C9 is the most descriptive topic in chemistry — there are almost no calculations, but a great deal to describe and explain precisely. It runs in three acts: how the atmosphere evolved over 4.6 billion years from volcanic carbon dioxide to today’s nitrogen and oxygen; how greenhouse gases warm the planet and how human activity is enhancing that effect; and how burning fuels pollutes the air. The marks live in the detail: the right gas, the right process, and a clear cause-and-effect chain.
- Everyone — the whole of C9 (4.9) is on both Combined Science (Trilogy) and Triple Chemistry papers. There is no Triple-only or Higher-only content, and no required practical.
- The skills examined are mostly extended writing: describe and explain the changes in the atmosphere, describe the greenhouse effect using short- and long-wavelength radiation, and evaluate evidence and actions on climate change — including the 6-mark levels-of-response questions this topic is famous for.
One distinction underpins the whole topic and earns marks throughout: a greenhouse gas (water vapour, CO2, methane) warms the planet; an atmospheric pollutant (CO, SO2, NOx, particulates) is something not found in clean air that causes problems near the ground. Don’t mix them up.
1Today’s Atmosphere
For about the last 200 million years the proportions of gases in the atmosphere have been roughly what they are now. Two gases dominate, and a thin sliver of everything else does the rest of the chemistry — including the small amount of carbon dioxide that matters far more than its share suggests.
Clean dry air is about four-fifths nitrogen and one-fifth oxygen, with roughly 1% other gases — carbon dioxide, water vapour and the noble gases.
- Nitrogen — about 80% (four-fifths).
- Oxygen — about 20% (one-fifth).
- Small proportions of other gases — carbon dioxide, water vapour and the noble gases (Group 0).
For the recent past — the last few hundred thousand years — scientists read the composition from air bubbles trapped in ice cores: each bubble is a sealed sample of the atmosphere from when that ice formed, and the same record underpins the carbon-dioxide graph in section 5. It does not reach back to the early atmosphere — for that, the evidence is far more limited (section 2).
Carbon dioxide is well under 1% of the air, but it is a powerful greenhouse gas (section 4). A tiny proportion can still have a large effect — so “it’s only a small percentage” is never a good argument in this topic.
🧪 Exam-style questions
Approximately what percentage of today’s atmosphere is nitrogen? Tick (✓) one box.
Which gas in the atmosphere is used up when hydrocarbons burn? Tick (✓) one box.
How do scientists know the composition of the atmosphere from thousands of years ago? Tick (✓) one box.
Dry air is about 78% nitrogen and 21% oxygen. Calculate the percentage made up of all the other gases.
Show answer
100% − 78% − 21% = 1%. 1 mark
That ~1% is mostly argon, with small amounts of carbon dioxide, water vapour and other noble gases.
2The Early Atmosphere & the Oceans
The Earth is about 4.6 billion years old, and for its first billion years its atmosphere was utterly different from today’s. Because it was so long ago, the evidence is limited — which is why there are several theories and why exam questions ask you to evaluate them rather than state a single fact.
Drag the slider — or press play — to watch the air change over 4.6 billion years. The volcanic theory below names each stage.
- For the Earth’s first billion years, intense volcanic activity released gases that formed the early atmosphere.
- The early atmosphere was mainly carbon dioxide, with water vapour and little or no oxygen — thought to be like the atmospheres of Mars and Venus today.
- Volcanoes also released nitrogen, which gradually built up, and there may have been small amounts of methane and ammonia.
- As the Earth cooled, the water vapour condensed and fell, forming the oceans.
- Carbon dioxide dissolved in the new oceans, and carbonates were precipitated as sediments — reducing the CO2 in the atmosphere.
That is the volcanic theory — how the early atmosphere and oceans formed. What changed the air next is a separate stage (and not really a “theory”): from about 2.7 billion years ago, algae and then plants used photosynthesis to add oxygen and remove carbon dioxide — the rest of the animation above, covered in section 3.
The great swap: carbon dioxide (red) falls as it dissolves in the oceans, forms carbonate sediments and limestone, and is taken up by photosynthesis; oxygen (blue) climbs once algae and plants appear, levelling off at today’s ~20% — the proportions have then stayed about constant for the last 200 million years.
Theories about the early atmosphere have changed over time as evidence and technology improve. Because it was 4.6 billion years ago, direct evidence is scarce, so models rely on clues like the gases trapped in ancient minerals and comparisons with Mars and Venus. In an exam you may be given an alternative theory and asked to evaluate it — weigh what the evidence does and doesn’t support, rather than declaring one theory simply “right”.
