13.2 Our Evolving Atmosphere

A free GCSE Chemistry lesson for AQA (8462) from OBA School: where the oxygen came from, where the carbon dioxide went, and how air reached its present make-up. 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

• Explain how algae and plants increased the amount of oxygen, using a symbol equation for photosynthesis
• Describe the ways carbon dioxide was removed from the atmosphere
• Describe how coal, crude oil, natural gas and limestone formed and locked up carbon
• Explain why the percentage of nitrogen increased
• Carry out simple calculations using equations and relative formula masses

Key words

• Photosynthesis: The reaction in which algae and plants use light energy to turn carbon dioxide and water into glucose and oxygen.
• Algae: Simple organisms that live in water and carry out photosynthesis. They first produced oxygen about 2.7 billion years ago.
• Sedimentary rock: Rock formed from layers of sediment pressed together over millions of years, e.g. limestone.
• Fossil fuel: A fuel formed over millions of years from the remains of living things: coal, crude oil and natural gas.
• Carbon store: A place where carbon is locked away, e.g. in rocks, fossil fuels or the oceans.

How oxygen entered the atmosphere

There was little or no oxygen in the early atmosphere. About 2.7 billion years ago, algae began to produce oxygen by photosynthesis:

carbon dioxide + water → glucose + oxygen (light energy needed)

6CO₂(g) + 6H₂O(l) → C₆H₁₂O₆(aq) + 6O₂(g)

Over the next billion years, plants evolved and the percentage of oxygen gradually increased. Eventually there was enough oxygen for animals to evolve.

• The first simple life (bacteria-like microorganisms) probably appeared about 3.4 billion years ago. They got their energy by breaking down chemicals, not from sunlight, so they did not make oxygen.
• Algae in the seas then began photosynthesising. As oxygen built up, many of the early microorganisms that had evolved without oxygen could not survive it and largely died out.
• Plants spread over the land, adding even more oxygen. Animals, which cannot make their own food, depend on plants for food and on oxygen for respiration.

Worked example: mass of oxygen from photosynthesis (Higher tier)

A forest takes in 33 tonnes of carbon dioxide in a year. What mass of oxygen does it give out?
Mr: CO₂ = 12 + 2 × 16 = 44; O₂ = 2 × 16 = 32. The equation shows 6 CO₂ : 6 O₂, a 1 : 1 mole ratio.
So every 44 tonnes of CO₂ gives 32 tonnes of O₂. Mass of O₂ = 33 × 32 ÷ 44 = 24 tonnes

Answer: 24 tonnes of oxygen

How carbon dioxide was removed

The percentage of carbon dioxide fell from most of the atmosphere to about 0.04% today. There were three main reasons:

• Dissolving in the oceans and forming carbonate precipitates (see 13.1).
• Photosynthesis by algae and plants used up carbon dioxide.
• Formation of sedimentary rocks and fossil fuels that contain carbon locked carbon away underground.

Rocks and fossil fuels as carbon stores

• Limestone; How it formed Shells and skeletons of marine organisms are made of calcium carbonate (CaCO₃). When the organisms died they sank, built up as sediment and were compressed into limestone over millions of years.
• Coal; How it formed A sedimentary rock made from thick layers of plant material (e.g. ancient forests) that were buried and compressed before they could decay.
• Crude oil and natural gas; How it formed Formed from deposits of plankton (tiny sea organisms) buried in mud on the sea bed, then heated and compressed over millions of years without oxygen.

Worked example: percentage of carbon in limestone

Mr of CaCO₃ = 40 + 12 + 3 × 16 = 100
% carbon = 12 ÷ 100 × 100 = 12%

Answer: Limestone is 12% carbon by mass

Key fact: Burning fossil fuels today releases carbon that has been locked away for millions of years. This is why it adds extra carbon dioxide to the atmosphere (see 13.3).

Practical: is there carbonate in a shell?

