AQA GCSE Chemistry 8462: Chemistry of the atmosphere – Revision Notes

These revision notes cover Topic 9, Chemistry of the atmosphere (sections 4.9.1 to 4.9.3), of the AQA GCSE Chemistry (8462) specification, for teaching from September 2016 with GCSE exams from June 2018 (version 1.1). The topic is examined on Paper 2 at both Foundation and Higher tier. No statement in this topic is Higher tier only.

For full explanations and worked examples, read the Chemistry of the atmosphere study guide first. Then test yourself with the practice questions. The course hub is AQA GCSE Chemistry and the printable checklist lets you tick off each statement. The next topic is Using resources.

Key definitions

Term	Meaning
Greenhouse gas	A gas that absorbs long wavelength radiation emitted by the Earth’s surface: water vapour, carbon dioxide, methane
Greenhouse effect	Greenhouse gases keep the lower atmosphere warm enough to support life
Peer review	Other scientists check methods and conclusions before results are published
Carbon footprint	Total amount of carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event
Particulates	Solid particles and unburned hydrocarbons released into the air when fuels burn
Global dimming	Less sunlight reaching the Earth’s surface because of particulates
Incomplete combustion	Burning with limited oxygen, forming carbon monoxide and/or soot

4.9.1 Composition and evolution

Today (last 200 million years): about 80% (four-fifths) nitrogen, about 20% (one-fifth) oxygen, small amounts of carbon dioxide, water vapour and noble gases.

The theory you must know, in order:

1. First billion years: intense volcanic activity.
2. Volcanoes released gases (mainly carbon dioxide) and water vapour; little or no oxygen, like Mars and Venus today.
3. Water vapour condensed to form the oceans.
4. Volcanoes also released nitrogen, which built up. Maybe small amounts of methane and ammonia.
5. Carbon dioxide dissolved in the oceans; carbonates precipitated as sediments.
6. About 2.7 billion years ago algae began photosynthesis; oxygen appeared.
7. Plants evolved; oxygen rose over the next billion years to a level that allowed animals to evolve.
8. Carbon dioxide fell further through photosynthesis and by forming sedimentary rocks and fossil fuels.

Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Change in each gas over time

Gas	Early atmosphere	Today	Main cause of the change
Carbon dioxide	Most of it	Small proportion	Dissolved in oceans; photosynthesis; locked in sedimentary rocks and fossil fuels
Oxygen	Little or none	About 20%	Photosynthesis by algae, then plants
Nitrogen	Small, building up	About 80%	Released by volcanoes; very unreactive, so it stayed
Water vapour	Large amount	Small, variable	Condensed to form the oceans

Deposits that lock up carbon

• Limestone: calcium carbonate shells and skeletons of sea creatures, compressed on the sea bed.
• Coal: buried plant material, compressed and heated over millions of years.
• Crude oil and natural gas: buried plankton in sea-bed mud, changed by heat and pressure.

Evaluating a theory – method in steps

1. State what the theory claims.
2. Pick out the evidence given that supports it.
3. Pick out any evidence against it, or gaps.
4. Explain why evidence is limited (4.6 billion years; few direct samples).
5. Give a judgement: supported, but not proven.

4.9.2 Greenhouse gases and climate change

Greenhouse effect in five steps

1. Sun emits short wavelength radiation.
2. It passes through the atmosphere; the surface absorbs it and warms.
3. Surface emits long wavelength (infrared) radiation.
4. Greenhouse gases absorb long wavelength radiation and re-emit it, some back towards the Earth.
5. Less energy escapes, so the atmosphere is warmer.

Two human activities per gas (learn at least two for each)

Carbon dioxide	Methane
Burning fossil fuels	Cattle farming (digestion)
Deforestation	Rice paddy fields
–	Decay of waste in landfill

Evidence and uncertainty

• Many scientists believe, based on peer-reviewed evidence, that human activities will warm the surface atmosphere and cause climate change.
• Climate is complex, so models are simplified.
• Media reports may use part of the evidence and may be biased.
• Uncertainty: past temperatures and gas levels are measured indirectly; models rely on assumptions; natural cycles also affect climate.
• Peer review and communicating results to a wide audience help others check and trust the science.

Percentage change – worked reminder. A reading rises from 60 units to 75 units. Change = 75 − 60 = 15. Percentage increase = 15 ÷ 60 × 100 = 25%. Always divide by the starting value, and show the subtraction so the method mark is safe.

Four effects of climate change (any four)

• Sea level rise: flooding, coastal erosion.
• More frequent or severe storms.
• Changes in rainfall: droughts or floods.
• Temperature and water stress for people and wildlife.
• Changes to food production in some regions.
• Changes to species distribution; some extinctions.

