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Detailed notes on Air and Water for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
CO₂ and CH₄ are the main greenhouse gases contributing to global warming. Cambridge tests their sources, properties, reactions, and the consequences of rising atmospheric concentrations.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
CO₂ and CH₄ are the major greenhouse gases — their rising atmospheric concentrations are driving global warming.
Carbon dioxide (CO₂):
Methane (CH₄):
The greenhouse effect:
Consequences: Sea level rise (ice melt + thermal expansion), extreme weather (floods, droughts), habitat loss, ocean acidification, agricultural disruption.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'State TWO human activities that increase the concentration of CO₂ in the atmosphere' (2 marks — burning fossil fuels, deforestation). 'Explain how CO₂ contributes to the greenhouse effect' (3 marks — CO₂ absorbs infrared/heat radiation from Earth, re-radiates in all directions, some returns to Earth surface, temperature rises). 'State ONE reason why methane is considered a more serious greenhouse gas than carbon dioxide' (1 mark — more potent/absorbs more infrared per molecule).
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C11); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on carbon dioxide and methane, written exactly the way a tutor would explain them at the board.
Question
Explain why increasing concentration of CO₂ in the atmosphere leads to global warming.
Step-by-step solution
Step 1
Short-wave radiation from the Sun passes through the atmosphere and warms the Earth's surface.
Step 2
The Earth's surface re-radiates energy as longer-wave infrared radiation.
Step 3
CO₂ (and other greenhouse gases) absorb this infrared radiation and re-radiate it in all directions, including back towards Earth.
Step 4
Higher concentrations of CO₂ mean more infrared is absorbed and returned to Earth; less energy escapes to space.
Step 5
The Earth's average surface temperature increases → global warming.
Answer
CO₂ absorbs infrared radiation from Earth's surface; re-radiates it back to Earth; higher CO₂ concentration → less heat escapes to space → global warming.
Examiner tip
Must state that CO₂ absorbs INFRARED radiation (not sunlight/UV). Say 'from Earth's surface' to show the radiation source.
Question
State TWO human activities that have increased the concentration of CO₂ in the atmosphere.
Step-by-step solution
Step 1
Activity 1: burning fossil fuels (coal, oil, natural gas) for energy/transport; releases CO₂ from carbon stored millions of years ago.
CH4+2O2→CO2+2H2O
Step 2
Activity 2: deforestation — trees absorb CO₂ in photosynthesis; removing them reduces the sink for CO₂ and releases stored carbon when wood is burned or decays.
Answer
Burning fossil fuels; deforestation (also: burning wood/biomass, cement manufacture from CaCO₃ → CaO + CO₂).
Question
Describe the test for carbon dioxide and explain the chemistry that produces the observed result.
Step-by-step solution
Step 1
Bubble the gas through limewater (calcium hydroxide solution, Ca(OH)₂).
Step 2
If CO₂ is present, the limewater turns milky/white (a white precipitate forms).
Step 3
The precipitate is calcium carbonate (CaCO₃), which is insoluble.
CO2+Ca(OH)2→CaCO3(s)+H2O
Step 4
If excess CO₂ is bubbled through, the precipitate dissolves as the slightly soluble calcium hydrogencarbonate forms: CaCO₃ + CO₂ + H₂O → Ca(HCO₃)₂ (solution becomes clear again).
Answer
Bubble through limewater; turns milky (white precipitate of CaCO₃); equation: CO2+Ca(OH)2→CaCO3+H2O.
The formulae you need to memorise for carbon dioxide and methane on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.
CH4+2O2→CO2+2H2O
When to use
Writing combustion equations for natural gas (methane).
Definitions to memorise and the exact keywords mark schemes credit for carbon dioxide and methane answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The process by which greenhouse gases (CO₂, CH₄, H₂O vapour) in the atmosphere absorb outgoing infrared radiation from Earth's surface and re-radiate it back, warming the Earth. Essential for life.
The intensification of the natural greenhouse effect due to increased concentrations of greenhouse gases (especially CO₂ and CH₄) from human activities; leads to global warming and climate change.
The total amount of CO₂ (and CO₂-equivalent greenhouse gases) emitted directly and indirectly by an individual, organisation, event, or product.
Natural gas extraction, landfill sites (decomposition of organic waste), livestock (cattle/sheep produce CH₄ during digestion), rice paddies (anaerobic decomposition of organic matter).
Rising sea levels (thermal expansion + melting ice caps); more extreme weather events; habitat loss (Arctic/coral reef); changes in crop yields; increased spread of tropical diseases.
The traps other students keep falling into on carbon dioxide and methane questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Students confuse the ozone hole (UV) with the greenhouse effect (infrared).
How to avoid it
Greenhouse effect: CO₂ absorbs INFRARED radiation from Earth's surface. Ozone depletion: O₃ is reduced, allowing MORE UV to reach Earth's surface. These are separate environmental issues.
Why it happens
Students know the test but not the further reaction with excess CO₂.
How to avoid it
Excess CO₂ + CaCO₃ + H₂O → Ca(HCO₃)₂ (soluble); solution goes clear again. Mention this if asked about 'excess' CO₂ or 're-dissolving' observations.
Why it happens
Students focus on the combustion of wood.
How to avoid it
Deforestation increases CO₂ by: (1) releasing CO₂ when trees are burned or decay; AND (2) reducing photosynthesis (fewer trees to absorb CO₂). Both points score marks.
The things students keep getting wrong in this sub-topic, answered.