OCR GCSE Chemistry A: Interpreting and interacting with earth systems (J248) – Revision Notes

For full explanations and five worked examples, read the C6.3 study guide first. Here, C6.3 Interpreting and interacting with earth systems from OCR’s J248 specification for GCSE (9–1) Chemistry A (Gateway Science), Version 4.0 of August 2026, is boiled down for final-weeks revision: statements C6.3a to C6.3g with maths outcomes CM6.3i and CM6.3ii. Both tiers are assessed on all of it, as nothing in C6.3 is Higher Tier only. Topic C6 sits in Paper 2 for Foundation entrants and Paper 4 for Higher entrants.

Useful links: the J248 course hub, a statement checklist to print, the C6.3 practice questions and the free 10-minute diagnostics.

Key terms

Term	Meaning
Outgassing	Release of gases from inside the Earth through volcanoes
Greenhouse gas	A gas that absorbs and re-emits infrared radiation, e.g. water vapour, carbon dioxide, methane
Enhanced greenhouse effect	Extra warming caused by human increases in greenhouse gases
Anthropogenic	Caused by human activity
Correlation	Two variables change together; not on its own proof that one causes the other
Mitigation	Action that reduces the size of a problem, here by cutting emissions or removing gases
Particulates	Tiny solid particles, mainly carbon (soot), suspended in air
Potable water	Water that is safe to drink; it still contains small amounts of dissolved substances
Desalination	Removing dissolved salts from sea water or salty water

C6.3a The first atmosphere

Thought to be: mainly carbon dioxide and water vapour, some nitrogen, possibly a little methane and ammonia, almost no oxygen. Source: volcanic outgassing.

How it changed, in order:

1. Earth cooled; water vapour condensed to form oceans.
2. Carbon dioxide dissolved in the oceans and was locked into carbonate rocks.
3. Photosynthesis removed more carbon dioxide (see C6.3b).
4. Carbon was stored in fossil fuels and sedimentary rocks.
5. Nitrogen, being unreactive, slowly built up to become the main gas.

Air today (dry): about 78% nitrogen, 21% oxygen, 0.9% argon, 0.04% carbon dioxide.

Evidence is indirect. No direct samples exist. Scientists use volcanic gases today, old rocks and models, so conclusions are “thought” or “suggested”, not known.

C6.3b Oxygen

• Cyanobacteria (and later algae and plants) photosynthesise: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
• Early oxygen reacted with dissolved iron, forming banded iron formations (evidence).
• Oxygen built up in the air once the iron was used up; the first big rise was about 2.4 billion years ago.
• An ozone layer formed, absorbing ultraviolet radiation.
• The build-up took billions of years (orders of magnitude, M2h).

C6.3c Greenhouse effect: method in steps

1. Sunlight, mostly short-wavelength, travels through the air largely unabsorbed.
2. Land and sea take in this energy and heat up.
3. The surface emits longer-wavelength infrared radiation.
4. Greenhouse gas molecules absorb infrared and re-emit it in all directions.
5. Some returns to the surface, so less energy escapes to space and the surface stays warmer.

The natural effect keeps the Earth habitable. Human activity enhances it. Nitrogen and oxygen hardly absorb infrared.

C6.3d Evidence and uncertainty

Evidence for a human cause:

• Carbon dioxide rose from about 280 ppm before industry to a monthly average over 430 ppm in 2025.
• The rise correlates with fossil fuel burning.
• Average temperatures have risen at the same time.
• Ice cores show carbon dioxide and temperature moving together in the past.
• There is a known mechanism (C6.3c).

Uncertainties:

• Correlation alone does not prove cause.
• Natural factors also change climate: the Sun’s output, volcanoes, ocean cycles.
• Past data come from proxies (ice cores, tree rings), with larger error.
• Fewer measuring sites existed in the past.
• Models simplify complex systems such as clouds.

Reading data (CM6.3i): for a table or graph, quote values with units, calculate a change or rate (change ÷ time), state the trend, then comment on whether the rate is increasing.

Converting units: 1% = 10 000 ppm. So ppm ÷ 10 000 = %, and % × 10 000 = ppm.

C6.3e Effects and mitigation

Effect of more CO₂ and CH₄	Mitigation
Higher average temperature	Renewable and nuclear electricity
Sea-level rise (thermal expansion, melting land ice)	Better insulation and efficient vehicles
More extreme weather: heatwaves, storms, floods	Carbon capture and storage
Drought and changed rainfall; lower crop yields in some places	Tree planting
Species and habitats shift	Capture methane from landfill; cut gas leaks

Evaluating a mitigation: say how it reduces emissions, then give a limitation (cost, scale, land use, reliability, public acceptance), then judge. Scale matters: actions by one person or one country are small against global emissions. Risk: rare but severe outcomes still need planning for.

Methane sources: cattle, rice paddies, landfill, gas extraction. Methane traps more heat per molecule than carbon dioxide.