- Saying the early atmosphere had oxygen. It had little or no oxygen — oxygen came later, from life.
- Saying volcanoes released oxygen. Volcanoes released CO2, water vapour and nitrogen (and possibly small amounts of methane and ammonia) — not O2.
- Forgetting the oceans’ role. Water vapour condensed to form oceans; CO2 then dissolved in them and formed carbonate sediments — a major way CO2 was removed.
🧪 Exam-style questions
What was the main gas in the Earth’s early atmosphere? Tick (✓) one box.
Where did the gases in the Earth’s early atmosphere mainly come from? Tick (✓) one box.
Billions of years ago there was no liquid water on the Earth’s surface. Suggest why. Tick (✓) one box.
Why are there several different theories about the Earth’s early atmosphere? Tick (✓) one box.
3Rising Oxygen, Falling Carbon Dioxide
Two changes turned the early CO2 atmosphere into the one we breathe: oxygen appeared, and carbon dioxide was removed. Both come back to the same cause — life, especially photosynthesis.
How oxygen increased
Algae and plants produced the oxygen now in the atmosphere, by photosynthesis. Algae first produced oxygen about 2.7 billion years ago, and soon after, oxygen began to build up in the atmosphere. Over the next billion years plants evolved and spread, the percentage of oxygen gradually increased, and eventually it reached a level that allowed animals to evolve.
6CO2 + 6H2O → C6H12O6 + 6O2
(carbon dioxide + water → glucose + oxygen, using light). This single reaction does both jobs: it removes CO2 and releases O2, and the equation must balance.
How carbon dioxide decreased
Carbon dioxide came out of the atmosphere by several routes — and much of that carbon is still locked away today:
- Dissolving in the oceans. CO2 is soluble; it dissolved in the new oceans and carbonates precipitated as sediments.
- Photosynthesis. Algae and plants absorbed CO2 as they grew.
- Sedimentary rocks. Marine organisms used carbonates to build shells and skeletons; when they died these formed limestone and other carbonate rocks. (This and the dissolving route above are two faces of the same idea — carbon locked into carbonate rock.)
- Fossil fuels (storage). Carbon that photosynthesis had already taken from the air — in the remains of plants and plankton — was buried under sediment and, over millions of years under heat and pressure, turned into coal, crude oil and natural gas, locking that carbon underground.
So the carbon from the early CO2 is now stored in oceans, limestone and fossil fuels. (Burning those fossil fuels today releases it again — section 5.)
- Saying plants produced the first oxygen. It was algae first (~2.7 billion years ago); plants came later.
- An unbalanced photosynthesis equation. Keep the 6s: 6CO2 + 6H2O → C6H12O6 + 6O2.
- Only giving one route for CO2. Higher-mark questions want several: dissolving in oceans, photosynthesis, sedimentary rocks and fossil fuels.
🧪 Exam-style questions
Which process increased the amount of oxygen in the atmosphere? Tick (✓) one box.
Which organisms first produced oxygen, about 2.7 billion years ago? Tick (✓) one box.
Which of these “locked up” carbon from the early atmosphere underground? Tick (✓) one box.
In photosynthesis, carbon dioxide and water react to form oxygen and which other product? Tick (✓) one box.
The percentages of carbon dioxide and oxygen have changed from Earth’s early atmosphere to Earth’s atmosphere today. Explain the processes that led to these changes. This is a levels-of-response question — identify the processes, then link them into a clear account of how CO2 fell and O2 rose. Plan, then compare.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): relevant points (reasons / causes) are identified, given in detail and logically linked to form a clear account. AO2
- Level 2 (3–4): relevant points (reasons / causes) are identified, and there are attempts at logical linking; the account is not fully clear. AO1
- Level 1 (1–2): points are identified and stated simply, but their relevance is not clear and there is no attempt at logical linking.
Indicative content — carbon dioxide decreased because:
- it dissolved in the newly-formed oceans;
- it was used by photosynthesising algae and plants, which removed it from the air (and that carbon was then locked away);
- it became locked in sedimentary rocks — carbonates such as limestone formed from the shells and skeletons of marine organisms;
- it became locked in fossil fuels (coal, crude oil and natural gas) formed from buried plant and plankton remains.
