Greenhouse Gas – IGCSE Chemistry Definition
IGCSE Chemistry definition of greenhouse gas: a gas that absorbs and re-emits infrared radiation. Covers CO₂, methane, water vapour, and exam tips for 0620.
Published by IGCSEChemistry.com.my
Chemistry teaching team: K. S. Tan (15+ years teaching IGCSE Chemistry) and Ms Yash (10+ years teaching IGCSE Chemistry) and Ms Kartini (15+ years teaching IGCSE Chemistry).
Mapped to Cambridge IGCSE Chemistry 0620 (2026–2028). Last updated 2026-08-19.
Greenhouse gases connect combustion chemistry, atmospheric chemistry, and environmental impact. Paper 2 and Paper 4 test both the identification of greenhouse gases and the mechanism by which they cause warming. Getting the mechanism right — infrared absorption and re-emission — is essential.
The 0620 definition
A greenhouse gas is a gas in the atmosphere that absorbs infrared radiation (heat energy) emitted by the Earth’s surface and re-emits it in all directions, including back toward the surface.
The three main greenhouse gases
| Gas | Formula | Sources |
|---|---|---|
| Carbon dioxide | CO₂ | Burning fossil fuels, deforestation, respiration |
| Methane | CH₄ | Agriculture (cattle, rice paddies), landfill, natural gas |
| Water vapour | H₂O | Evaporation (natural water cycle) |
Why their concentrations are increasing
- CO₂: burning coal, oil, and natural gas for energy; deforestation removes trees that absorb CO₂ through photosynthesis
- CH₄: increased livestock farming, more landfill waste, extraction of natural gas
Water vapour concentration increases indirectly — as temperatures rise, more water evaporates, adding to the greenhouse effect (a positive feedback loop).
The mechanism
- Short-wavelength radiation from the Sun passes through the atmosphere and warms the Earth’s surface
- The warm surface emits longer-wavelength infrared radiation
- Greenhouse gases absorb this infrared radiation
- The gases re-emit the radiation in all directions, including back toward the Earth
- This traps energy in the atmosphere, raising the temperature
Worked exam question
Carbon dioxide is a greenhouse gas. (a) State one human activity that increases the amount of CO₂ in the atmosphere. [1] (b) Explain how greenhouse gases cause the temperature of the atmosphere to increase. [2]
Mark scheme
(a) Burning fossil fuels / deforestation [1]
(b) Greenhouse gases absorb infrared radiation / heat emitted by the Earth [1]; and re-emit / re-radiate it back toward the Earth’s surface / in all directions [1]
Common exam mistakes
- Saying greenhouse gases “trap heat” without explaining how — the mark requires “absorb infrared radiation” and “re-emit.”
- Confusing the greenhouse effect with the ozone layer — they are different phenomena. The ozone layer absorbs ultraviolet radiation; greenhouse gases absorb infrared radiation.
- Listing oxygen or nitrogen as greenhouse gases — O₂ and N₂ do not absorb infrared radiation significantly.
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