C6.3f Pollutants

Pollutant	Formed by	Problem
CO	Incomplete combustion of carbon fuels	Toxic; reduces oxygen carried by blood; no colour or smell
SO₂	Burning sulfur impurities in fuels: S + O₂ → SO₂	Acid rain; lung irritation
NO, NO₂	N₂ + O₂ → 2NO at high engine temperatures; 2NO + O₂ → 2NO₂	Acid rain; breathing problems; smog
Particulates	Incomplete combustion, especially diesel	Lung and heart damage; blackened buildings; global dimming

Small worked reminder. The products of burning a hydrocarbon depend on the oxygen supply. Plenty of oxygen gives CO₂ and H₂O; less gives CO; very little gives C (soot). Balance each by counting oxygen atoms last.

Mass reminder. Sulfur dioxide has Mr 64 and sulfur has Ar 32, so each gram of sulfur burned gives 2 g of sulfur dioxide.

C6.3g Potable water

Source	Treatment	Ease
Ground water (aquifers)	Little needed; sterilise (may filter)	Easiest
Fresh surface water	Sedimentation → filtration (sand, gravel) → sterilisation (chlorine, ozone or UV)	Fairly easy
Waste water	Screening, settling, biological breakdown of organic matter, then filtration and sterilisation	Harder, more stages
Salt water	Desalination: distillation or reverse osmosis	Hardest; needs most energy

Distillation separates water by boiling and condensing. Reverse osmosis pushes water through a membrane under high pressure. Distillation and filtration are separation techniques from C2.1 (see the C2.1 separation methods page).

Must-know distinctions

• Greenhouse effect vs ozone depletion. Infrared absorbed by greenhouse gases warms the Earth; ozone loss lets in more ultraviolet. Different problems.
• Natural vs enhanced greenhouse effect. The first is essential for life; the second is the human addition.
• Correlation vs causation. Data that rise together need a mechanism before you claim a cause.
• Potable vs pure. Potable water is safe to drink; pure water contains only water molecules.
• Sulfur dioxide vs oxides of nitrogen. SO₂ comes from sulfur in the fuel; NOₓ comes from nitrogen in the air.

Quick self-test

1. Name the process that released the gases of the early atmosphere.
2. Why did the oceans form as the Earth cooled?
3. Give two ways carbon dioxide was removed from the early atmosphere.
4. Write the balanced equation for photosynthesis.
5. Which type of radiation is absorbed by greenhouse gases?
6. Convert 0.038% carbon dioxide into ppm.
7. Convert 1.9 ppm of methane into a percentage.
8. A 500 cm³ sample of dry air is 78% nitrogen. What volume is nitrogen?
9. What mass of carbon dioxide forms when 6 g of carbon burns completely? (C = 12, O = 16)
10. What mass of sulfur dioxide forms when 16 g of sulfur burns? (S = 32, O = 16)
11. Why do car engines release oxides of nitrogen even though petrol contains no nitrogen?
12. Which source of water needs the most energy to make it potable, and why?

Answers

1. Volcanic activity (outgassing).
2. Water vapour condensed when the temperature fell below its boiling point.
3. Any two: dissolving in oceans; forming carbonate rocks; photosynthesis; locking into fossil fuels.
4. 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
5. Infrared (long wavelength).
6. 0.038 × 10 000 = 380 ppm.
7. 1.9 ÷ 10 000 = 0.00019%.
8. 0.78 × 500 = 390 cm³.
9. 6 g is 0.5 mol of carbon, giving 0.5 mol of CO₂ = 0.5 × 44 = 22 g.
10. 16 g is 0.5 mol of sulfur, giving 0.5 mol of SO₂ = 0.5 × 64 = 32 g.
11. Inside the hot engine, atmospheric nitrogen combines with oxygen.
12. Salt water; desalination by distillation or reverse osmosis uses a lot of energy.

Where marks are usually lost

• Writing “greenhouse gases reflect heat” instead of “absorb and re-emit infrared radiation”.
• Missing out the change in wavelength: incoming short wavelength, outgoing infrared.
• Saying the early atmosphere contained oxygen from volcanoes.
• Forgetting that oxygen first reacted with iron before building up in the air.
• Describing a data trend without quoting any values or units.
• Concluding that a correlation proves humans cause climate change, with no mention of mechanism or uncertainty.
• Giving a mitigation with no limitation when the command word is “evaluate”.
• Mixing up the sources of SO₂ (fuel) and NOₓ (air).
• Calling potable water “pure” or “clean water with nothing in it”.
• Saying the ozone hole causes global warming.

Official syllabus

This page follows OCR’s GCSE (9–1) Chemistry A (Gateway Science) specification J248, Version 4.0 (August 2026), first assessed in 2018, published by OCR. Content used: C6.3 Interpreting and interacting with earth systems (statements C6.3a–C6.3g; maths outcomes CM6.3i and CM6.3ii).