Indicative content — oxygen increased because:
- algae first, and later plants, produced oxygen by photosynthesis: 6CO2 + 6H2O → C6H12O6 + 6O2;
- as algae and plants spread, oxygen gradually built up in the atmosphere.
Conclusion (needed for Level 3): the same process — photosynthesis — both removed carbon dioxide and released oxygen; alongside CO2 dissolving in the oceans and being locked into rocks and fossil fuels, this is why CO2 fell from a high level in the early atmosphere to a small proportion today while O2 rose to about one-fifth of the air.
Source: AQA GCSE Chemistry.
4The Greenhouse Effect
Some gases in the atmosphere act like the glass of a greenhouse: they let the Sun’s energy in but slow its escape, keeping the surface warm enough for life. The examiner’s favourite phrase here is “short and long wavelength radiation” — get that sequence right and the marks follow.
Water vapour, carbon dioxide and methane. They keep the Earth’s temperature high enough to support life — without them the surface would be far colder.
Short-wavelength radiation comes in from the Sun (tight waves); the surface absorbs it and re-emits long-wavelength infrared (stretched waves — about double the wavelength). A greenhouse-gas molecule absorbs that infrared and re-radiates it in all directions — some back down, trapping heat.
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface, warming it.
- The surface re-emits energy as longer-wavelength (infrared) radiation.
- Greenhouse gases absorb this long-wavelength radiation and re-radiate it in all directions — some back to the surface, trapping heat and keeping the Earth warm.
- Mixing up the wavelengths. Short-wavelength radiation comes in; the Earth re-emits long-wavelength (infrared); greenhouse gases absorb the long-wavelength radiation.
- Saying greenhouse gases “reflect” radiation. They absorb and re-radiate it — not mirror-like reflection.
- Confusing this with the ozone layer or with pollution. The greenhouse effect is about trapping heat, not blocking UV or smog near the ground.
The Enhanced Greenhouse Effect
The natural greenhouse effect above is essential — it keeps the planet warm enough for life. The problem is the enhanced greenhouse effect: as human activity adds extra CO2 and methane, more infrared is trapped, the average global temperature rises, and the climate changes (section 5). In the animation below, drag the greenhouse-gas slider to add more molecules to the air — watch more infrared get sent back to the surface, and the surface thermometer climb.
Press play to follow the energy: short-wavelength radiation in from the Sun, long-wavelength infrared back out, and some of it trapped. Drag the slider to add greenhouse gas.
🧪 Exam-style questions
Which two of these are greenhouse gases? Tick (✓) two boxes, then press Check.
What do greenhouse gases do to keep the Earth warm? Tick (✓) one box.
Which statement describes the radiation in the greenhouse effect correctly? Tick (✓) one box.
Describe the greenhouse effect in terms of short- and long-wavelength radiation. Build the answer in your head (or on paper), then compare it with the model answer below.
Show a model answer
- Short-wavelength radiation from the Sun passes through the atmosphere and is absorbed by the Earth’s surface, warming it. 1 mark
- The surface re-emits energy as longer-wavelength (infrared) radiation. 1 mark
- Greenhouse gases (water vapour, carbon dioxide, methane) absorb this long-wavelength radiation. 1 mark
- They re-radiate it in all directions — some back down to the surface — trapping heat and keeping the Earth warm. 1 mark
Watch the wording: greenhouse gases absorb and re-radiate — never “reflect”.
5Human Activity & Climate Change
A growing population, burning more fuel and clearing more land, is adding extra carbon dioxide and methane to the air. That enhances the greenhouse effect and, most scientists agree, is driving global climate change. You need the human activities, the evidence, and a fair view of its uncertainties.
Carbon dioxide — recall two:
- Burning fossil fuels (for electricity, heating and transport).
- Deforestation — fewer trees means less CO2 removed by photosynthesis.
Methane — recall two (each of these is a separate activity):
- Farming livestock (cattle and other animals produce methane in digestion).
- Rice paddy fields (flooded fields release methane).
- Landfill / decomposing waste (rotting rubbish gives off methane).
By comparing the percentage of CO2 in the atmosphere with global temperature over many years, scientists find a clear correlation: as CO2 rises, temperature rises. This evidence is peer-reviewed — checked by other scientists — which is why most scientists now agree human activity is warming the surface. A correlation this strong is powerful evidence, but a correlation on its own does not prove that CO2 causes the warming — the case rests on the physics of the greenhouse effect (section 4) and the correlation together.