If shells were made from calcium carbonate, they should react with dilute acid like any carbonate, giving off carbon dioxide. You can test this in the lab.

• Put a spatula of crushed shells in a test tube (crushing gives a bigger surface area, so the reaction is faster).
• Add dilute hydrochloric acid and quickly fit the bung and delivery tube. Look for fizzing (effervescence).
• Bubble the gas through limewater (calcium hydroxide solution). If it turns milky (cloudy), the gas is carbon dioxide, so the shells contain carbonate.

CaCO₃(s) + 2HCl(aq) → CaCl₂(aq) + H₂O(l) + CO₂(g)

Limewater test: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l) (the solid makes it cloudy)

Safety and limits: Wear eye protection: the acid is an irritant. The test only shows that a carbonate is present. It does not show which carbonate, or how much of the shell is carbonate.

Nitrogen, methane and ammonia

Nitrogen came from volcanoes. It is very unreactive, so once it was in the atmosphere it stayed there and its percentage rose as carbon dioxide was removed. Any methane and ammonia in the early atmosphere reacted with the new oxygen:

CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l)

4NH₃(g) + 3O₂(g) → 2N₂(g) + 6H₂O(l) (this added more nitrogen)

By about 200 million years ago the proportions of gases had settled to roughly what they are today. Since then, carbon has kept moving between the air, oceans, living things and rocks in the natural carbon cycle, which kept the carbon dioxide level fairly steady until humans began burning fossil fuels on a large scale.

• nitrogen, N₂; Approximate percentage 78% (about four-fifths)
• oxygen, O₂; Approximate percentage 21% (about one-fifth)
• argon, Ar (a noble gas); Approximate percentage 0.9%
• carbon dioxide, CO₂; Approximate percentage 0.04%
• other noble gases (neon, helium, krypton, xenon); Approximate percentage tiny traces, well under 0.1% in total
• water vapour (not in dry air); Approximate percentage varies from place to place and day to day

Exam tip: For a 6-mark “how did the atmosphere change” question, cover both gases: oxygen up (algae, then plants, photosynthesis) and carbon dioxide down (photosynthesis, dissolving in oceans, locked in sedimentary rocks and fossil fuels). Use the photosynthesis equation.

Common mistakes

• Saying plants made the first oxygen. Algae started it about 2.7 billion years ago; plants came later.
• Writing the photosynthesis equation the wrong way round. CO₂ and H₂O go in; glucose and O₂ come out.
• Thinking fossil fuels removed carbon dioxide by “absorbing” it. The carbon was taken in by living things, which were then buried before they decayed.
• Saying limestone formed from volcanic rock. It is a sedimentary rock from shells and skeletons.

Revision questions

Try these, then check your answers below each one.

Question 1. About how long ago did algae first start to produce oxygen? (Choose from: 4.6 billion years ago, 2.7 billion years ago, 200 million years ago, 10 000 years ago.)

Answer: 2.7 billion years ago

Working: Algae began photosynthesising about 2.7 billion years ago. Over the next billion years plants evolved and oxygen gradually increased.

Question 2. (Higher tier) Photosynthesis uses CO₂ and makes O₂ in a 1 : 1 mole ratio. What mass of oxygen, in tonnes, is made when a forest takes in 22 tonnes of carbon dioxide? (Mr: CO₂ = 44, O₂ = 32)

Answer: 16 tonnes

Working: 22 × 32/44 = 16 tonnes of oxygen.

Question 3. Calculate the percentage by mass of carbon in calcium carbonate, CaCO₃. (Ar: Ca = 40, C = 12, O = 16)

Answer: 12 %

Working: Mr = 100. % carbon = 12 ÷ 100 × 100 = 12%.

Related lessons

• Previous lesson: 13.1 History of Our Atmosphere
• Next lesson: 13.3–13.4 Greenhouse Gases and Global Climate Change
• In this chapter: 13.5 Atmospheric Pollutants

Continue in the full course

This lesson is section 13.2 (AGCCH1302) 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.