4.9 Chemistry of the atmosphere — revision question pack

10 specification points · notes, questions, answers and worked methods

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.

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4.9.1.1 · The proportions of different gases in the atmosphere

Explanation

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.

Tier 1 · Easy

Estimate the volume of oxygen in a $240\,\text{cm}^3$ sample of air.
[1 mark]
Total for this question: 1
State the approximate fraction of the atmosphere that is nitrogen.
[1 mark]
Total for this question: 1

Tier 2 · Standard

A weather balloon contains $2.5\,\text{dm}^3$ of air. Estimate the volumes of nitrogen and oxygen in the balloon.
[2 marks]
Total for this question: 2
A gas jar contains $730\,\text{cm}^3$ of air. The air is 79% nitrogen and 20% oxygen. Calculate the volume occupied by all the other gases.
[3 marks]
Total for this question: 3
A $500\,\text{cm}^3$ air sample contains $390\,\text{cm}^3$ of nitrogen, $105\,\text{cm}^3$ of oxygen and $5\,\text{cm}^3$ of other gases. Calculate the percentage of nitrogen and the percentage of oxygen in the sample. Compare the results with the approximate proportions of these gases in air.
[4 marks]
Total for this question: 4

Tier 3 · Hard

A student passes an air sample over a substance that removes oxygen. Its volume falls from $480$ to $386\,\text{cm}^3$ . Calculate the percentage of the original sample that was oxygen and compare it with the accepted approximate value.
[4 marks]
Total for this question: 4
A sealed chamber contains $3.12\,\text{dm}^3$ of nitrogen. Nitrogen is 78% of the air in the chamber. Calculate the total volume of air. The air is also 21% oxygen; calculate the volume occupied by gases other than nitrogen and oxygen.
[4 marks]
Total for this question: 4
A $412\,\text{cm}^3$ sample of air contains $12\,\text{cm}^3$ of gases other than nitrogen and oxygen. The volume ratio nitrogen : oxygen in the rest of the sample is $4:1$ . Determine the volumes of nitrogen and oxygen and calculate the percentage of the complete sample that is oxygen to 1 decimal place.
[5 marks]
Total for this question: 5
A technician mixes air A, which is 20% oxygen, with gas B, which is 8% oxygen and contains no nitrogen. The final $750\,\text{cm}^3$ mixture is 15.2% oxygen. Let $x$ be the volume of air A in $\text{cm}^3$ and form an equation for the total volume of oxygen. Use it to determine the volumes of air A and gas B, and state the volume of oxygen contributed by each gas. Air A is 79% nitrogen; calculate the volume of nitrogen in the final mixture.
[5 marks]
Total for this question: 5
An oxygen sensor gives readings that are 10% greater than the true oxygen volume. For a $660\,\text{cm}^3$ air sample it reports $138.6\,\text{cm}^3$ of oxygen. A separate measurement finds $520\,\text{cm}^3$ of nitrogen. Correct the oxygen reading, calculate the true oxygen percentage to 1 decimal place, and determine the volume of gases other than nitrogen and oxygen. Compare your percentage with the accepted approximate value, and explain the error made by a student who corrects the reading by subtracting 10% of $138.6$ .
[5 marks]
Total for this question: 5

4.9.1.2 · The Earth's early atmosphere

Explanation

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.

Tier 1 · Easy

Name the gas thought to have made up most of the Earth's early atmosphere.
[1 mark]
Total for this question: 1
State one gas, other than carbon dioxide, released by early volcanic activity.
[1 mark]
Total for this question: 1

Tier 2 · Standard

Scientists use gases released by modern volcanoes and the present atmospheres of Mars and Venus when developing a theory of the Earth's early atmosphere. Explain why this evidence is useful but cannot make the theory certain.
[3 marks]
Total for this question: 3
Describe how cooling of the early Earth formed oceans and then helped to lower atmospheric carbon dioxide.
[3 marks]
Total for this question: 3
Describe one theory of how volcanic activity formed the Earth's early atmosphere. Include its likely main gas, one gas that may have been present in a smaller proportion, and the likely abundance of oxygen.
[4 marks]
Total for this question: 4

Tier 3 · Hard

A report claims that the exact composition of the atmosphere 4.6 billion years ago is known because modern volcanoes release carbon dioxide and water vapour. Evaluate this claim.
[4 marks]
Total for this question: 4
Laboratory measurements show that carbon dioxide dissolves in water. Venus has an atmosphere containing a high proportion of carbon dioxide. Evaluate how these observations contribute to a theory of the Earth's early atmosphere.
[4 marks]
Total for this question: 4
New geological evidence suggests that liquid oceans formed earlier than one theory had predicted. Evaluate how this evidence could affect the theory that atmospheric carbon dioxide decreased as the Earth cooled.
[4 marks]
Total for this question: 4
Scientists find volcanic rock containing trapped carbon dioxide and water vapour dated to 4.1 billion years ago, minerals that formed in liquid water dated to 4.0 billion years ago, and carbonate sediments dated to 3.8 billion years ago. Use the evidence to construct a possible sequence for atmospheric development and explain one conclusion that the evidence cannot justify.
[5 marks]
Total for this question: 5
A mineral formed 3.5 billion years ago contains trapped carbon dioxide and nitrogen. Heating during later geological activity may have changed the trapped gases. Evaluate the claim that this sample proves the Earth's first atmosphere was entirely carbon dioxide.
[5 marks]
Total for this question: 5

4.9.1.3 · How oxygen increased

Explanation

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.

Tier 1 · Easy

Name the process by which algae first increased atmospheric oxygen.
[1 mark]
Total for this question: 1
Complete the word equation for photosynthesis: carbon dioxide + water $\rightarrow$ ____ + ____.
[2 marks]
Total for this question: 2

Tier 2 · Standard

Put algae, plants and animals in the order in which they appeared in the account of atmospheric development, and state the atmospheric change that made the final group possible.
[3 marks]
Total for this question: 3
Describe how algae and plants contributed to the increase in atmospheric oxygen, including when algae began this process.
[3 marks]
Total for this question: 3
Rock evidence indicates that atmospheric oxygen began increasing before land plants appeared but after algae became widespread. Explain how this evidence supports one account of the increase in oxygen.
[3 marks]
Total for this question: 3

Tier 3 · Hard

A model shows atmospheric oxygen rising from $0.20\%\text{ to }8.6\%$ during a period when algae spread widely. Calculate how many times greater the final percentage is and explain why the data are consistent with the accepted account of atmospheric change.
[4 marks]
Total for this question: 4
A model shows atmospheric oxygen increasing from 1.0% to 15.0% over $700$ million years. Calculate the average increase in oxygen percentage, in percentage points per $100$ million years. Explain why the result is consistent with, but does not prove, the accepted account of oxygen increasing.
[5 marks]
Total for this question: 5
Algae in a shallow pond produce an average of $18\,\text{cm}^3$ of oxygen per day for each $1\,\text{dm}^2$ of pond surface. The pond has a surface area of $240\,\text{dm}^2$ . Calculate the volume of oxygen produced in $30$ days, in $\text{dm}^3$ . Only 75% of this oxygen reaches the atmosphere; calculate the volume that reaches the atmosphere. Suggest why this result cannot be used to determine the exact rate of oxygen increase in the early atmosphere.
[5 marks]
Total for this question: 5
Two sealed vessels contain equal amounts of the same algae at the same temperature. Vessel L is kept in light and its oxygen reading rises from 40 to 68 units. Vessel D is kept in darkness and its reading falls from 40 to 37 units. Calculate the change in each vessel and the difference between the changes. Explain what the comparison suggests and give one limitation.
[5 marks]
Total for this question: 5
In an atmospheric model, carbon dioxide falls from 18% to 7% while oxygen rises from 2% to 9%. A student claims that photosynthesis means the percentage-point fall in carbon dioxide must equal the percentage-point rise in oxygen. Evaluate the claim using the data and the accepted account of atmospheric change.
[5 marks]
Total for this question: 5

4.9.1.4 · How carbon dioxide decreased

Explanation

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.