CO2 and global temperature have risen together since the Industrial Revolution — a strong correlation. Be careful in exams: a correlation is powerful evidence, but on its own it does not prove cause.
- Peer review — results are checked by other scientists before they are trusted; this filters out errors and bias.
- Why it’s uncertain — the climate is hugely complex and hard to model, and historical data (from tree rings, ice cores) is less precise than modern measurements. This leads to simplified models and, in the media, speculation and opinions that may be based on only part of the evidence and may be biased.
- Communication matters — results should be shared with a wide range of audiences so decisions are based on the full picture, not the headlines.
- Rising sea levels (melting ice caps and glaciers) → coastal flooding and loss of land.
- More frequent and severe storms (warmer oceans give storms more energy).
- Changes to rainfall — droughts and floods → water shortages and reduced food production.
- Temperature and habitat changes → shifts in the distribution of species and loss of habitats.
You should be able to describe four effects and discuss the scale, risk and environmental implications.
🧪 Exam-style questions
Which human activity increases the amount of methane in the atmosphere? Tick (✓) one box.
Explain how deforestation increases the amount of carbon dioxide in the atmosphere.
Show answer
- Trees remove carbon dioxide from the air by photosynthesis. 1 mark
- With fewer trees, less CO2 is absorbed, so more stays in the atmosphere. 1 mark
Allow: burning or rotting the felled wood also releases CO2.
A graph shows atmospheric CO2 from 1850 to 2020. Which best describes the change? Tick (✓) one box.
Why is it important that evidence about climate change is peer-reviewed? Tick (✓) one box.
A newspaper article claims that human activity is causing climate change. Evaluate the evidence for this claim. This is a levels-of-response question — a good answer weighs evidence on both sides and reaches a justified conclusion. Plan, then compare.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a balanced answer using evidence for human-caused change and the uncertainties/limitations, ending with a justified conclusion.
- Level 2 (3–4): describes evidence for the claim and gives at least one limitation.
- Level 1 (1–2): one or two simple relevant points.
Evidence the claim is correct:
- CO2 and average global temperature have risen together since the Industrial Revolution — a strong correlation.
- There is a mechanism: extra CO2 and methane enhance the greenhouse effect, trapping more long-wavelength radiation.
- Human activities (burning fossil fuels, deforestation, farming) raise CO2 and methane, and the evidence is peer-reviewed.
Reasons to be cautious (uncertainty):
- A correlation does not prove cause on its own.
- The climate is very complex and hard to model, so models are simplified; older data is less precise.
- Media reports may be biased or based on only part of the evidence.
Conclusion (needed for Level 3): based on peer-reviewed evidence, most scientists agree human activity is the main cause — the mechanism and the correlation together make a strong case, even though some uncertainty remains. 6 marks
6The Carbon Footprint
If climate change is driven by greenhouse-gas emissions, one useful way to measure the impact of anything we make or do is its carbon footprint.
The carbon footprint is the total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, a service or an event.
“Full life cycle” means everything — getting the raw materials, manufacturing, transport and distribution, use, and disposal at the end.
The carbon footprint adds up the greenhouse gases emitted at every stage of a product’s life — not just while it is being used.
Emissions of CO2 and methane can be reduced by, for example:
- using renewable energy (solar, wind) instead of burning fossil fuels, and improving energy efficiency;
- using public transport or electric vehicles and cutting unnecessary journeys;
- carbon capture and storage — trapping CO2 and storing it underground;
- government taxes, targets and incentives to encourage low-carbon choices.
Exam questions love the “why is this hard?” side. Reasons actions are limited include:
- Cost — renewable technology and changes can be expensive.
- Incomplete or developing technology — some solutions (eg large-scale carbon capture) are still being developed.
- Economic and political concerns — governments fear harming their economies or jobs, and international agreement is hard.
- Lifestyle resistance — people are reluctant to change habits (travel, diet).
🧪 Exam-style questions
What is meant by the “carbon footprint” of a product? Tick (✓) one box.
Which action would reduce a person’s carbon footprint? Tick (✓) one box.
Give one reason why actions to reduce carbon emissions may be limited.
Show answer
Any one of: the changes are often expensive (cost); the technology is still developing; economic or political concerns (governments fear harming the economy or jobs); or people are reluctant to change their lifestyles. 1 mark
A company wants to reduce the carbon footprint of its products. Suggest actions it could take, and explain why such actions may be limited. Aim for a balance of actions and reasons, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): a balanced answer giving several actions and several reasons they may be limited, logically linked, ending with a justified conclusion.
