4.9 Chemistry of the atmosphere — notes on one page

Every AQA GCSE Chemistry 8462 specification point in 4.9, read straight through: 10 points with definitions, explanations, mark-scheme wording and worked examples. Practice questions are in the printable question pack.

Checked against AQA 8462 section 4.9. Review basis: the qualification registry sourced from the AQA GCSE Chemistry (8462) specification; registry verification recorded 17 July 2026.

How this checking works

4.9.1.1 · The proportions of different gases in the atmosphere

For about 200 million years, air has contained approximately 80% nitrogen and 20% oxygen, with much smaller proportions of carbon dioxide, noble gases and water vapour.
Use a percentage as a fraction of 100: volume of a gas $=(\text{percentage}/100)\times\text{total volume}$ .
For example, a $350\,\text{cm}^3$ air sample contains about $0.20\times350=70\,\text{cm}^3$ of oxygen.
The 80:20 split is an approximation: nitrogen and oxygen do not make exactly 100%, and the proportion of water vapour varies.
The accepted modern values are close to four-fifths nitrogen and one-fifth oxygen, with argon, carbon dioxide and variable water vapour making up the remainder.

Worked example

Estimate the volume of oxygen in $625\,\text{cm}^3$ of air using the specification's approximate composition.

1.Use the approximate oxygen proportion of $20\%$ .
2.Convert the percentage to a decimal: $20\div100=0.20$ .
3.Calculate $0.20\times625=125\,\text{cm}^3$ .

Answer: $125\,\text{cm}^3$ of oxygen

Common mistakes

Don't fall into the trap of stating that the atmosphere is exactly $80\%$ nitrogen and $20\%$ oxygen, leaving no room for other gases.
Don't fall into the trap of using the percentage as a whole-number multiplier instead of dividing by $100$ when finding a gas volume.

Exam tip. Use ‘approximately’ for atmospheric proportions and show $\text{percentage}\div100\times\text{total volume}$ in calculations.

4.9.1.2 · The Earth's early atmosphere

Theories of the early atmosphere have changed and developed as evidence has been interpreted; one theory proposes that intense volcanic activity released the gases that formed it.
The atmosphere may initially have resembled those of Mars and Venus today: mainly carbon dioxide, with little or no oxygen; volcanic nitrogen accumulated, and small proportions of methane and ammonia may also have been present.
As the Earth cooled, volcanic water vapour condensed to form oceans; carbon dioxide then dissolved and carbonates precipitated as sediments, lowering its atmospheric proportion.
Evidence from 4.6 billion years ago is limited, so this is a supported theory rather than a certain, directly observed account; detailed knowledge of other theories is not required.
Scientific explanations are revised when new geological or planetary evidence becomes available, so conclusions must match the strength of the evidence.

Worked example

Explain how volcanic activity and cooling could produce an early atmosphere and then oceans.

1.Give the events in causal order: volcanic gases were released, accumulated as an atmosphere, and then the falling temperature allowed gaseous water to condense into liquid oceans.

Answer: Volcanic activity released gases, including carbon dioxide and water vapour. These gases formed the early atmosphere. As the Earth cooled, water vapour condensed to form oceans.

Common mistakes

Don't fall into the trap of presenting one model of the early atmosphere as directly observed fact despite the limited evidence.
Don't fall into the trap of putting abundant oxygen into the earliest atmosphere instead of recognising that little or no oxygen was present.

Exam tip. Use cautious language such as ‘one theory suggests’ and connect volcanic gases, cooling, ocean formation and carbon-dioxide removal.

4.9.1.3 · How oxygen increased

Algae and plants released oxygen by photosynthesis: carbon dioxide and water form glucose and oxygen using light energy.
Link the biological timeline: algae began producing oxygen about 2.7 billion years ago, oxygen then appeared in the atmosphere, and later plants increased its proportion further.
Photosynthesis simultaneously removes carbon dioxide and supplies oxygen, so a growth in photosynthetic organisms changes both gases in opposite directions.
Do not attribute the oxygen rise to volcanoes: in this model the sustained increase came from algae and plants, eventually allowing animals to evolve.
The photosynthesis equation provides the chemical link between the removal of carbon dioxide and the release of oxygen.