Tier 1 · Easy

State one process carried out by plants that lowered atmospheric carbon dioxide.
[1 mark]
Total for this question: 1
State the name of the sedimentary rock that stores carbon from carbonate sediments.
[1 mark]
Total for this question: 1

Tier 2 · Standard

Explain why fossil fuels are a long-term carbon store only while they remain unburned.
[2 marks]
Total for this question: 2
Describe how carbon atoms from atmospheric carbon dioxide can become stored in crude oil or natural gas.
[3 marks]
Total for this question: 3
An ancient rock layer contains a large limestone deposit but no evidence of plants. A researcher claims that the limestone proves plants removed carbon dioxide from the atmosphere. Evaluate the claim.
[4 marks]
Total for this question: 4

Tier 3 · Hard

Compare the formation of limestone, coal, and crude oil or natural gas, and explain how each contributed to a lower proportion of atmospheric carbon dioxide.
[5 marks]
Total for this question: 5
A model begins with $250$ units of carbon in atmospheric carbon dioxide. It transfers $95$ units into carbonate rocks and $60$ units into biomass and fossil fuels. At the end, $55$ units remain in the atmosphere. Calculate the units transferred by other processes and the percentage of the original atmospheric carbon removed. Explain why the model does not show carbon being destroyed.
[5 marks]
Total for this question: 5
A student makes three claims: 'Carbon was destroyed when limestone formed. Coal formed mainly from marine organisms. Crude oil formed mainly from land plants.' Evaluate the claims and give a corrected account of how these carbon stores contributed to lower atmospheric carbon dioxide.
[5 marks]
Total for this question: 5
Natural gas is burned and releases a carbon atom as carbon dioxide. Describe a possible route by which that carbon atom could have travelled from the early atmosphere into the natural gas. Explain why burning the gas does not create a new carbon atom.
[5 marks]
Total for this question: 5
Model world X has algae and plants but no oceans. Model world Y has oceans but no photosynthetic organisms. Compare the long-term carbon stores that could form in each world and explain why neither world includes all the accepted routes by which atmospheric carbon dioxide decreased on Earth.
[6 marks]
Total for this question: 6

4.9.2.1 · Greenhouse gases

Explanation

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.

Tier 1 · Easy

Name two greenhouse gases present in the Earth's atmosphere.
[2 marks]
Total for this question: 2
State one benefit of the natural greenhouse effect.
[1 mark]
Total for this question: 1

Tier 2 · Standard

Explain why the natural greenhouse effect is necessary for life but an increase in greenhouse-gas concentrations can still be a concern.
[3 marks]
Total for this question: 3
Two planets receive the same amount of radiation from their star. Planet A has greenhouse gases in its atmosphere; planet B does not. Explain why planet A is expected to have a higher average surface temperature.
[3 marks]
Total for this question: 3
A gas cell transmits most visible light from a lamp but absorbs some infrared radiation from a warm surface. Explain how this observation supports the greenhouse-gas model.
[3 marks]
Total for this question: 3

Tier 3 · Hard

Explain why increasing the concentration of carbon dioxide can raise the Earth's average surface temperature even though sunlight can still enter the atmosphere.
[4 marks]
Total for this question: 4
A student states, 'Methane warms the Earth by stopping all radiation from leaving the atmosphere.' Evaluate this statement and give a scientifically accurate explanation.
[4 marks]
Total for this question: 4
Two model atmospheres each receive $320$ units of infrared radiation from a warm surface. Atmosphere X absorbs 25% and atmosphere Y absorbs 60%. Calculate how many more units atmosphere Y absorbs. Predict which surface is likely to reach the higher average temperature and explain the prediction. Give one limitation of the model.
[5 marks]
Total for this question: 5
In a simplified energy model, 300 units of short-wavelength radiation enter the Earth-atmosphere system. The surface emits 300 units of infrared radiation. Of this infrared, 90 units pass directly to space and greenhouse gases absorb 210 units, re-emitting 120 units to space and 90 units towards the surface. Calculate the total energy leaving for space and the net energy gain. Predict the initial temperature change.
[5 marks]
Total for this question: 5
Gas cells A, B and C transmit 96%, 95% and 96% of visible light, respectively. They absorb 10%, 65% and 5% of infrared radiation from a warm surface. Choose the gas cell that best models the strongest greenhouse effect, calculate how many percentage points more infrared it absorbs than cell A, explain your choice, and give one limitation of the comparison.
[5 marks]
Total for this question: 5

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

Explanation

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.

Tier 1 · Easy

Give one human activity that increases carbon dioxide and one that increases methane in the atmosphere.
[2 marks]
Total for this question: 2
Explain one way that deforestation can increase atmospheric carbon dioxide.
[2 marks]
Total for this question: 2

Tier 2 · Standard

One data set contains one week of weather from one town. Another contains $50$ years of global temperature measurements from several independent organisations. Which is stronger evidence for a climate trend? Give two reasons.
[3 marks]
Total for this question: 3
A farm clears woodland and then increases the number of cattle it keeps. Explain how both changes can increase greenhouse-gas concentrations, naming the gas linked to each change.
[4 marks]
Total for this question: 4
A town sends more food waste to landfill and opens a power station that burns natural gas. Identify the main greenhouse gas increased by each activity and explain how each gas is produced.
[4 marks]
Total for this question: 4

Tier 3 · Hard

An online article uses six years of temperatures from one town to claim that human activity cannot affect global climate. The article was written by an energy company, gives no uncertainty, and has not been peer reviewed. Evaluate the quality of this evidence.
[5 marks]
Total for this question: 5
Over several decades, atmospheric carbon dioxide rises from $325$ ppm to $366$ ppm while a global temperature anomaly also rises. Calculate the percentage increase in carbon dioxide to 1 decimal place. Explain why the paired trends support, but do not by themselves prove, that human activity caused the temperature change.
[5 marks]
Total for this question: 5
Over ten years, annual carbon dioxide emissions from energy production fall from $600$ to $480$ units, while annual methane emissions from livestock rise from $80$ to $120$ units. Calculate the percentage change in each emission. Explain why these two results alone cannot determine the change in atmospheric greenhouse-gas concentrations.
[5 marks]
Total for this question: 5
A region attributes 800 units of annual carbon dioxide emissions to power generation and 200 units to deforestation. A plan cuts these sources by 25% and 40%, respectively. Its annual methane emissions are 120 units from cattle and 80 units from landfill; methane capture cuts the landfill source by 75%, while cattle emissions are unchanged. Calculate the new total and percentage decrease for each gas. Identify the largest remaining named source of each gas.
[6 marks]
Total for this question: 6
Researchers match 12 landfill sites by size and climate. Six install methane capture and six do not. Over five years, mean methane emissions fall by 40% at the capture sites and by 3% at the comparison sites. Measurements are taken in the same seasons, but no uncertainties are reported. Evaluate how strongly the study supports the effectiveness of methane capture.
[5 marks]
Total for this question: 5

4.9.2.3 · Global climate change

Explanation

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.

Tier 1 · Easy

State one potential effect of global climate change.
[1 mark]
Total for this question: 1
A model predicts sea-level rise. State one item of information needed to judge the scale of this change.
[1 mark]
Total for this question: 1

Tier 2 · Standard

A model predicts less rainfall in region A but more intense rainfall in region B as the climate changes. Explain why the two predictions are not contradictory and why a single rainfall effect should not be claimed for every region.
[3 marks]
Total for this question: 3
A region is predicted to receive less rainfall as its average temperature rises. Explain one possible environmental consequence using a complete cause-and-effect chain.
[3 marks]
Total for this question: 3
A climate model predicts that the annual probability of coastal flooding in a region will rise from $0.10$ to $0.35$ . Calculate the increase in percentage points. State two possible environmental implications of the increased flood risk.
[3 marks]
Total for this question: 3

Tier 3 · Hard

A coastal wetland supports rare birds and protects a nearby town from storm waves. Discuss the scale, risk and environmental implications if climate change raises sea level in this region.
[5 marks]
Total for this question: 5
A newspaper claims that climate change will make every region drier and will reduce the population of every species. Evaluate both parts of this claim.
[4 marks]
Total for this question: 4
A coastal reserve currently has $800$ hectares of habitat. Different climate models predict that between $500$ and $620$ hectares will remain after sea-level rise. Calculate the smallest and largest predicted percentage loss of habitat. Evaluate why the range should be used when planning protection for the reserve.
[5 marks]
Total for this question: 5
For a simplified planning measure, expected habitat at risk equals annual flood probability multiplied by the habitat area exposed. Region A has probability 0.30 with 200 hectares exposed. Region B has probability 0.15 with 500 hectares exposed. Calculate the expected habitat at risk for each region, choose which is greater, and explain why this measure alone cannot set the protection priority.
[5 marks]
Total for this question: 5
A coastal town currently has an annual flood probability of 0.04. Climate models predict a future probability between 0.10 and 0.18. Calculate the smallest and largest scale factors for the increase and the corresponding range of increases in percentage points. Evaluate the claim that the town is certain to flood every year and must therefore relocate immediately.
[6 marks]
Total for this question: 6

4.9.2.4 · The carbon footprint and its reduction

Explanation

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.