- Level 2 (3–4): gives some actions and at least one reason they may be limited, with some linking.
- Level 1 (1–2): one or two simple relevant points (an action or a limitation).
Actions to reduce the carbon footprint (any three):
- Use renewable energy and improve energy efficiency in manufacturing.
- Cut transport emissions — source locally, use electric vehicles, reduce journeys.
- Use carbon capture and storage, or use less / recyclable materials.
Why actions may be limited (any three):
- Cost — renewable technology and changes can be expensive.
- Some technology is still developing (e.g. large-scale carbon capture).
- Economic/political concerns (higher prices, competitiveness, hard international agreement) and people’s reluctance to change habits.
Conclusion (needed for Level 3): the company could cut its carbon footprint in several ways, but in practice cost, developing technology and economic, political and lifestyle factors limit how far and how quickly these can be applied. 6 marks
7Pollutants from Burning Fuels
Burning fuels is the major source of atmospheric pollutants. Most fuels contain carbon and/or hydrogen, and many — coal especially — also contain some sulfur. What comes out of the chimney or exhaust depends on the fuel and on how much oxygen is available. You first met complete and incomplete combustion of hydrocarbons in C7: with plenty of oxygen a hydrocarbon burns to carbon dioxide and water; starved of oxygen it gives carbon monoxide and soot instead.
With plenty of oxygen, a hydrocarbon burns cleanly to CO2 and water. Starve it of oxygen and you get a sooty yellow flame producing carbon monoxide and carbon particles instead.
- Carbon dioxide (CO2) — from complete combustion of the carbon in the fuel. (A greenhouse gas, not a “classic” pollutant, but a product of burning.)
- Carbon monoxide (CO) and soot (carbon particles) — from incomplete combustion, when there is not enough oxygen. Unburned hydrocarbons are released too; soot and unburned hydrocarbons form particulates.
- Sulfur dioxide (SO2) — from the sulfur impurities in the fuel burning.
- Oxides of nitrogen (NOx) — nitrogen and oxygen from the air react together at the high temperatures inside an engine.
You don’t need to recall combustion equations, but you should be able to predict the products from the fuel and the conditions: a hydrocarbon with plenty of oxygen → CO2 + water; with limited oxygen → CO and/or carbon (soot) + water; if the fuel contains sulfur → also SO2.
- Thinking NOx comes from the fuel. It comes from nitrogen and oxygen in the air reacting at the high temperature of the engine.
- Getting incomplete combustion wrong. With too little oxygen a fuel gives carbon monoxide and/or carbon (soot) as well as water — and often some CO2 too. The exam point is that CO and soot only appear when oxygen is limited.
- Forgetting the sulfur. SO2 only forms if the fuel contains sulfur — it isn’t made from the air.
🧪 Exam-style questions
Explain how carbon monoxide is produced when petrol is burned in a car engine.
Show answer
- There is not enough oxygen for complete combustion (incomplete combustion). 1 mark
- So the carbon is only partly oxidised, forming carbon monoxide (CO) instead of carbon dioxide. 1 mark
Describe how oxides of nitrogen are produced when fuel is burned in an engine.
Show answer
- The high temperature inside the engine 1 mark
- makes nitrogen and oxygen from the air react together to form oxides of nitrogen (NOx). 1 mark
Which pollutant is produced from sulfur impurities in a fuel? Tick (✓) one box.
A hydrocarbon burns in plenty of oxygen. What are the products? Tick (✓) one box.
8Properties & Effects of Pollutants
Each pollutant causes a specific problem, and the exam wants the property linked to the effect. Learn them as a table — and keep them separate from greenhouse gases.
| Pollutant | Property | Problems it causes |
|---|---|---|
| Carbon monoxide (CO) | toxic, colourless and odourless | binds to haemoglobin so blood carries less oxygen; not easily detected; fainting, coma, death |
| Sulfur dioxide (SO2) | acidic gas | respiratory problems; acid rain |
| Oxides of nitrogen (NOx) | acidic gases | respiratory problems; acid rain |
| Particulates (soot & unburned hydrocarbons) | tiny solid particles | global dimming (reflect sunlight); respiratory/health problems |
Carbon monoxide binds to the haemoglobin in red blood cells in place of oxygen (and more strongly), so the blood carries less oxygen — which is why this colourless, odourless gas is so dangerous.
