Clean, dry air — 78% nitrogen, 21% oxygen
Nitrogen is approximately 78%, oxygen approximately 21%, and the remaining ~1% is a mixture of noble gases (mainly argon) and carbon dioxide. Learn the syllabus figures themselves, not a rough impression of them. Carbon dioxide is a genuine natural component of clean air — it only becomes a concern when human activity pushes its level above that background. Carbon monoxide, particulates, oxides of nitrogen and sulfur dioxide are never part of clean air.
Pollutant sources — five fixed pairs
Carbon dioxide: complete combustion of carbon-containing fuels. Carbon monoxide and particulates: incomplete combustion of the same fuels (limited oxygen). Methane: decomposition of vegetation (landfill, wetlands) and waste gases from digestion in animals. Oxides of nitrogen: car engines. Sulfur dioxide: combustion of fossil fuels containing sulfur compounds. Complete combustion gives carbon dioxide and water; incomplete gives carbon monoxide and carbon.
Adverse effects and reduction strategies
Carbon dioxide and methane raise global warming, leading to climate change; carbon monoxide is toxic; particulates cause respiratory problems and cancer; oxides of nitrogen give acid rain and respiratory problems; sulfur dioxide gives acid rain. Strategies: plant trees (photosynthesis absorbs carbon dioxide); reduce livestock farming (less methane from digestion); burn less fossil fuel; increase hydrogen and renewables (wind, solar), releasing no carbon dioxide.
Drawn from real examiner reports.
78 and 21 — no rounding, no swapping
Mark schemes want the syllabus figures: nitrogen about 78%, oxygen about 21%. Rounded guesses such as 80 and 20, and answers that reverse the two gases so oxygen becomes the majority gas, are not credited. The remainder, roughly 1%, must be described as a mixture of noble gases and carbon dioxide rather than left out of the answer altogether.
Flagged Jun 2022 P42 Q2a; Jun 2023 P32 Q5a
Pollutants are not part of clean air
Nitrogen dioxide, sulfur dioxide, carbon monoxide and particulates are added to the atmosphere by human activity, so none of them belongs in an answer about the composition of clean air. Carbon dioxide is the exception: it genuinely is present in clean air, and examiners class it as a greenhouse gas rather than as an air pollutant.
Flagged Nov 2023 P12 Q24; P11 Q25
Carbon dioxide harms climate, not lungs
Carbon dioxide and methane have one listed adverse effect: higher levels leading to increased global warming, which leads to climate change. Answers such as it is poisonous to breathe, or it causes acid rain, describe other pollutants: toxicity belongs to carbon monoxide, respiratory and cancer risk to particulates, and acid rain to sulfur dioxide and oxides of nitrogen.
Sulfur dioxide: its source is not acid rain
Asked where sulfur dioxide comes from, candidates commonly write acid rain — but that is its effect, not its source. The source is the combustion of fossil fuels which contain sulfur compounds. Source and effect are separate marking points, so an effect can never earn a source mark, however true the statement is in itself.
Flagged Nov 2023 P31 Q2bi/bii
Carbon monoxide is toxic, not just bad
The required word for carbon monoxide is toxic or poisonous. Vague wording such as it makes breathing hard, or it causes health problems, does not gain the mark. Note also that carbon monoxide and particulates come from incomplete combustion — a limited oxygen supply — not from complete combustion, which gives carbon dioxide and water.
Flagged Jun 2023 P31 Q8cii; Nov 2023 P41 Q2d
Global warming is not climate change
Global warming is the rise in the average temperature of the Earth's atmosphere caused by increased greenhouse gas levels. Climate change is the wider set of consequences that follows — changed weather patterns, rising sea levels. In an explain answer, put global warming as the cause and climate change as its result; the two terms are not interchangeable.
(Extended) Low-sulfur fuel vs desulfurisation
These are two separate strategies, not one. Low-sulfur fuels act before combustion: sulfur compounds are removed from the fuel, so less sulfur dioxide ever forms. Flue gas desulfurisation acts after combustion: sulfur dioxide is removed from the flue gases by calcium oxide, . Asked for two, give one of each.
Answer the part actually asked
Source means where the pollutant comes from; effect means the harm it causes; strategy means the action taken to reduce that harm. Each is a separate marking point. A scientifically true statement filed under the wrong one of the three earns nothing.
(Extended) Keep both 2s in the converter equation
Learn exactly — dropping either 2 loses the balancing mark. Say that a hot catalyst is needed. Oxides of nitrogen form from nitrogen and oxygen in the air at the engine's high temperature, not from the fuel.
(Extended) Three steps in a greenhouse answer
A three-mark describe needs three linked steps: the gases absorb thermal energy radiated from the Earth's surface; they re-emit some of it in all directions, including downwards; this reduces the thermal energy lost to space. Trap heat alone scores nothing.
State and explain means two things
For a reduction strategy, the action and its mechanism are separate marks. Plant trees on its own is half an answer; add that trees absorb carbon dioxide during photosynthesis. Count your points against the mark tariff before moving on.
Cambridge 0654 spec reference: Section C10 "Chemistry of the environment", sub-topic C10.2 "Air quality and climate". Core: composition of clean, dry air; sources of five named air pollutants; adverse effects of those pollutants; strategies to reduce the effects of climate change. Supplement (Extended): strategies to reduce acid rain; how CO2 and methane cause global warming; how oxides of nitrogen form in car engines and are removed by catalytic converters.
| Term | Mark-scheme-precise meaning |
|---|---|
| Clean, dry air | Air with the natural composition of approximately 78% nitrogen, 21% oxygen, and a remainder of noble gases and carbon dioxide -- no added pollutants |
| Air pollutant | A substance released into the atmosphere (usually by human activity) that is harmful to health, other organisms or the environment |
| Global warming | A rise in the average temperature of the Earth's atmosphere, caused by increased levels of greenhouse gases such as carbon dioxide and methane |
| Climate change | The broader, longer-term changes in the Earth's climate (e.g. weather patterns, sea levels) that result from global warming |
| Greenhouse gas | A gas (e.g. carbon dioxide, methane) that absorbs and re-emits thermal energy radiated from the Earth's surface, reducing the amount lost to space |
| Acid rain | Rain that is more acidic than normal, caused by pollutant gases (sulfur dioxide and oxides of nitrogen) dissolving in atmospheric water to form acids |
| Flue gas desulfurisation | A process that removes sulfur dioxide from waste (flue) gases before they are released, e.g. by reacting them with calcium oxide |
| Complete combustion | Burning of a carbon-containing fuel in plenty of oxygen, producing carbon dioxide (and water) |
| Incomplete combustion | Burning of a carbon-containing fuel with a limited oxygen supply, producing carbon monoxide and/or solid carbon particulates |
Define a greenhouse gas.
Methane, CH4, is released into the atmosphere from the decomposition of vegetation and from animal digestion.
Calculate the relative formula mass (Mr) of CH4. (Ar: C = 12, H = 1) (2 marks)