Tier 1 · Easy

Define the carbon footprint of a product.
[2 marks]
Total for this question: 2
Give one action a household could take to reduce its carbon footprint.
[1 mark]
Total for this question: 1

Tier 2 · Standard

A company compares two products using only the electricity consumed while each product is in use. Explain why this is not a complete carbon-footprint comparison and name the three other canonical life-cycle stages that should be included.
[4 marks]
Total for this question: 4
The life-cycle emissions of cup A are $3,\ 7,\ 2,\ 4$ and $1\,\text{kg}$ for raw materials, manufacture, transport, use and disposal, respectively. For cup B the corresponding values are $5,\ 2,\ 3,\ 10$ and $1\,\text{kg}$ . Calculate which cup has the smaller carbon footprint and the difference between the footprints.
[3 marks]
Total for this question: 3
Company A reports only the emissions from manufacturing a food container. Company B reports emissions from raw materials, manufacture, transport, use and disposal of a different container. Explain why the reported footprints cannot be compared fairly and state how the comparison should be improved.
[3 marks]
Total for this question: 3

Tier 3 · Hard

A reusable product has life-cycle contributions of $18,\ 4,\ 30\text{ and }2\,\text{kg}$ from manufacture, transport, use and disposal, respectively. A redesign adds $3\,\text{kg}$ in manufacture but cuts transport emissions by $25\%$ and use emissions by $40\%$ . Calculate the new footprint and the percentage reduction.
[5 marks]
Total for this question: 5
A council compares two carbon-footprint projects using greenhouse-gas reductions expressed on the same scale. Home insulation is estimated to reduce emissions by $500$ tonnes each year and costs $\pounds1.2$ million. Landfill methane capture is estimated to reduce emissions by $310$ tonnes each year and costs $\pounds0.70$ million. Calculate the estimated annual reduction per $\pounds1$ million for each project. State which estimate is greater and give two reasons why these figures alone are not enough to choose a project.
[6 marks]
Total for this question: 6
Making a reusable bag produces $2.40\,\text{kg}$ of greenhouse-gas emissions, and cleaning it adds $0.02\,\text{kg}$ per use. Making and disposing of one single-use bag produces $0.08\,\text{kg}$ per use. Determine the minimum whole number of uses after which the reusable bag has the smaller footprint. Give two reasons why the real break-even point may differ from this estimate.
[6 marks]
Total for this question: 6
Using the same complete life-cycle boundary, product A has a reported carbon footprint of $46\pm4\,\text{kg}$ and product B has a reported footprint of $52\pm3\,\text{kg}$ . Determine the possible range for each footprint. Evaluate the claim that A definitely has the smaller footprint and state one further check needed before choosing a product.
[5 marks]
Total for this question: 5
The raw-material, manufacture, transport and use stages of a product total $91\,\text{kg}$ of greenhouse-gas emissions. Disposal accounts for 12.5% of the complete footprint. Calculate the complete footprint and the disposal contribution. A redesign halves disposal emissions but adds $1\,\text{kg}$ to transport. Calculate the redesigned footprint and its percentage reduction to 1 decimal place. Give one limitation of the estimate.
[6 marks]
Total for this question: 6

4.9.3.1 · Atmospheric pollutants from fuels

Explanation

Burning carbon- and hydrogen-containing fuels can release carbon dioxide and water vapour; limited oxygen can also produce carbon monoxide, soot and unburned hydrocarbons.
Identify sulfur dioxide from sulfur impurities in a fuel, and oxides of nitrogen from nitrogen and oxygen reacting at the high temperatures inside engines.
Given a fuel's composition and conditions, first list its elements, then use oxygen supply and combustion temperature to predict the possible gaseous and particulate products.
Carbon monoxide and carbon dioxide are different products: carbon monoxide and soot indicate incomplete combustion, whereas complete combustion of carbon produces carbon dioxide.
Incomplete combustion can produce both carbon monoxide and carbon particulates, while unburned hydrocarbons may also escape.

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.

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.

Tier 1 · Easy

Name the toxic gas formed when a carbon-containing fuel burns with too little oxygen.
[1 mark]
Total for this question: 1
State the pollutant formed when sulfur in a fuel burns.
[1 mark]
Total for this question: 1

Tier 2 · Standard

A hydrocarbon fuel burns with too little oxygen inside a hot engine. Explain why carbon monoxide and soot may form, and state how oxides of nitrogen are produced in the same engine.
[4 marks]
Total for this question: 4
During a cold start, a petrol engine receives too little air and some fuel droplets leave the engine without burning. Explain why the exhaust may contain carbon monoxide and unburned hydrocarbons.
[3 marks]
Total for this question: 3
An engine first runs with too little oxygen at a moderate temperature. It is then supplied with excess oxygen and runs at a much higher temperature. Predict how the production of carbon monoxide, soot and oxides of nitrogen is likely to change. Explain the predictions.
[4 marks]
Total for this question: 4

Tier 3 · Hard

A $2.0\,\text{kg}$ fuel sample contains 0.80% sulfur by mass. Assume every sulfur atom forms sulfur dioxide when the fuel burns. Calculate the mass of sulfur dioxide produced. Use $A_r(\text{S})=32$ and $A_r(\text{O})=16$ .
[5 marks]
Total for this question: 5
A sulfur-free hydrocarbon fuel burns in a hot engine with too little oxygen. Predict whether carbon monoxide, soot, sulfur dioxide and oxides of nitrogen could be present in the exhaust. Explain each prediction.
[5 marks]
Total for this question: 5
Fuel A contains a sulfur impurity and burns with excess oxygen at a high temperature. Fuel B is sulfur-free and burns with too little oxygen at a lower temperature. Compare the likely production of sulfur dioxide, carbon monoxide, soot and oxides of nitrogen from the two fuels. Explain each comparison.
[6 marks]
Total for this question: 6
An engine is tested twice with the same sulfur-containing fuel. From test 1 to test 2, carbon monoxide falls from 42 to 11 units, soot falls from 18 to 4 units, oxides of nitrogen rise from 9 to 31 units, and sulfur dioxide remains at 6 units. Suggest how the oxygen supply and combustion temperature probably changed, and explain why the sulfur dioxide result is different.
[5 marks]
Total for this question: 5
A fuel contains $500\,\text{g}$ of carbon. Exhaust analysis accounts for $360\,\text{g}$ of that carbon in carbon dioxide, $55\,\text{g}$ in carbon monoxide and $25\,\text{g}$ as soot. Assume all remaining carbon leaves in unburned hydrocarbons. Calculate the unaccounted carbon mass and the percentage of the fuel's carbon that leaves in incomplete-combustion products. Explain the chemical condition responsible.
[5 marks]
Total for this question: 5

4.9.3.2 · Properties and effects of atmospheric pollutants

Explanation

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.

Tier 1 · Easy

Give two properties that make a carbon monoxide leak difficult for a person to detect.
[2 marks]
Total for this question: 2
A beam of sunlight becomes weaker after passing through smoky air. State the type of pollutant responsible.
[1 mark]
Total for this question: 1

Tier 2 · Standard

Explain why a carbon monoxide leak can be difficult for a person to detect without an alarm, even though the gas is dangerous.
[3 marks]
Total for this question: 3
State one health effect of carbon monoxide and one health effect of sulfur dioxide and identify which gas causes acid rain.
[3 marks]
Total for this question: 3
After a factory installs a particle filter, more sunlight reaches the ground but the acidity of a nearby lake does not change. Suggest what the observations indicate about particulate emissions and emissions of gases that cause acid rain.
[3 marks]
Total for this question: 3

Tier 3 · Hard

Near an industrial area, residents report breathing problems, a lake becomes more acidic, and less sunlight reaches the ground. Identify the likely pollutant groups and explain how the observations support your choices.
[5 marks]
Total for this question: 5
A factory filter reduces particulate emissions from $80$ to $12\,\text{mg}$ per unit of product. Over the same period, sulfur dioxide falls from $50$ to $45\,\text{mg}$ and nitrogen oxides fall from $70$ to $66\,\text{mg}$ . Calculate the percentage reduction in particulates. Explain which environmental effect is most directly reduced and why acid-rain and respiratory risks may remain.
[5 marks]
Total for this question: 5
A company removes carbon monoxide from an exhaust and claims that the exhaust is now harmless. Evaluate the claim if sulfur dioxide, oxides of nitrogen and particulates are still released.
[5 marks]
Total for this question: 5
On day A, a particulate-haze index is 5 and a carbon monoxide sensor reads 2 units. On day B, the haze index is 1 and the carbon monoxide sensor reads 18 units. Calculate how many times greater the carbon monoxide reading is on day B. Evaluate the claim that day A must be more dangerous because its pollution is more visible.
[5 marks]
Total for this question: 5
A study reports an average of 42 respiratory admissions on days with high sulfur dioxide and nitrogen oxide levels, compared with 28 on matched lower-pollution days. On days with high particulate levels, the sunlight reaching the ground averages 72 units, compared with 90 units on matched lower-pollution days. Calculate the numerical and percentage changes. Evaluate how well the data support the stated effects of these pollutants.
[6 marks]
Total for this question: 6

Answer key

Answers begin on a new printed page so the question pack can be completed without the solutions alongside it.

