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

This study guide teaches C6.3 Interpreting and interacting with earth systems, the final sub-topic of Topic C6 Global challenges in OCR’s J248 specification for GCSE (9–1) Chemistry A (Gateway Science), Version 4.0 of August 2026. It works through learning outcomes C6.3a to C6.3g plus the mathematical learning outcomes CM6.3i and CM6.3ii. Every C6.3 statement is assessed on both tiers; none is Higher Tier only. According to the assessment overview, Topic C6 is tested in Paper 2 for Foundation entrants and Paper 4 for Higher entrants. No practical suggestions are listed for C6.3.

Every other J248 topic is reachable from the course hub, and each statement appears on the J248 printable checklist. A free 10-minute chemistry diagnostic shows where to start.

What C6.3 asks you to do

Ref	Skill you need	Tier
C6.3a	Interpret evidence for how the first atmosphere formed and changed	Both tiers
C6.3b	Describe how an oxygen-rich atmosphere is thought to have developed	Both tiers
C6.3c	Describe the greenhouse effect as radiation interacting with matter in the atmosphere	Both tiers
C6.3d	Evaluate evidence for human causes of climate change; describe the uncertainties	Both tiers
C6.3e	Describe effects of more carbon dioxide and methane, and how they may be mitigated	Both tiers
C6.3f	Sources of, and problems caused by, CO, SO₂, oxides of nitrogen and particulates	Both tiers
C6.3g	Increase the supply of potable water using separation techniques	Both tiers
CM6.3i	Extract and interpret information from charts, graphs and tables	Both tiers
CM6.3ii	Use orders of magnitude to judge how significant data are	Both tiers

The specification flags three misconceptions: that small rises in carbon dioxide cannot matter in so large an atmosphere, that the ozone hole causes global warming, and that humans alone cause the greenhouse effect. Each is corrected below.

How the first atmosphere formed (C6.3a)

The Earth formed about 4.5 billion years ago. Its early atmosphere is thought to have come from gases released by volcanoes, and to have been mostly carbon dioxide and water vapour, with some nitrogen, possibly small amounts of methane and ammonia, and little or no oxygen.

As the surface cooled below 100 °C, water vapour condensed and fell as rain, forming the oceans. Carbon dioxide then fell steadily because:

1. it dissolved in the oceans and ended up in carbonate rocks such as limestone;
2. photosynthesis by early organisms turned it into carbon compounds;
3. carbon from living things was locked away in fossil fuels and sedimentary rocks.

Unreactive nitrogen built up slowly and was not removed. Today dry air is about 78% nitrogen, 21% oxygen, just under 1% argon and roughly 0.04% carbon dioxide.

Why “it is thought”. Nobody measured the early atmosphere. The evidence is indirect: gases from volcanoes today, the chemistry of very old rocks, and models of a cooling planet. Use cautious words such as “suggests”.

Worked example 1 (orders of magnitude, CM6.3ii). One model of the early atmosphere (an illustrative figure) gives carbon dioxide as 70% of the gas. The air at Mauna Loa in May 2025 averaged about 430 parts per million (ppm) of carbon dioxide. Compare the two.

• 1% = 10 000 ppm, so 430 ppm = 430 ÷ 10 000 = 0.043%
• ratio = 70 ÷ 0.043 ≈ 1600
• 1600 is between 10³ and 10⁴, closest to 10³

The early proportion was roughly three orders of magnitude (about a thousand times) larger.

How oxygen built up (C6.3b)

Early oceans contained simple organisms. When cyanobacteria and later algae evolved, they began photosynthesis:

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

At first the oxygen reacted with dissolved iron in seawater, so little reached the air. Iron oxides settled out as banded iron formations, layered rocks that are part of the evidence. Once the iron was used up, oxygen collected in the atmosphere. The first large rise, called the Great Oxidation Event, is dated to about 2.4 billion years ago.

Oxygen then rose slowly to levels that could support animals. On time-scales (M2h): the oxygen build-up took billions of years, while the recent rise in carbon dioxide took about two hundred years, roughly seven orders of magnitude shorter. Ideas about this history have changed as new evidence, such as rock dating, appeared (Working Scientifically 1.1a).

The greenhouse effect (C6.3c)

The greenhouse effect is about how radiation interacts with molecules in the atmosphere.

1. The Sun emits mostly short-wavelength radiation (visible light and some ultraviolet). Most of it passes through the atmosphere and is absorbed by the Earth’s surface, which warms up.
2. The warm surface emits longer-wavelength infrared radiation.
3. Greenhouse gases, such as water vapour, carbon dioxide and methane, absorb some of this infrared radiation. Their molecules then re-emit the energy, sending part of it down to the ground again.
4. Less energy escapes to space, so the lower atmosphere and the surface are warmer than they would otherwise be.

