4CH1

Gases in the Atmosphere

Inorganic Chemistry · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 5% of your exam marks.

stable
Rare
Stable5%

Air composition, greenhouse effect and climate implications appear in most series.

How the natural greenhouse effect works

  • Short-wavelength radiation from the Sun (visible and ultraviolet) passes through the atmosphere and warms the Earth's surface
  • The warmed surface re-emits energy as infrared radiation with a longer wavelength
  • Some of this outgoing infrared is absorbed by greenhouse gases in the atmosphere and re-radiated in all directions, including back down toward the surface
  • The trapped energy keeps the average surface temperature at about +15 °C — without it, the planet would freeze
Diagram of the greenhouse effect: the Sun's rays pass through the atmosphere and warm the Earth, which re-emits infrared radiation, some of which escapes to space while some is absorbed by carbon dioxide and re-radiated back toward the surface
Source: Greenhouse effect by Save My Exams

The main greenhouse gases

  • Carbon dioxide, CO2
  • Methane, CH4
  • Water vapour, H2O

Sources of atmospheric carbon dioxide

  • Burning fossil fuels (coal, oil, natural gas) and wood
  • Heating limestone and other carbonate rocks during cement manufacture, releasing CO2 by thermal decomposition
  • Natural weathering, where acids react with carbonate rocks
  • Respiration: plants and animals exhale CO2 as a metabolic by-product

Enhanced greenhouse effect

  • Human activity has raised CO2 and CH4 concentrations above their long-term natural values
  • More greenhouse gas means more outgoing infrared is trapped, so the average surface temperature drifts upward — the enhanced
  • This is the basis of the link between greenhouse gas emissions and global warming and
Exam tip

Explaining why rising CO₂ levels are an environmental problem

What comes up: a 2-mark question asking you to explain the environmental problem caused by an increasing concentration of CO₂ in the atmosphere.

Write (two marks): (1) Carbon dioxide is a greenhouse gas — it traps infrared radiation in the atmosphere (or: absorbs outgoing infrared and re-radiates it back toward the surface). (2) This causes global warming / climate change / a rise in global temperature. Specific consequences such as melting polar ice caps, flooding, or wildfires are also credited for the second mark.

Watch out: the mark scheme rejects any reference to the ozone layer — that is a separate issue from the greenhouse effect. Acid rain is ignored (not credited). You need to link the greenhouse mechanism (M1) to a named consequence (M2) to collect both marks.