4.9.1.1 · The proportions of different gases in the atmosphere

Tier 1 · Easy

Mark scheme for 4.9.1.1 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	$48\,\text{cm}^3$	Oxygen is approximately 20% of air, so its volume is $0.20\times240=48\,\text{cm}^3$ .	1
Total Question 1			1
02.1	Four-fifths (or about 80%).	Nitrogen makes up approximately four-fifths, or 80%, of the atmosphere.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.1.1 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	$2.0\,\text{dm}^3\text{ nitrogen};\ 0.50\,\text{dm}^3\text{ oxygen}$	Apply each approximate percentage to the total: $0.80\times2.5=2.0\,\text{dm}^3\text{ nitrogen};\ 0.20\times2.5=0.50\,\text{dm}^3\text{ oxygen}$ .	2
Total Question 1			2
02.1	$7.3\,\text{cm}^3$	The other gases occupy $100-79-20=1\%$ of the sample. Their volume is $0.01\times730=7.3\,\text{cm}^3$ .	3
Total Question 2			3
03.1	$78\%$ nitrogen $21\%$ oxygen The results are close to the approximate proportions of 80% nitrogen and 20% oxygen.	Calculate $(390/500)\times100=78\%$ and $(105/500)\times100=21\%$ . These values are close to, but not exactly, the approximate 80:20 composition.	4
Total Question 3			4

Tier 3 · Hard

Mark scheme for 4.9.1.1 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	$19.6\%$ oxygen This is close to the accepted approximate value of 20%.	Calculate the volume loss and then its percentage of the starting volume: $480-386=94\,\text{cm}^3;\ (94/480)\times100=19.58\ldots\%=19.6\%$ . This differs from 20% by only $0.4$ percentage points, so it is close to the accepted approximation.	4
Total Question 1			4
02.1	$4.00\,\text{dm}^3$ of air $0.040\,\text{dm}^3$ of other gases	The total volume is $3.12/0.78=4.00\,\text{dm}^3$ . Nitrogen and oxygen account for $78+21=99\%$ , so the other gases occupy $0.01\times4.00=0.040\,\text{dm}^3$ .	4
Total Question 2			4
03.1	$320\,\text{cm}^3$ of nitrogen $80\,\text{cm}^3$ of oxygen $19.4\%$ oxygen	Nitrogen and oxygen occupy $412-12=400\,\text{cm}^3$ . Five ratio parts therefore represent $400\,\text{cm}^3$ , so one part is $80\,\text{cm}^3$ : oxygen is $80\,\text{cm}^3$ and nitrogen is $320\,\text{cm}^3$ . The oxygen percentage is $(80/412)\times100=19.417\ldots\%=19.4\%$ .	5
Total Question 3			5
04.1	The mixture contains $114\,\text{cm}^3$ of oxygen. $450\,\text{cm}^3$ of air A was used. $300\,\text{cm}^3$ of gas B was used. Air A contributes $90\,\text{cm}^3$ of oxygen and gas B contributes $24\,\text{cm}^3$ . The final mixture contains $355.5\,\text{cm}^3$ of nitrogen.	The total oxygen volume is $0.152\times750=114\,\text{cm}^3$ . With $x\,\text{cm}^3$ of air A, the oxygen equation is $0.20x+0.08(750-x)=114$ . This gives $0.12x=54$ , so $x=450\,\text{cm}^3$ and gas B occupies $750-450=300\,\text{cm}^3$ . The oxygen contributions are $0.20\times450=90\,\text{cm}^3$ and $0.08\times300=24\,\text{cm}^3$ . Only air A supplies nitrogen, giving $0.79\times450=355.5\,\text{cm}^3$ .	5
Total Question 4			5
05.1	The true oxygen volume is $126\,\text{cm}^3$ . The true oxygen percentage is 19.1%. The other gases occupy $14\,\text{cm}^3$ . The corrected result is close to the approximate 20% oxygen value. The reported value must be divided by 1.10 rather than reduced by 10% of 138.6, because the 10% error is of the true volume, not of the reading; subtracting gives $124.74\,\text{cm}^3$ instead of $126\,\text{cm}^3$ .	A reading 10% high is 110% of the true value, so the true oxygen volume is $138.6/1.10=126\,\text{cm}^3$ . Its percentage is $(126/660)\times100=19.0909\ldots\%=19.1\%$ . The unassigned volume is $660-520-126=14\,\text{cm}^3$ . The corrected percentage is about 0.9 percentage points below the approximate 20% value, so it is reasonably close.	5
Total Question 5			5

4.9.1.2 · The Earth's early atmosphere

Tier 1 · Easy

Mark scheme for 4.9.1.2 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Carbon dioxide	Recall the volcanic-atmosphere model: the early atmosphere is thought to have contained mainly carbon dioxide.	1
Total Question 1			1
02.1	Water vapour (or nitrogen, methane or ammonia).	The proposed volcanic gases include water vapour and nitrogen, with smaller amounts of methane and ammonia.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.1.2 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	The evidence supports the possibility of a carbon-dioxide-rich atmosphere formed by volcanic gases and provides useful comparisons. It cannot show the exact atmosphere of the Earth's first billion years because no direct atmospheric sample from that time is available. Any statement of the timescale in billions of years is acceptable.	Separate support from proof. Present-day observations can make a proposed process plausible, but they are indirect evidence for conditions billions of years ago.	3
Total Question 1			3
02.1	Water vapour condensed as the Earth cooled, forming oceans. Carbon dioxide dissolved in the ocean water, so its amount in the atmosphere decreased.	Give the changes in order: cooling causes condensation, the liquid water forms oceans, and carbon dioxide then dissolves in those oceans.	3
Total Question 2			3
03.1	One theory suggests that gases released by intense volcanic activity formed the early atmosphere. Carbon dioxide was probably the main gas. A smaller proportion may have included methane or ammonia. There was probably little or no oxygen.	Use a hedged account because the atmosphere was not observed directly: give the proposed volcanic source, the main gas, one possible minor gas and the likely oxygen abundance.	4
Total Question 3			4

Tier 3 · Hard

Mark scheme for 4.9.1.2 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	Modern volcanic gases support the idea that early volcanoes supplied carbon dioxide and water vapour. The evidence is indirect and conditions on the early Earth may have differed from conditions today. Very little direct evidence survives over 4.6 billion years. The evidence supports a theory but does not establish an exact composition with certainty.	Separate support from certainty. The observation gives a plausible mechanism, but extrapolating modern activity across an immense time interval introduces uncertainty, so the word 'exact' is not justified.	4
Total Question 1			4
02.1	The laboratory evidence supports the idea that oceans removed some carbon dioxide from the atmosphere. Venus provides a possible comparison supporting a carbon-dioxide-rich early atmosphere. Neither observation is a direct sample of the Earth's early atmosphere. The observations support parts of the theory but cannot establish its exact composition with certainty.	Link each observation to the part of the model it supports, then limit the conclusion because the evidence is indirect and Venus is not the Earth.	4
Total Question 2			4
03.1	Earlier oceans would allow carbon dioxide to start dissolving in water earlier. Carbonates could then form and become sediments earlier, transferring carbon from the atmosphere. The evidence could support an earlier decrease in atmospheric carbon dioxide than the theory predicted. The theory should be revised if the evidence is reliable, but the evidence does not establish the exact early atmosphere with certainty.	Link the new evidence to the proposed mechanism, then use an evidence-aware conclusion: earlier liquid water could bring forward carbon-dioxide removal, but indirect geological evidence does not make the entire account certain.	4
Total Question 3			4
04.1	One theory suggests that volcanic activity released carbon dioxide and water vapour into the early atmosphere. The liquid-water minerals are consistent with water vapour condensing as the Earth cooled. Carbon dioxide could then dissolve in the oceans. The later carbonate sediments are consistent with carbon becoming stored in sediments. The evidence cannot establish the exact composition of the atmosphere at 4.6 billion years ago because the samples are later and the evidence is incomplete.	Order the dated evidence from oldest to youngest, then link each observation to a proposed process: volcanic release, cooling and condensation, followed by carbon-dioxide removal into carbonate sediments. Use cautious language because the rocks constrain a possible sequence but are not a complete sample of the earliest atmosphere.	5
Total Question 4			5
05.1	The trapped carbon dioxide supports the possibility that carbon dioxide was abundant in an early atmosphere. The trapped nitrogen contradicts the claim that this sample contains only carbon dioxide. The mineral formed about 1.1 billion years after the Earth formed, so it does not directly sample the first atmosphere. Later heating may have altered the gases, reducing confidence that they preserve the original composition. The evidence may lead scientists to revise a theory, but it does not prove an exact composition for the first atmosphere.	Test the absolute claim against the content, age and reliability of the sample. Carbon dioxide is supporting evidence, but nitrogen, the later formation date and possible alteration all prevent the sample from establishing that the first atmosphere was entirely carbon dioxide.	5
Total Question 5			5

