Chemistry of the atmosphere — AQA GCSE Chemistry revision

How the early atmosphere formed

• Around 4.6 billion years ago volcanic activity released huge amounts of gas, forming an early atmosphere thought to be mostly carbon dioxide with little or no oxygen.
• Intense volcanic activity also released water vapour, which condensed as the Earth cooled to form the early oceans.
• Nitrogen built up gradually as ammonia reacted with oxygen, and today makes up about four-fifths of the atmosphere.
• Because evidence from billions of years ago is limited, scientists' models of the early atmosphere involve a good deal of uncertainty.

How oxygen levels increased

• Early oceans allowed algae and later plants to evolve; these organisms used photosynthesis, taking in carbon dioxide and releasing oxygen.
• Photosynthesis gradually removed carbon dioxide from the air and increased oxygen levels to around the one-fifth seen today.
• Dissolved carbon dioxide combined with calcium ions in seawater to form insoluble carbonates, which settled and became sedimentary rock, locking away carbon.
• Some carbon became locked in fossil fuels formed from the buried remains of ancient plants and sea organisms over millions of years.

Key terms

Greenhouse effect

Natural warming caused by gases in the atmosphere absorbing and re-emitting infrared radiation from Earth's surface.

Carbon footprint

Total greenhouse gas emissions associated with a product, service or activity over its whole life cycle.

Photosynthesis

Process where plants and algae use carbon dioxide and water plus light energy to make glucose and oxygen.

Global dimming

Reduction in sunlight reaching Earth's surface due to particulates reflecting or scattering light.

Incomplete combustion

Burning a fuel with insufficient oxygen, producing carbon monoxide and/or soot instead of only carbon dioxide and water.

Acid rain

Rain made acidic by dissolved sulfur dioxide and nitrogen oxides from burning fossil fuels.

Common exam traps

• Don't say the early atmosphere was 'mostly oxygen' — it was mostly carbon dioxide with little or no oxygen.
• Photosynthesis increased oxygen and decreased carbon dioxide; don't reverse this.
• Carbon monoxide is colourless and odourless — you cannot detect it by smell, which is what makes it dangerous.

Practice questions with answers

1. 1. What was the early Earth's atmosphere thought to be mostly composed of?

   • • Oxygen
   • • Carbon dioxide
   • • Nitrogen
   • • Methane

   Answer: Carbon dioxide

   Volcanic activity released gases forming an early atmosphere thought to be mostly carbon dioxide, similar to Mars and Venus today.

2. 2. Name the process that removed carbon dioxide and produced oxygen in the early atmosphere.

   Answer: Photosynthesis

   Algae and later plants used photosynthesis, absorbing CO2 and releasing O2 over billions of years.

3. 3. Which gas makes up roughly four-fifths of today's atmosphere?

   • • Oxygen
   • • Carbon dioxide
   • • Nitrogen
   • • Argon

   Answer: Nitrogen

   Nitrogen built up gradually and now makes up about 80% of the atmosphere; oxygen is about 20%.

4. 4. How did carbon dioxide become locked into sedimentary rocks?

   Answer: It dissolved in oceans and reacted with calcium to form insoluble carbonates that settled as sediment.

   Dissolved CO2 combined with calcium ions to form carbonate precipitates, later compacted into rock like limestone.

5. 5. Which of these is NOT a greenhouse gas?

   • • Carbon dioxide
   • • Methane
   • • Water vapour
   • • Nitrogen

   Answer: Nitrogen

   Nitrogen does not absorb infrared radiation the way CO2, methane and water vapour do.

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