Worked example

Explain why a large increase in photosynthetic organisms changes the proportions of both oxygen and carbon dioxide in the atmosphere.

1.Use the word equation for photosynthesis as the reasoning chain: carbon dioxide is a reactant and oxygen is a product, so more photosynthesis removes more of the former and produces more of the latter.

Answer: Photosynthetic organisms take in carbon dioxide. They use it with water to make glucose. They release oxygen, so carbon dioxide decreases while oxygen increases.

Common mistakes

Don't fall into the trap of claiming volcanic activity caused the sustained rise in atmospheric oxygen.
Don't fall into the trap of writing photosynthesis as consuming oxygen rather than producing it from carbon dioxide and water.

Exam tip. Link the timeline explicitly: algae first, atmospheric oxygen rising, then plants contributing further oxygen.

4.9.1.4 · How carbon dioxide decreased

Atmospheric carbon dioxide decreased because algae and plants used it in photosynthesis and because it dissolved in the oceans.
Trace where the carbon went: dissolved carbon dioxide contributed to carbonate sediments and limestone, while carbon in organisms was eventually locked into fossil fuels.
Coal formed mainly from buried plant material; crude oil and natural gas formed mainly from buried remains of marine organisms over millions of years.
A common error is to say carbon disappeared: it was transferred into biomass, sedimentary rocks and fossil fuels rather than destroyed.
Each mechanism transfers carbon out of the atmosphere while conserving the carbon atoms in another reservoir.

Worked example

Explain two routes by which carbon from the early atmosphere became stored for long periods.

1.Follow carbon atoms rather than saying the gas vanished. One pathway is atmosphere to ocean to carbonate rock; the other is atmosphere to living material to buried organic matter and then fossil fuels.

Answer: Carbon dioxide dissolved in oceans and formed carbonate sediments that became sedimentary rocks such as limestone. Photosynthetic organisms took in carbon dioxide, and some buried remains eventually formed coal, crude oil or natural gas.

Common mistakes

Don't fall into the trap of saying carbon disappeared: it was transferred into biomass, sedimentary rocks and fossil fuels rather than destroyed.
Don't fall into the trap of saying all fossil fuels formed from plants, when crude oil and natural gas formed mainly from marine organisms.

Exam tip. Trace carbon into named stores—oceans, carbonate rocks, biomass and fossil fuels—rather than saying it vanished.

4.9.2.1 · Greenhouse gases

Water vapour, carbon dioxide and methane are greenhouse gases that maintain a temperature on Earth high enough to support life.
Describe the greenhouse effect by wavelength: short-wavelength radiation from the Sun reaches and warms the surface, which emits longer-wavelength infrared radiation.
Greenhouse-gas molecules absorb some outgoing long-wavelength radiation and re-emit it in all directions, reducing the rate at which energy escapes to space.
Do not confuse the greenhouse effect with ozone depletion, and do not claim that greenhouse gases stop all radiation from leaving Earth.
The natural greenhouse effect is necessary for life; enhanced greenhouse warming concerns changes in greenhouse-gas concentrations.

Short-wave solar radiation warms Earth, which emits infrared radiation that greenhouse gases absorb and re-emit.

Worked example

Describe how short-wavelength and long-wavelength radiation are involved in the greenhouse effect.

1.Follow the energy transfer in order: incoming short wavelength, surface absorption and warming, outgoing longer wavelength, then interaction with greenhouse-gas molecules.

Answer: Short-wavelength radiation from the Sun passes through the atmosphere and warms the surface. The warm surface emits longer-wavelength infrared radiation. Greenhouse gases absorb and re-emit some of this outgoing radiation.

Common mistakes

Don't fall into the trap of explaining the greenhouse effect as a hole in the ozone layer rather than absorption and re-emission of infrared radiation.
Don't fall into the trap of claiming greenhouse gases reflect all incoming solar radiation and prevent any energy escaping to space.