4.9.1.3 · How oxygen increased

Tier 1 · Easy

Mark scheme for 4.9.1.3 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Photosynthesis	Algae use light energy to make glucose and release oxygen; this process is photosynthesis.	1
Total Question 1			1
02.1	Glucose Oxygen	Photosynthesis uses carbon dioxide and water to produce glucose and oxygen.	2
Total Question 2			2

Tier 2 · Standard

Mark scheme for 4.9.1.3 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	Algae appeared first, followed by plants and then animals. The rise in atmospheric oxygen made the evolution of animals possible.	Use the chronology rather than re-explaining the photosynthesis equation: algae increased oxygen first, plants appeared later and added more oxygen, and animals evolved after oxygen became sufficiently abundant.	3
Total Question 1			3
02.1	Algae began producing oxygen by photosynthesis about 2.7 billion years ago. Plants appeared later and also released oxygen by photosynthesis, increasing its atmospheric proportion further.	Give the timing, the oxygen-producing process and the later contribution from plants.	3
Total Question 2			3
03.1	Algae were present before the oxygen increase. Algae release oxygen by photosynthesis. Land plants cannot explain the start of the increase because they appeared later, although they could increase oxygen further later on.	Use the order of events as evidence. The timing is consistent with algae starting the increase through photosynthesis and with plants contributing only after they evolved.	3
Total Question 3			3

Tier 3 · Hard

Mark scheme for 4.9.1.3 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	The final oxygen percentage is 43 times greater. Algae release oxygen by photosynthesis, so their spread could cause the increase.	Calculate the scale factor: $8.6/0.20=43$ . The direction of the change agrees with the known product of photosynthesis, although the data alone show consistency rather than proving that algae were the only cause.	4
Total Question 1			4
02.1	$2.0$ percentage points per $100$ million years Photosynthesis by algae and later plants releases oxygen, so the direction of change is consistent with the accepted account. An average from a model does not show that the increase was constant or prove that photosynthetic organisms were the only cause.	The increase is $15.0-1.0=14.0$ percentage points over seven periods of $100$ million years, so the average is $14.0/7=2.0$ . Agreement with a predicted direction supports an account but is not direct proof of every cause or rate.	5
Total Question 2			5
03.1	$129.6\,\text{dm}^3$ produced $97.2\,\text{dm}^3$ reaches the atmosphere Conditions, algal coverage or oxygen uptake in the early oceans may have differed, so a modern short-term average cannot give an exact ancient global rate.	The volume produced is $18\times240\times30=129600\,\text{cm}^3=129.6\,\text{dm}^3$ . The atmospheric volume is $0.75\times129.6=97.2\,\text{dm}^3$ . Extrapolating one modern pond to ancient global conditions introduces substantial uncertainty.	5
Total Question 3			5
04.1	The oxygen reading in vessel L changes by $+28$ units. The oxygen reading in vessel D changes by $-3$ units. The difference between the changes is $31$ units. Light enables photosynthesis, which releases oxygen, while in darkness respiration consumes it. One pair of vessels or one short investigation is not enough to determine the exact effect under all conditions.	Subtract each starting reading from its final reading: $68-40=+28$ units in light and $37-40=-3$ units in darkness. The difference between the changes is $28-(-3)=31$ units. In light the algae photosynthesise faster than they respire, so oxygen accumulates; in darkness only respiration occurs, so oxygen is consumed. Replication and a longer investigation would strengthen the evidence.	5
Total Question 4			5
05.1	Carbon dioxide falls by 11 percentage points. Oxygen rises by 7 percentage points. The changes are not equal; they differ by 4 percentage points. Photosynthesis does use carbon dioxide and release oxygen, so it supports the opposite directions of change. The percentages need not change equally because carbon dioxide also dissolved in oceans and formed carbonates, while other processes affected oxygen and the total atmosphere.	Calculate $18-7=11$ percentage points and $9-2=7$ percentage points, leaving a difference of $4$ percentage points. The photosynthesis equation links the directions of change but does not make atmospheric percentages a one-for-one record, because gases also move through other stores and processes.	5
Total Question 5			5

4.9.1.4 · How carbon dioxide decreased

Tier 1 · Easy

Mark scheme for 4.9.1.4 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Photosynthesis	Plants consume carbon dioxide as a reactant in photosynthesis, so this process lowers its atmospheric proportion.	1
Total Question 1			1
02.1	Limestone	Carbonate sediments can become limestone, storing carbon in sedimentary rock.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.1.4 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	Burial keeps carbon in the remains of organisms and the fossil fuels formed from them for millions of years. Burning the fuels transfers that stored carbon back to the atmosphere as carbon dioxide.	Follow the carbon in both directions. Formation and burial move it into a geological store; combustion oxidises the fuel and releases carbon dioxide, so the store is not permanent once used.	2
Total Question 1			2
02.1	Photosynthetic organisms remove carbon dioxide from the atmosphere. Carbon enters marine organisms, whose remains may be buried. Over millions of years the buried material can form crude oil or natural gas.	Trace the carbon through three stores: atmospheric carbon dioxide, marine biomass and buried remains that form fossil fuels.	3
Total Question 2			3
03.1	The limestone supports the idea that carbon dioxide dissolved in oceans and carbonates precipitated to form sediments. Formation of the limestone stored carbon outside the atmosphere. The layer gives no direct evidence that plants were present or carrying out photosynthesis. The limestone alone therefore does not prove the researcher's claim; evidence of photosynthetic organisms would also be needed.	Separate what the rock supports from what it cannot establish. Limestone is evidence for carbonate storage, but the absence of plant evidence means it cannot by itself establish biological removal by photosynthesis.	4
Total Question 3			4

Tier 3 · Hard

Mark scheme for 4.9.1.4 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	Carbon dioxide dissolved in oceans and carbonates precipitated as sediments that formed limestone. Plants removed carbon dioxide by photosynthesis; buried plant material formed coal over millions of years. Carbon taken into marine organisms was buried and formed crude oil and natural gas over millions of years. Each route stored carbon outside the atmosphere for a long time.	Distinguish the inorganic carbonate route from the two organic routes. Then connect all three stores to the same atmospheric outcome: carbon is locked in rock or fuel instead of remaining as carbon dioxide gas.	5
Total Question 1			5
02.1	$40$ units $78\%$ removed The carbon atoms were transferred to other carbon stores rather than destroyed.	Other transfers account for $250-95-60-55=40$ units. The amount removed is $250-55=195$ units, so $(195/250)\times100=78\%$ . The model conserves carbon by moving it between reservoirs.	5
Total Question 2			5
03.1	Carbon was not destroyed; carbon dioxide dissolved and carbon became stored in carbonate sediments and limestone. Coal formed mainly from buried plant material. Crude oil and natural gas formed mainly from buried marine organisms. Photosynthesis and ocean processes transferred carbon from atmospheric carbon dioxide into these long-term stores.	Test each claim separately, correct the source of each store, and finish by tracing conserved carbon atoms from the atmosphere into rock or buried organic material.	5
Total Question 3			5
04.1	The carbon atom could begin in atmospheric carbon dioxide. A photosynthetic marine organism could take in the carbon dioxide and store the carbon in biomass. The organism's remains could be buried in sediments. Over millions of years the buried material could form natural gas. Combustion transfers the existing carbon back to atmospheric carbon dioxide; it does not create carbon.	Trace the same atom through named stores in reverse order from the fuel: natural gas formed from buried marine remains, whose carbon entered living material after photosynthetic uptake of atmospheric carbon dioxide. Combustion changes the carbon-containing substance but conserves the carbon atom.	5
Total Question 4			5
05.1	In world X, algae and plants could remove carbon dioxide by photosynthesis. Buried plant material in world X could form coal. Without oceans, world X would lack the route in which carbon dioxide dissolves and forms carbonate sediments and limestone. In world Y, carbon dioxide could dissolve in ocean water. Carbonate sediments and limestone could therefore store carbon in world Y. Without photosynthetic organisms, world Y would lack biological uptake and the main route to fossil-fuel carbon stores, so each model omits one major set of transfers.	Apply each missing condition separately. Photosynthetic organisms enable biological uptake and later organic stores; oceans enable dissolution and carbonate-rock storage. Earth had both pathways, whereas each model world removes one and therefore cannot reproduce the complete accepted account.	6
Total Question 5			6

