate composition of dry air today? (Choose from: 80% oxygen, 20% nitrogen, 80% nitrogen, 20% oxygen, 50% nitrogen, 50% oxygen, 78% nitrogen, 21% carbon dioxide.)

Answer: 80% nitrogen, 20% oxygen

Working: Dry air is about four-fifths (80%) nitrogen and one-fifth (20%) oxygen, with small amounts of argon, carbon dioxide and water vapour.

Question 2. According to one theory, which gas made up most of Earth's early atmosphere? Give its name or formula.

Answer: carbon dioxide

Working: The early atmosphere was probably mainly carbon dioxide, with little or no oxygen, like Mars and Venus today.

Question 3. True or false? The oceans formed when water vapour released by volcanoes condensed as Earth cooled.

Answer: True

Working: True. As Earth cooled, water vapour condensed into liquid water, forming the oceans.

Related lessons

Continue in the full course

This lesson is section 13.1 (AGCCH1301) of our free GCSE Chemistry course for AQA. In the full course you get every video for this section, a self-marking practice quiz, a printable worksheet with a mark scheme, chapter tests, and your progress saved. Go to the full GCSE Chemistry course.

 

 

 

 

 

 

 

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