Exam tip. Use the wavelength sequence: short-wave solar radiation in, long-wave infrared from Earth, then absorption and re-emission by greenhouse gases.

4.9.2.2 · Human activities which contribute to an increase in greenhouse gases in the atmosphere

Burning fossil fuels and deforestation increase atmospheric carbon dioxide; livestock farming, rice cultivation, landfill and decay of organic waste can increase methane.
For a recall question, give two distinct human activities for carbon dioxide and two for methane, linking each activity to the correct gas.
A strong climate-evidence evaluation checks sample size and duration, uncertainty, peer review, agreement with other data and whether the source communicates the full evidence.
A single weather event or a short local record cannot by itself establish a global climate trend; correlation also needs a scientifically plausible explanation.
Correlation between emissions and climate variables becomes more persuasive when supported by a plausible mechanism and several independent data sets.

Worked example

State two human activities that increase atmospheric carbon dioxide and two that increase atmospheric methane.

1.Keep the gases separate. Combustion releases stored carbon and deforestation reduces carbon dioxide uptake; digestion in livestock and anaerobic decay of buried organic waste release methane.

Answer: Carbon dioxide: burning fossil fuels and deforestation. Methane: livestock farming and decomposition of organic waste in landfill.

Common mistakes

Don't fall into the trap of using one short local weather record as proof of a long-term global climate trend.
Don't fall into the trap of assigning landfill methane emissions to carbon dioxide without linking decay of organic waste to methane.

Exam tip. For each activity, name the greenhouse gas it increases; for evidence evaluation, address duration, sample size, uncertainty and source quality.

4.9.2.3 · Global climate change

An increase in average global temperature is a major cause of climate change, but the consequences differ between regions.
Use a cause-and-effect chain: warming can melt land ice and expand seawater, raise sea level, alter rainfall and extreme-weather patterns, and change habitats or species distributions.
For example, sea-level rise may increase coastal flooding, while changed rainfall may produce drought in one region and greater flood risk in another.
Do not present every projected effect as certain: discuss its scale, likelihood, risk and environmental implication using the evidence supplied.
Different regions may experience opposite rainfall changes, so answers must use the evidence supplied rather than claiming one universal outcome.

Worked example

Explain how an increase in average global temperature can increase the risk of coastal flooding.

1.Link temperature to two mechanisms and then to the hazard: land ice adds water and thermal expansion increases ocean volume, so sea level rises and flooding becomes more likely.

Answer: Higher temperatures can melt land-based ice. They can also cause seawater to expand. Both processes raise sea level, increasing coastal flood risk.

Common mistakes

Don't fall into the trap of stating every projected regional consequence as certain rather than discussing probability, scale and risk.
Don't fall into the trap of explaining sea-level rise only by melting sea ice and omitting expansion of warmer seawater and melting land ice.

Exam tip. Build one complete chain from warming to a physical change and then to a specific environmental or social consequence.

4.9.2.4 · The carbon footprint and its reduction

A carbon footprint is the total carbon dioxide and other greenhouse gases emitted across the full life cycle of a product, service or event.
Include raw materials, manufacture, transport, use and end-of-life when comparing footprints; omitting a stage can reverse a decision.
Footprints can be reduced through lower energy use, renewable energy, less travel, reduced waste, recycling, methane capture and changes in farming or diet.
Reductions may be limited by cost, available technology, infrastructure, public acceptance, convenience and incomplete or uncertain life-cycle data.
Because data across a complete life cycle can be uncertain, carbon-footprint comparisons should state the same boundaries and assumptions.

Worked example

A music festival wants to reduce its carbon footprint. Describe two suitable actions and give one reason why each action may be limited.

1.For each pair, name a change that reduces carbon dioxide or methane emissions, then attach a realistic economic, technical or social limitation to that same change.

Answer: For example, provide shared electric transport, but vehicles and charging infrastructure may be expensive or unavailable. For example, replace diesel generators with renewable electricity, but supply may be intermittent or the necessary connection may be costly.