4.9.2.1 · Greenhouse gases

Tier 1 · Easy

Mark scheme for 4.9.2.1 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Any two from water vapour, carbon dioxide and methane.	Select two gases from the specified set: water vapour, carbon dioxide and methane.	2
Total Question 1			2
02.1	It keeps the Earth warm enough to support life.	Greenhouse gases maintain an average temperature high enough for life.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.2.1 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	The natural greenhouse effect keeps Earth warm enough for life. Higher greenhouse-gas concentrations absorb and re-emit more outgoing infrared radiation, reducing energy loss and increasing average temperature.	Distinguish the normal effect from its enhancement. Some retained energy gives a habitable temperature; increasing the gases changes the balance so less energy escapes to space.	3
Total Question 1			3
02.1	The surface of each planet emits long-wavelength infrared radiation. Greenhouse gases on planet A absorb and re-emit some of this radiation, so energy escapes more slowly and planet A has a higher average surface temperature.	The difference is the interaction with outgoing infrared radiation: absorption and re-emission reduce the rate of energy loss from planet A.	3
Total Question 2			3
03.1	Visible light models shorter-wavelength solar radiation that can pass through the atmosphere. The warm surface emits longer-wavelength infrared radiation. Absorption of some infrared by the gas is consistent with greenhouse gases reducing the rate at which energy escapes.	Match the two laboratory observations to the two wavelength stages of the greenhouse effect, then state what the infrared absorption changes.	3
Total Question 3			3

Tier 3 · Hard

Mark scheme for 4.9.2.1 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	Incoming solar radiation is mainly shorter wavelength. The warmed surface emits longer-wavelength infrared radiation. Additional carbon dioxide absorbs and re-emits more of this outgoing infrared radiation. Energy escapes more slowly, so the surface warms until energy transfers balance again.	The key distinction is wavelength, not a solid 'blanket'. Carbon dioxide interacts with part of the outgoing long-wavelength radiation, so increasing its concentration changes the rate of energy loss while incoming sunlight can continue to reach the surface.	4
Total Question 1			4
02.1	The statement is inaccurate because greenhouse gases do not stop all radiation from leaving. The warmed surface emits long-wavelength infrared radiation. Methane absorbs and re-emits some of this outgoing radiation. This reduces the rate of energy loss and can increase average surface temperature.	Replace the absolute claim with the specified mechanism: partial absorption and re-emission of outgoing infrared radiation slows energy transfer to space.	4
Total Question 2			4
03.1	Atmosphere X absorbs $80$ units and atmosphere Y absorbs $192$ units, so Y absorbs $112$ more units. The surface below atmosphere Y is likely to be warmer because more outgoing infrared is absorbed and can be re-emitted, reducing energy loss. The model may omit factors such as clouds, changing incoming radiation or other energy transfers.	Calculate $0.25\times320=80$ and $0.60\times320=192$ , then subtract to obtain $112$ units. Use the absorption difference to predict relative energy loss, while recognising that a simplified model does not include every climate variable.	5
Total Question 3			5
04.1	90 units leave directly for space. A further 120 units are re-emitted to space. The total energy leaving for space is 210 units. The net energy gain is 90 units. The system will initially warm because incoming energy exceeds outgoing energy.	Add both routes to space: $90+120=210$ units leave the system. Compare this with the 300 incoming units: $300-210=90$ units are gained. A positive energy imbalance causes warming until the energy transfers change enough to approach a new balance.	5
Total Question 4			5
05.1	Cell B best models the strongest greenhouse effect. It absorbs 55 percentage points more infrared than cell A. Cell B still transmits most visible light, modelling short-wavelength solar radiation entering. Its greater infrared absorption would allow more outgoing energy to be absorbed and re-emitted, reducing energy loss. A gas cell does not include atmospheric concentration, clouds or all other energy transfers, so it cannot determine the actual temperature change.	All three cells transmit similar large proportions of visible light, but cell B has the greatest infrared absorption. The difference from A is $65-10=55$ percentage points. This combination best matches the wavelength-dependent greenhouse mechanism, while the laboratory comparison omits important features of a real atmosphere.	5
Total Question 5			5

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

Tier 1 · Easy

Mark scheme for 4.9.2.2 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Carbon dioxide: for example, burning a fossil fuel or deforestation. Methane: for example, livestock farming, rice cultivation or sending organic waste to landfill.	Choose one activity from each gas-specific list and make the pairing explicit. One correct carbon dioxide source and one correct methane source earn the two marks.	2
Total Question 1			2
02.1	Fewer trees remove carbon dioxide from the atmosphere by photosynthesis.	Deforestation reduces the number of photosynthetic plants, so less carbon dioxide is taken from the air.	2
Total Question 2			2

Tier 2 · Standard

Mark scheme for 4.9.2.2 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	The $50$ -year global data set is stronger because it covers a much longer period and many locations, and agreement between independent organisations makes the pattern less dependent on one source.	Climate is a long-term, large-scale pattern. Prefer evidence with suitable duration, geographic coverage and independent replication over a short local weather record.	3
Total Question 1			3
02.1	Clearing woodland reduces carbon dioxide removal by photosynthesis and may release carbon dioxide if the wood is burned or decays. Cattle release methane, so keeping more cattle can increase atmospheric methane.	Treat the activities separately and pair each with its gas: deforestation with carbon dioxide and livestock farming with methane.	4
Total Question 2			4
03.1	Food waste decomposing in landfill can release methane. Burning natural gas releases carbon dioxide because the fuel contains carbon.	Treat the activities independently: anaerobic decay of organic waste is linked to methane, while combustion of a carbon-containing fossil fuel is linked to carbon dioxide.	4
Total Question 3			4

Tier 3 · Hard

Mark scheme for 4.9.2.2 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	One town is not representative of the whole globe. Six years is too short to establish a long-term climate trend reliably. No uncertainty is reported, so the reliability and significance of the measurements cannot be judged. The company may have a conflict of interest or present only selected evidence. The claim should be compared with peer-reviewed evidence from wider and longer datasets.	Test the report against five evidence-quality questions: Is the dataset broad, long, uncertainty-aware, independent and peer reviewed? It fails each check, so its conclusion is not well supported.	5
Total Question 1			5
02.1	$12.6\%$ Carbon dioxide is a greenhouse gas, so there is a scientifically plausible mechanism linking its increase to warming. A correlation alone does not prove causation; evidence about human emissions, uncertainty and other independent long-term data is also needed.	The increase is $366-325=41$ ppm, so $(41/325)\times100=12.615\ldots\%=12.6\%$ . Use both correlation and mechanism as support, but identify the extra evidence needed for a causal conclusion.	5
Total Question 2			5
03.1	Carbon dioxide emissions decrease by $20\%$ . Methane emissions increase by $50\%$ . The data omit other emissions, starting concentrations and processes that remove gases, so these emission changes alone do not give the concentration change.	For carbon dioxide, $((600-480)/600)\times100=20\%$ decrease. For methane, $((120-80)/80)\times100=50\%$ increase. Atmospheric concentration also depends on starting amounts, other sources and processes that remove gases.	5
Total Question 3			5
04.1	Power generation falls to 600 units and deforestation falls to 120 units. The new carbon dioxide total is 720 units. Carbon dioxide emissions decrease by 28%. Landfill methane falls to 20 units, giving a new methane total of 140 units. Methane emissions decrease by 30%. Power generation remains the largest named carbon dioxide source, and cattle remain the largest named methane source.	The new carbon dioxide contributions are $800\times0.75=600$ and $200\times0.60=120$ , totalling $720$ units. The fall is $1000-720=280$ units, or $(280/1000)\times100=28\%$ . Landfill methane becomes $80\times0.25=20$ units, so methane totals $120+20=140$ units. The fall is $60$ out of $200$ units, giving $30\%$ . Compare the remaining contributions within each gas to identify the largest source.	6
Total Question 4			6
05.1	The much larger fall at capture sites supports the conclusion that capture reduces methane emissions. Comparison sites help show that a general change affecting every landfill is unlikely to explain the whole 40% fall. Matching sites and measuring in the same seasons reduce the effects of size, climate and seasonal differences. Using several sites over five years is stronger than using one short local record. Without uncertainties, the reliability and significance of the difference cannot be judged fully, so the evidence supports but does not prove the conclusion.	Compare the change in the intervention group with the matched comparison group, then assess design quality. Replication, duration, matching and common measurement seasons strengthen the causal interpretation, whereas the missing uncertainty prevents a fully confident conclusion.	5
Total Question 5			5