Sulfur dioxide and oxides of nitrogen dissolve in rainwater to make it acidic. Acid rain damages buildings and statues (especially limestone and metals), harms trees and aquatic life in lakes and rivers, and damages crops. Limestone is a carbonate, so acid rain reacts with it.
Sulfur dioxide comes from sulfur in fuels (especially coal); oxides of nitrogen form from nitrogen and oxygen in the air at the high temperature of car engines. Both dissolve in rain to make it acidic — eroding limestone buildings, harming trees and crops, and making lakes too acidic for fish.
- Greenhouse gas vs pollutant. A greenhouse gas (CO2, methane, water vapour) causes global warming. A pollutant (CO, SO2, NOx, particulates) is not part of clean air and causes problems near the ground — toxicity, smog, breathing problems, acid rain.
- Global dimming, not warming. Particulates cause global dimming by reflecting sunlight back into space — a different effect from the greenhouse warming caused by gases.
- CO is dangerous because it’s undetectable. Colourless and odourless means people don’t notice it — that’s the exam point, alongside it being toxic.
🧪 Exam-style questions
Give two reasons why people may be unaware that carbon monoxide is present. Tick (✓) one box.
Which two pollutants cause acid rain? Tick (✓) two boxes, then press Check.
What environmental problem is caused by particulates (soot) in the atmosphere? Tick (✓) one box.
Four coal samples contain different amounts of sulfur: A 0.4%, B 1.8%, C 0.9%, D 2.5%. Which sample produces the most acid rain per kilogram, and why? Tick (✓) one box.
★Capstone: Sort the Gases
One distinction runs through the whole topic and earns marks everywhere: a greenhouse gas warms the planet; an atmospheric pollutant is something not in clean air that causes problems near the ground; and the two main gases of clean air are neither. Sort all nine to check you can keep them apart.
Drag each gas into a box — or tap it to step through the boxes. Then press Check.
- Today’s atmosphere — about 80% nitrogen and 20% oxygen, with small amounts of carbon dioxide, water vapour and noble gases; roughly constant for the last 200 million years.
- The early atmosphere — formed by intense volcanic activity: mainly CO2 with water vapour and little or no oxygen (like Mars and Venus). As the Earth cooled, water vapour condensed to form the oceans, CO2 dissolved and carbonates precipitated. Evidence is limited because it was ~4.6 billion years ago.
- Oxygen up, carbon dioxide down — algae (then plants) added oxygen by photosynthesis from ~2.7 bya (6CO2 + 6H2O → C6H12O6 + 6O2); CO2 was removed and locked into oceans, sedimentary rocks (limestone) and fossil fuels.
- The greenhouse effect — water vapour, CO2 and methane. Short-wavelength radiation in → absorbed by the surface → re-emitted as long-wavelength (infrared) → greenhouse gases absorb and re-radiate it (not “reflect”), trapping heat.
- Human activity & climate change — CO2 from burning fossil fuels and deforestation; methane from livestock, rice fields and landfill. CO2 correlates with temperature (peer-reviewed evidence). Four effects: rising sea levels, more severe storms, droughts/floods, changes to habitats and species. A correlation does not prove cause.
- The carbon footprint — the total CO2 and other greenhouse gases over the full life cycle of a product, service or event; reduce it with renewables, efficiency, public transport/EVs and carbon capture — limited by cost, developing technology, economics/politics and lifestyle.
- Pollutants from fuels — complete combustion → CO2 + water; incomplete (too little oxygen) → carbon monoxide + soot; sulfur in the fuel → SO2; the high temperature of engines makes nitrogen and oxygen from the air react → oxides of nitrogen.
- Effects of pollutants — CO is toxic, colourless and odourless (binds haemoglobin, so blood carries less oxygen); SO2 and NOx cause acid rain and respiratory problems; particulates cause global dimming and health problems.
That is the arc of C9: a planet that built its own breathable atmosphere over billions of years, a delicate greenhouse balance that keeps it warm enough for life, and the pollutants and extra greenhouse gases that burning fuels now adds. For Paper 2 it sits between C8 Chemical Analysis (the tests that identify gases such as carbon dioxide) and C10 Using Resources (how we respond) — and the combustion chemistry here links straight back to C7 Organic Chemistry, the fuels themselves.