Common mistakes

Don't fall into the trap of counting only emissions during product use and omitting manufacture, transport or disposal from the carbon footprint.
Don't fall into the trap of claiming a proposed reduction is automatically practical without considering cost, infrastructure or public acceptance.

Exam tip. Evaluate a footprint measure by naming the emission it reduces and one realistic limitation to implementation.

4.9.3.1 · Atmospheric pollutants from fuels

Definition. The combustion of fuels is a major source of atmospheric pollutants. Most fuels, including coal, contain carbon and/or hydrogen and may also contain some sulfur, so burning them can release carbon dioxide, water vapour, carbon monoxide, sulfur dioxide, oxides of nitrogen and particulates.

Which pollutants appear depends on the composition of the fuel and on the conditions in which it is burned, and examiners expect both to be used when predicting the products. Complete combustion of a hydrocarbon in plenty of oxygen produces only carbon dioxide and water vapour.
When the oxygen supply is insufficient, combustion is incomplete: carbon monoxide is produced, and solid carbon particles — soot — and unburned hydrocarbons may be released as particulates. Carbon monoxide is toxic, colourless and odourless, which is why it is not detected by smell.
Any sulfur in the fuel burns to sulfur dioxide, which causes acid rain and respiratory problems.
Oxides of nitrogen do not come from the fuel at all, but from nitrogen and oxygen in the air reacting at the high temperatures inside an engine.
Particulates cause global dimming and health problems.

Worked example

A fuel contains carbon, hydrogen and a small amount of sulfur. It burns in excess oxygen. Predict three products released by combustion and identify the element responsible for each.

1.Excess oxygen favours complete combustion. Match each stated fuel element to its oxidised product: carbon to carbon dioxide, hydrogen to water, and sulfur to sulfur dioxide.

Answer: Carbon dioxide from carbon. Water vapour from hydrogen. Sulfur dioxide from sulfur.

Mark-scheme wording to use

“incomplete combustion … (because of) insufficient oxygen” — AQA 8462, November 2020 Paper 2H, Q06.1
“air … oxygen … oxides of nitrogen … must be in this order” — AQA 8462, November 2020 Paper 2F, Q02.5
“soot. 1 mark: carbon monoxide” — AQA 8462, June 2024 Paper 2F, Q01.7

What the examiner wants. Oxides of nitrogen traced to the air and the high temperature, never to the fuel. That distinction is the most common single mark lost on this point.

Common mistakes

Don't fall into the trap of treating carbon monoxide and carbon dioxide as interchangeable products of combustion.
Don't fall into the trap of saying fuel nitrogen alone causes nitrogen oxides instead of linking their formation to nitrogen and oxygen reacting at high engine temperatures.

Exam tip. Start from fuel composition and oxygen supply, then name each possible gaseous or particulate product.

4.9.3.2 · Properties and effects of atmospheric pollutants

Carbon monoxide is toxic, colourless and odourless, so a dangerous concentration is not easily detected by human senses.
Sulfur dioxide and oxides of nitrogen cause respiratory problems and acid rain; connect each pollutant to both human-health and environmental effects when asked.
Particulates damage health and cause global dimming by reducing the amount of sunlight reaching the Earth's surface.
Do not assign global dimming to carbon dioxide or acid rain to soot: name the pollutant before explaining its specific effect.
Carbon monoxide's lack of colour and smell increases its danger because toxic exposure cannot be detected reliably by human senses.

Worked example

Describe one effect of sulfur dioxide or oxides of nitrogen on humans, one effect on the environment, and one effect of particulates.

1.Allocate one statement to each requested category: human respiratory harm, environmental acid rain, and either the health or light-reduction effect of particulates.

Answer: Sulfur dioxide or oxides of nitrogen can cause respiratory problems. They can cause acid rain. Particulates can cause health problems or global dimming.

Common mistakes

Don't fall into the trap of assigning global dimming to carbon dioxide rather than to particulates that reduce sunlight reaching the surface.
Don't fall into the trap of linking soot to acid rain instead of identifying sulfur dioxide and nitrogen oxides as acid-rain pollutants.

Exam tip. Name the pollutant before each property or effect; unsupported lists of effects do not show the required link.