Reducing the carbon footprint vs why limited

Action	Limitation
Renewable and nuclear energy	Cost; opposition; weather-dependent supply
Energy saving	Up-front cost; habits
Carbon capture and storage	Expensive; still developing
Carbon taxes and limits	Unpopular; needs international agreement
Off-setting and tree planting	Land needed; slow

4.9.3 Pollutants

Which element makes which pollutant

Element in fuel or air	Product
Carbon, plenty of oxygen	Carbon dioxide
Carbon, limited oxygen	Carbon monoxide, soot (C)
Hydrogen	Water vapour
Sulfur impurity	Sulfur dioxide
Nitrogen from the air, high temperature	Oxides of nitrogen (NO, NO₂)

Balancing a combustion equation – method in steps

1. Write the fuel and O₂ on the left and the products on the right.
2. Balance C, then H.
3. Count O atoms on the right. Divide by 2 for O₂.
4. If you get a half, double everything.

Small reminder: C₂H₆ burning to carbon monoxide gives 2C₂H₆ + 5O₂ → 4CO + 6H₂O (right side: 4 + 6 = 10 O atoms, so 5O₂).

Effects

Pollutant	Effect
Carbon monoxide	Toxic; colourless and odourless, so hard to detect
Sulfur dioxide	Respiratory problems; acid rain
Oxides of nitrogen	Respiratory problems; acid rain
Particulates	Global dimming; health problems

Must-know distinctions

• Greenhouse effect vs climate change. The greenhouse effect is natural and needed for life. An increase in greenhouse gases enhances it and raises average temperature.
• Short vs long wavelength. Incoming sunlight is short; radiation from the Earth is long. Greenhouse gases absorb the long.
• Carbon monoxide vs carbon dioxide. CO is toxic and forms from incomplete combustion; CO₂ is a greenhouse gas from complete combustion.
• Acid rain vs global dimming. Acid rain: SO₂ and NOₓ. Global dimming: particulates.
• Oxygen increase vs carbon dioxide decrease. Oxygen rose only by photosynthesis. Carbon dioxide fell by photosynthesis, dissolving in oceans, and forming rocks and fossil fuels.

Quick self-test

1. What fraction of today’s atmosphere is oxygen?
2. A 500 cm³ sample of dry air is analysed. Estimate the volume of nitrogen.
3. Name the main gas in the early atmosphere.
4. How did the oceans form?
5. Name the process that increased oxygen, and the organisms that first carried it out.
6. Name three greenhouse gases.
7. What type of radiation do greenhouse gases absorb?
8. Give two human activities that increase methane.
9. Define carbon footprint.
10. Why does carbon monoxide often go unnoticed?
11. Where does the nitrogen in nitrogen oxides from a car engine come from?
12. Balance: CH₄ + O₂ → C + H₂O.

Answers

1. About one-fifth (about 20%).
2. Four-fifths of 500 = 400 cm³.
3. Carbon dioxide.
4. Water vapour from volcanoes condensed.
5. Photosynthesis; algae.
6. Water vapour, carbon dioxide, methane.
7. Long wavelength (infrared) radiation from the Earth’s surface.
8. Any two of: cattle farming, rice paddy fields, landfill waste decay.
9. The total carbon dioxide and other greenhouse gases emitted over the full life cycle of a product, service or event.
10. It is colourless and odourless.
11. The air – nitrogen and oxygen react at high temperature.
12. CH₄ + O₂ → C + 2H₂O (1 C, 4 H and 2 O on each side).

Where marks are usually lost

• Writing “the early atmosphere had lots of oxygen”. It had little or none.
• Describing the greenhouse effect as “heat bouncing off gases” or “the ozone layer trapping heat” instead of absorption of long wavelength radiation.
• Naming only one human activity for a gas when the question asks for two.
• Giving “car exhaust” for methane. Car engines are a carbon dioxide source.
• Linking carbon monoxide to acid rain, or sulfur dioxide to global dimming.
• Forgetting that nitrogen oxides need high temperature and air, not nitrogen in the fuel.
• Listing climate effects with no consequence when asked to “describe” (e.g. “sea levels rise” with no flooding or erosion).
• Giving reasons why carbon footprint actions are limited that are too vague (“it’s hard”). Name cost, cooperation or lifestyle.
• Percentage change divided by the final value instead of the starting value.
• Describing coal formation as “from dead animals”. Coal came from plants.

Official syllabus

AQA GCSE Chemistry (8462) specification, for teaching from September 2016, GCSE exams June 2018 onwards, version 1.1, published by AQA – section 4.9 Chemistry of the atmosphere.