4.9.2.3 · Global climate change

Tier 1 · Easy

Mark scheme for 4.9.2.3 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	For example: sea-level rise, coastal flooding, altered rainfall, more extreme weather, habitat change, changed species distribution or reduced biodiversity.	Give one specific consequence rather than repeating 'the temperature increases'. Sea-level rise is one valid example.	1
Total Question 1			1
02.1	The predicted amount of sea-level rise (or the rate or timescale of the rise).	Judging scale requires the size of the predicted rise and may also use how quickly or over what time it occurs.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.2.3 Tier 2 · Standard
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01.1	Climate change can alter atmospheric and rainfall patterns differently in different places. Region A may face greater drought risk while region B faces greater flood risk, so consequences must be based on regional evidence.	Global average warming does not imply identical local outcomes. Use the supplied regional direction and connect reduced or intensified rainfall to the relevant local risk.	3
Total Question 1			3
02.1	Less rainfall can cause drought, reducing the water available to plants. This may damage habitats or change the abundance and distribution of species.	Connect the predicted physical change to an intermediate effect and then to an environmental outcome: less rainfall, water shortage, then habitat or species change.	3
Total Question 2			3
03.1	An increase of 25 percentage points. Any two from habitat loss, salt-water damage, coastal erosion, reduced biodiversity or changes in species distribution.	Convert the probabilities to 10% and 35%, then subtract to obtain 25 percentage points. Give two distinct environmental consequences rather than repeating the flooding prediction.	3
Total Question 3			3

Tier 3 · Hard

Mark scheme for 4.9.2.3 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	Higher sea level increases the likelihood or severity of flooding and erosion. Salt water may change or destroy the wetland habitat. Rare-bird populations or their distribution may decline or shift. Loss of the wetland could remove natural protection and increase risk to the town. The size of the effect depends on the amount and rate of sea-level rise and on local protection or adaptation.	Cover all three command areas. Scale concerns how large and rapid the change is; risk combines likelihood with severity; environmental implications include habitat, biodiversity and the wetland's protective function.	5
Total Question 1			5
02.1	Climate change can alter rainfall differently between regions, so some places may become drier while others receive more or more intense rainfall. Species may change their distribution as conditions change; some populations may fall, but the evidence does not justify saying every population will fall. The scale and likelihood of each effect depend on the region, species and evidence used.	Challenge each absolute word using the specified regional nature of climate effects. Warming changes risks and distributions, but it does not impose one identical outcome everywhere.	4
Total Question 2			4
03.1	The smallest predicted loss is $22.5\%$ . The largest predicted loss is $37.5\%$ . The range shows uncertainty between models, so planners should consider both likely and more severe outcomes rather than treating one value as certain.	The smallest loss is $800-620=180$ hectares, giving $(180/800)\times100=22.5\%$ . The largest is $800-500=300$ hectares, giving $(300/800)\times100=37.5\%$ . A protection plan should account for the uncertainty and the environmental risk near the upper end.	5
Total Question 3			5
04.1	Region A has 60 hectares of expected habitat at risk. Region B has 75 hectares of expected habitat at risk. Region B has the greater value by 15 hectares. The measure does not include the rarity or ecological importance of the habitats and species. It also omits uncertainty, flood severity and the effects of local protection, so priority needs more evidence.	Apply the defined measure: $0.30\times200=60$ hectares for A and $0.15\times500=75$ hectares for B. The difference is $75-60=15$ hectares. This combines likelihood and exposed area but not every consequence or uncertainty needed for a protection decision.	5
Total Question 4			5
05.1	The smallest scale factor is 2.5. The largest scale factor is 4.5. The increase ranges from 6 to 14 percentage points. The models predict a substantial increase in risk. A probability of 0.10 to 0.18 is not certainty, and the range shows uncertainty between models. A relocation decision also needs evidence about flood severity, local defences, environmental impacts, costs and the timescale.	Divide each predicted probability by the current value: $0.10/0.04=2.5$ and $0.18/0.04=4.5$ . In percentage terms the probability rises from 4% to between 10% and 18%, increases of 6 to 14 percentage points. The risk is higher, but neither endpoint means a flood is certain in every year, so a major decision needs wider local evidence.	6
Total Question 5			6

4.9.2.4 · The carbon footprint and its reduction

Tier 1 · Easy

Mark scheme for 4.9.2.4 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	The total amount of carbon dioxide and other greenhouse gases emitted over the product's full life cycle.	Include both required ideas: more than carbon dioxide alone is counted, and emissions are totalled across the complete life cycle rather than only manufacture.	2
Total Question 1			2
02.1	For example: use less energy, use renewable energy, travel less, reduce waste or recycle more.	Give one practical action that lowers emissions of carbon dioxide or another greenhouse gas.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.2.4 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	A carbon footprint includes greenhouse-gas emissions across the whole life cycle, not just use. The other stages are obtaining raw materials, manufacture and packaging, and disposal at the end of life.	Use the four canonical stages: raw materials; manufacture and packaging; use; disposal. Transport can contribute at each stage, but it is not a separate stage in this list.	4
Total Question 1			4
02.1	Cup A $4\,\text{kg}$ smaller	Add every stage using the same boundary: cup A is $3+7+2+4+1=17\,\text{kg}$ and cup B is $5+2+3+10+1=21\,\text{kg}$ . The difference is $21-17=4\,\text{kg}$ .	3
Total Question 2			3
03.1	The two reports use different life-cycle boundaries. Company A omits emissions from raw materials, transport, use and disposal. Both products should be assessed using the same complete life-cycle stages and assumptions.	A fair comparison holds the accounting boundary constant. Identify the omitted stages, then require both totals to cover equivalent life cycles.	3
Total Question 3			3

Tier 3 · Hard

Mark scheme for 4.9.2.4 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	$\text{new footprint}=44\,\text{kg};\ \text{percentage reduction}=18.5\%$	Keep every life-cycle stage in the total: $18+4+30+2=54\,\text{kg};\ 4(0.75)=3\,\text{kg};\ 30(0.60)=18\,\text{kg};\ 21+3+18+2=44\,\text{kg};\ ((54-44)/54)\times100=18.5\%$ . The redesigned manufacture value is $18+3=21\,\text{kg}$ .	5
Total Question 1			5
02.1	Home insulation: $417$ tonnes per year per $\pounds1$ million Methane capture: $443$ tonnes per year per $\pounds1$ million, so its estimate is greater. Any two valid limitations, such as uncertainty in the estimates, different project lifetimes, technical feasibility, public acceptance, infrastructure needs or emissions from installation and maintenance.	Calculate $500/1.2=416.7$ and $310/0.70=442.9$ , giving $417$ and $443$ tonnes per year per $\pounds1$ million. Cost-effectiveness estimates do not include every life-cycle contribution or practical limitation.	6
Total Question 2			6
03.1	At 40 uses both footprints are $3.20\,\text{kg}$ . The reusable bag first has the smaller footprint at 41 uses. Any two valid reasons, such as uncertain manufacturing data, different cleaning methods, transport emissions, repair or early disposal, or different single-use disposal routes.	At $n$ uses, the reusable footprint is $2.40+0.02n$ and the single-use footprint is $0.08n$ . Equality occurs when $2.40=0.06n$ , so $n=40$ ; the reusable option becomes smaller at the next whole use, $41$ . The estimate depends on its life-cycle data and assumptions.	6
Total Question 3			6
04.1	Product A has a possible range from $42$ to $50\,\text{kg}$ . Product B has a possible range from $49$ to $55\,\text{kg}$ . The ranges overlap from $49$ to $50\,\text{kg}$ . The central estimate for A is lower, but the overlap means the data do not show that A is definitely lower. The comparison should check that the data quality, assumptions and treatment of every life-cycle stage are equivalent.	Subtract and add each uncertainty: A spans $46-4=42$ to $46+4=50\,\text{kg}$ , while B spans $52-3=49$ to $52+3=55\,\text{kg}$ . Because both products could lie between $49$ and $50\,\text{kg}$ , the ranking is uncertain even though A has the lower central value.	5
Total Question 4			5
05.1	The four stated stages account for 87.5% of the complete footprint. The complete footprint is $104\,\text{kg}$ . Disposal contributes $13\,\text{kg}$ . The redesigned footprint is $98.5\,\text{kg}$ . The percentage reduction is 5.3%. One valid limitation is uncertainty in the life-cycle data or whether the redesign changes another omitted assumption.	If disposal is 12.5%, the stated stages are 87.5% of the total. Therefore the complete footprint is $91/0.875=104\,\text{kg}$ and disposal is $104-91=13\,\text{kg}$ . The redesign changes disposal to $6.5\,\text{kg}$ and adds $1\,\text{kg}$ elsewhere, so the new total is $91+6.5+1=98.5\,\text{kg}$ . The reduction is $104-98.5=5.5\,\text{kg}$ , giving $(5.5/104)\times100=5.288\ldots\%=5.3\%$ .	6
Total Question 5			6

