Chemistry of the atmosphere — GCSE Chemistry Revision
Everything you need to revise chemistry of the atmosphere for GCSE Chemistry: clear notes, the key facts and terms to learn, the mistakes that cost students marks, and practice questions with answers.
Revision notes
Composition of the Earth's Early Atmosphere
Scientists believe the Earth's early atmosphere was very different from today's. It was likely formed from volcanic eruptions, releasing large amounts of carbon dioxide, water vapour, methane, and ammonia. There would have been little to no oxygen. As the Earth cooled, water vapour condensed to form oceans. Carbon dioxide dissolved in these oceans and reacted to form carbonate rocks, reducing its atmospheric concentration. This set the stage for the evolution of life.
Changes to the Atmosphere: Role of Algae and Plants
Over billions of years, primitive algae and, later, plants evolved. These organisms began to photosynthesise, a process that uses carbon dioxide and water to produce glucose and oxygen. This crucial biological activity dramatically increased the oxygen content in the atmosphere and simultaneously decreased the carbon dioxide levels. This change was essential for the evolution of more complex life forms that respire aerobically, leading to the atmosphere we know today.
Greenhouse Gases and the Greenhouse Effect
Greenhouse gases, such as carbon dioxide, methane, and water vapour, absorb and re-emit long-wavelength infrared radiation from the Earth's surface. This process warms the Earth's atmosphere, known as the greenhouse effect. Without it, the Earth would be too cold to support life. However, increased concentrations of these gases due to human activity are enhancing this natural effect, leading to global warming and climate change.
Human Activities and Climate Change
Human activities are significantly altering the composition of the atmosphere. The burning of fossil fuels (coal, oil, gas) for energy releases vast amounts of carbon dioxide. Deforestation reduces the amount of CO2 absorbed by plants. Agriculture (e.g., cattle farming, rice paddies) produces methane. These increased greenhouse gas concentrations lead to an enhanced greenhouse effect, causing global warming and associated climate change impacts like rising sea levels and extreme weather.
Key facts
- Earth's early atmosphere mainly CO2, H2O vapour, N2.|Algae and plants increased O2 through photosynthesis.|Today's atmosphere is ~78% N2, 21% O2, <1% Ar, 0.04% CO2.|Greenhouse gases absorb infrared radiation, warming Earth.|Burning fossil fuels increases CO2, enhancing greenhouse effect.|Consequences of climate change include sea level rise and extreme weather.
Key terms
- Photosynthesis::The process by which plants and algae use carbon dioxide, water, and sunlight to create glucose and oxygen.|Greenhouse effect::The natural process where certain gases in Earth's atmosphere trap heat, warming the planet.|Global warming::The long-term heating of Earth's climate system observed since the pre-industrial period due to human activities, primarily fossil fuel burning.|Fossil fuels::Natural fuels such as coal, oil, and natural gas, formed from the remains of living organisms over millions of years.|Combustion::A high-temperature exothermic redox chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidised gaseous products.|Climate change::Long-term shifts in temperatures and weather patterns, largely driven by human activities like the burning of fossil fuels.
Common mistakes
- Confusing the greenhouse effect (natural and essential) with the enhanced greenhouse effect (caused by humans and problematic).|Believing the early atmosphere had significant oxygen before photosynthesis evolved.|Incorrectly stating that nitrogen was a major component of the early atmosphere, rather than building up later.|Attributing all atmospheric changes solely to human activity, ignoring natural processes like volcanic outgassing.
Exam tips
- Be able to describe the formation and changes of the Earth's early atmosphere, linking it to volcanic activity and the evolution of life.|Understand the role of algae and plants in decreasing CO2 and increasing O2 through photosynthesis.|Know the major greenhouse gases and explain how they contribute to the greenhouse effect.|Describe human activities that increase greenhouse gas concentrations and the potential consequences of climate change.
Quick quiz
1. Which gas made up the largest proportion of Earth's early atmosphere?
- Oxygen
- Nitrogen
- Carbon dioxide
- Methane
Show answer
Carbon dioxide — Scientists believe the Earth's early atmosphere was primarily composed of carbon dioxide, released from intense volcanic activity, along with water vapour.
2. Which process was responsible for the increase in oxygen levels in the Earth's atmosphere?
- Respiration
- Combustion
- Photosynthesis
- Volcanic outgassing
Show answer
Photosynthesis — Algae and early plants evolved to carry out photosynthesis, a process that consumes carbon dioxide and releases oxygen, gradually increasing atmospheric oxygen levels.
3. Approximately what percentage of the Earth's current atmosphere is nitrogen?
- 21%
- 0.04%
- 78%
- 1%
Show answer
78% — The Earth's current atmosphere is roughly 78% nitrogen, 21% oxygen, and about 1% other gases like argon and carbon dioxide.
4. Which of the following is NOT a greenhouse gas?
- Carbon dioxide
- Methane
- Nitrogen
- Water vapour
Show answer
Nitrogen — Carbon dioxide, methane, and water vapour are all greenhouse gases because they absorb and re-emit infrared radiation. Nitrogen does not.
5. What is the primary human activity that contributes to increased carbon dioxide levels in the atmosphere?
- Deforestation
- Volcanic eruptions
- Burning of fossil fuels
- Respiration
Show answer
Burning of fossil fuels — The combustion of fossil fuels (coal, oil, gas) for energy is the largest single contributor to increased atmospheric carbon dioxide from human activities.
Exam-style questions
Describe how the Earth's early atmosphere formed and changed over time to become the atmosphere we have today. Refer to specific gases and processes. [4 marks]
Show mark scheme
- Volcanic activity released gases (CO2, H2O vapour, methane, ammonia).
- As Earth cooled, water vapour condensed to form oceans.
- CO2 dissolved in oceans and formed carbonate rocks / sedimentary rocks.
- Algae and plants evolved and carried out photosynthesis, absorbing CO2 and releasing O2.
Explain the enhanced greenhouse effect and its link to human activities. Give two examples of these activities. [3 marks]
Show mark scheme
- Human activities increase concentration of greenhouse gases (e.g., CO2, methane).
- This traps more long-wavelength infrared radiation / heat, causing global warming.
- Example 1: Burning fossil fuels (for energy/transport) releases CO2.
- Example 2: Deforestation reduces CO2 absorption / Agriculture (cattle/rice) releases methane.
Compare the composition of the Earth's early atmosphere with its current atmosphere, identifying the key differences in major gas proportions. [3 marks]
Show mark scheme
- Early atmosphere: high proportion of carbon dioxide / water vapour / methane / ammonia.
- Current atmosphere: high proportion of nitrogen (~78%) and oxygen (~21%).
- Early atmosphere: very little or no oxygen; Current atmosphere: significant oxygen.
- Early atmosphere: very high CO2; Current atmosphere: very low CO2 (approx 0.04%).
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