4.9.3.1 · Atmospheric pollutants from fuels

Tier 1 · Easy

Mark scheme for 4.9.3.1 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	Carbon monoxide	Too little oxygen causes incomplete combustion, which can form carbon monoxide rather than only carbon dioxide.	1
Total Question 1			1
02.1	Sulfur dioxide	Sulfur impurities are oxidised during combustion to form sulfur dioxide.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.3.1 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	Limited oxygen causes incomplete combustion, producing carbon monoxide and carbon particulates or soot. At the engine's high temperature, nitrogen and oxygen from the air react to form oxides of nitrogen.	Keep the two causes separate. Oxygen shortage changes combustion products from the fuel's carbon; high temperature enables normally unreactive nitrogen and oxygen in the air to react.	4
Total Question 1			4
02.1	Too little oxygen causes incomplete combustion, which can produce carbon monoxide. Fuel that does not burn leaves the engine as unburned hydrocarbons.	Use a different cause for each pollutant: restricted oxygen produces carbon monoxide, while fuel that escapes combustion remains as hydrocarbons.	3
Total Question 2			3
03.1	Carbon monoxide and soot are likely to decrease because excess oxygen makes complete combustion more likely. Oxides of nitrogen are likely to increase because the higher temperature allows nitrogen and oxygen from the air to react.	Use oxygen supply to predict the carbon-containing products and temperature to predict nitrogen oxides. The two changes can therefore affect different pollutants in opposite directions.	4
Total Question 3			4

Tier 3 · Hard

Mark scheme for 4.9.3.1 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	$32\,\text{g}$ of sulfur dioxide	Use the sulfur percentage and the fact that each sulfur atom forms one sulfur dioxide molecule: $m(\mathrm{S})=2000(0.0080)=16\,\text{g};\ M_r(\mathrm{SO_2})=32+2(16)=64;\ m(\mathrm{SO_2})=16(64/32)=32\,\text{g}$ . The product-to-sulfur mass ratio is therefore $2:1$ .	5
Total Question 1			5
02.1	Carbon monoxide and soot could form because too little oxygen causes incomplete combustion. Sulfur dioxide should not form from the fuel because the fuel contains no sulfur. Oxides of nitrogen could form because nitrogen and oxygen from the air react at the high engine temperature.	Use oxygen supply for carbon products, fuel composition for sulfur dioxide, and combustion temperature plus air composition for nitrogen oxides.	5
Total Question 2			5
03.1	Fuel A can produce sulfur dioxide because it contains sulfur; fuel B should not produce sulfur dioxide from the fuel. Fuel B is more likely to produce carbon monoxide and soot because too little oxygen causes incomplete combustion. Fuel A is more likely to produce oxides of nitrogen because its high combustion temperature allows nitrogen and oxygen from the air to react.	Compare one controlling variable at a time: sulfur content controls sulfur dioxide, oxygen supply controls incomplete-combustion products, and temperature controls the formation of nitrogen oxides from air.	6
Total Question 3			6
04.1	The oxygen supply probably increased. More oxygen makes complete combustion more likely, explaining the falls in carbon monoxide and soot. The combustion temperature probably increased. A higher temperature allows more nitrogen and oxygen from the air to react, explaining the rise in oxides of nitrogen. The same fuel supplied the same sulfur impurity, so changing oxygen supply and temperature need not remove the sulfur dioxide source.	Reason backwards from each pollutant pattern. Lower carbon monoxide and soot indicate less incomplete combustion and therefore a better oxygen supply. Higher nitrogen oxides indicate a higher temperature. Sulfur dioxide depends primarily on sulfur in the fuel, which was unchanged between tests.	5
Total Question 4			5
05.1	The accounted carbon mass is $440\,\text{g}$ . The unaccounted carbon mass is $60\,\text{g}$ . Incomplete-combustion products contain $140\,\text{g}$ of carbon in total. This is 28% of the carbon in the fuel. Too little oxygen causes incomplete combustion, producing carbon monoxide, soot and unburned hydrocarbons.	The measured products contain $360+55+25=440\,\text{g}$ of carbon, so unburned hydrocarbons contain $500-440=60\,\text{g}$ of carbon. Carbon in incomplete-combustion products totals $55+25+60=140\,\text{g}$ . The percentage is $(140/500)\times100=28\%$ . These products are expected when the oxygen supply is insufficient for complete combustion.	5
Total Question 5			5

4.9.3.2 · Properties and effects of atmospheric pollutants

Tier 1 · Easy

Mark scheme for 4.9.3.2 Tier 1 · Easy
Question	Answers	Extra information	Mark
01.1	It is colourless. It is odourless.	Select the two properties linked to detection by sight and smell. Toxicity explains the danger but not why the gas is hard to notice.	2
Total Question 1			2
02.1	Particulates	Particulates reduce the amount of sunlight reaching the surface and can cause global dimming.	1
Total Question 2			1

Tier 2 · Standard

Mark scheme for 4.9.3.2 Tier 2 · Standard
Question	Answers	Extra information	Mark
01.1	Carbon monoxide is toxic, but it is colourless and odourless, so a person may neither see nor smell a leak.	Award one point for each distinct property: toxic explains the danger, while colourless and odourless explain why normal sight and smell do not warn a person.	3
Total Question 1			3
02.1	Carbon monoxide is toxic. Sulfur dioxide causes respiratory problems. Sulfur dioxide causes acid rain.	Assign each health effect to the correct gas, then distinguish sulfur dioxide as the acid-rain pollutant.	3
Total Question 2			3
03.1	Particulate emissions probably decreased because less global dimming allowed more sunlight to reach the ground. Emissions of sulfur dioxide or oxides of nitrogen may not have decreased enough because the lake acidity did not change.	Use each observation separately: increased sunlight is evidence for fewer particulates, while unchanged acidity gives no evidence of a substantial fall in the gases that cause acid rain.	3
Total Question 3			3

Tier 3 · Hard

Mark scheme for 4.9.3.2 Tier 3 · Hard
Question	Answers	Extra information	Mark
01.1	Sulfur dioxide and/or oxides of nitrogen are consistent with the breathing problems. The same gases can cause acid rain, which is consistent with the lake becoming more acidic. Particulates are consistent with less sunlight reaching the ground because they cause global dimming. The observations indicate more than one type of pollutant rather than a single gas causing every effect.	Match each observation to a specified effect: respiratory problems and acid rain point to sulfur dioxide or nitrogen oxides, while reduced incoming sunlight points to particulates. Because those effect profiles differ, infer a mixture of pollutants.	5
Total Question 1			5
02.1	$85\%$ reduction in particulates Global dimming is most directly reduced because fewer particulates block sunlight from reaching the surface. Substantial sulfur dioxide and nitrogen oxide emissions remain; these gases can cause acid rain and respiratory problems.	The particulate reduction is $80-12=68\,\text{mg}$ , so $(68/80)\times100=85\%$ . Match the large particulate reduction to global dimming, and the remaining gases to acid rain and breathing problems.	5
Total Question 2			5
03.1	Removing carbon monoxide reduces the risk from a toxic, colourless and odourless gas. Sulfur dioxide and oxides of nitrogen can still cause respiratory problems. Sulfur dioxide and oxides of nitrogen can still cause acid rain. Particulates can still harm health and cause global dimming. The exhaust is less harmful in one respect but cannot be described as harmless while these pollutants remain.	Credit the benefit of removing carbon monoxide, then assess the independent health and environmental effects of every pollutant that remains before reaching a qualified conclusion.	5
Total Question 3			5
04.1	The carbon monoxide reading is 9 times greater on day B. The greater visible haze on day A is consistent with more particulates, which can harm health and cause global dimming. Carbon monoxide is colourless and odourless, so little visible haze does not show that its concentration is low. The higher carbon monoxide reading on day B indicates a greater risk from this toxic gas. The two days cannot be ranked as definitely more dangerous without exposure limits, duration and measurements of the other pollutants.	The carbon monoxide factor is $18/2=9$ . Visible haze provides information about particulates, but human sight and smell do not reveal carbon monoxide. The observations therefore identify different hazards, and further exposure and pollutant data are needed before comparing the overall danger.	5
Total Question 4			5
05.1	Respiratory admissions are 14 higher on the high-gas days. This is a 50% increase relative to 28 admissions. Sunlight reaching the ground is 18 units lower on high-particulate days. This is a 20% decrease relative to 90 units. Both associations are consistent with respiratory effects from sulfur dioxide or nitrogen oxides and global dimming from particulates. The observations support but do not prove causation because other changing factors or measurement uncertainty could affect the results.	Admissions increase by $42-28=14$ , and $(14/28)\times100=50\%$ . Sunlight decreases by $90-72=18$ units, and $(18/90)\times100=20\%$ . The directions agree with the specified pollutant effects, while matched observational data still cannot exclude every alternative cause.	6
Total Question 5			6