Nitrogen and oxygen, the main gases in air, absorb almost none of this infrared radiation.

Two corrections:

• The greenhouse effect is natural. Without it the average surface temperature would be around −18 °C instead of about 15 °C. Humans are enhancing it.
• The ozone hole lets more ultraviolet reach the ground. It does not cause global warming.

Evidence for human-caused climate change (C6.3d)

The key evidence in the specification is the correlation between carbon dioxide in the atmosphere and the burning of fossil fuels. Carbon dioxide was about 280 ppm before the Industrial Revolution; its monthly average passed 430 ppm in 2025. Over the same period, fossil fuel use rose steeply.

Other evidence supports the link: average temperatures have risen alongside carbon dioxide, and air bubbles in ice cores show past carbon dioxide levels and temperatures moving together.

Worked example 2 (reading a table, CM6.3i). The table gives rounded annual mean carbon dioxide levels at one monitoring station.

Year	Carbon dioxide / ppm
1970	326
1990	354
2010	390
2020	414

Calculate the average yearly rise in each interval and describe the trend.

• 1970–1990: (354 − 326) ÷ 20 = 28 ÷ 20 = 1.4 ppm per year
• 1990–2010: (390 − 354) ÷ 20 = 36 ÷ 20 = 1.8 ppm per year
• 2010–2020: (414 − 390) ÷ 10 = 24 ÷ 10 = 2.4 ppm per year

The level rises in every interval and the rate of rise is increasing. Overall, the 1970–2020 increase is 88 ppm, about 27% of the 1970 value.

Uncertainties you must be able to describe:

• Correlation is not proof of cause. Two quantities can rise together for other reasons. The case is strong because there is also a mechanism (C6.3c).
• Natural factors also change climate: variation in the Sun’s output, large volcanic eruptions and ocean cycles.
• Older data are indirect. Ice cores, tree rings and sediments are proxies, and early thermometer records covered fewer places.
• Climate models are simplifications. Clouds and feedbacks, such as melting ice reflecting less sunlight, are hard to model precisely.

Effects and mitigation (C6.3e)

Carbon dioxide and methane are both greenhouse gases. Methane comes from cattle, rice paddies, landfill and gas leaks. Its level has more than doubled since pre-industrial times, and each molecule warms more than a carbon dioxide molecule.

Potential effects of increased levels:

• rising average temperatures;
• sea-level rise, from water expanding as it warms and from melting land ice, flooding low-lying coasts;
• more frequent or more severe extreme weather, such as heatwaves, droughts and storms;
• changes in where and how much rain falls, affecting food production and water supplies;
• changes to habitats, so some species move or decline.

Mitigation:

Approach	How it helps	A limitation
Renewable or nuclear electricity	Less fossil fuel burned	Cost, intermittency, public concern
Energy efficiency (insulation, efficient vehicles)	Less energy needed	Needs investment by households and firms
Carbon capture and storage	Carbon dioxide stored underground instead of released	Expensive; not yet used widely
Planting trees	Photosynthesis removes carbon dioxide	Needs large areas of land; slow
Cutting methane (capturing landfill gas, fixing gas leaks)	Methane has a strong warming effect per molecule	Hard to control emissions from farming
Carbon taxes and emission limits	Make low-carbon choices cheaper	Political disagreement; need international agreement

Weigh scale, risk and environmental implications (Working Scientifically 1.1f, Working Scientifically 1.1h): one country acting alone changes little; some effects are unlikely but very serious; some fixes, such as large dams, bring problems of their own.

Air pollutants (C6.3f)

Pollutant	Main source	Problems caused
Carbon monoxide, CO	Incomplete combustion of carbon-containing fuels, e.g. engines, faulty boilers	Toxic: combines with haemoglobin so blood carries less oxygen; colourless and odourless
Sulfur dioxide, SO₂	Sulfur impurities in fossil fuels, especially coal and some fuel oils, burning: S + O₂ → SO₂	Causes acid rain, which damages buildings, trees and lakes; irritates the lungs
Oxides of nitrogen, NO and NO₂	At engine temperatures, atmospheric nitrogen combines with oxygen: N₂ + O₂ → 2NO, then 2NO + O₂ → 2NO₂	Acid rain; breathing problems; photochemical smog
Particulates (soot)	Incomplete combustion, especially in diesel engines	Lung and heart disease; darken buildings; can reduce sunlight reaching the ground (global dimming)

Oxides of nitrogen do not come from nitrogen in the fuel. They form from the air.

Worked example 3. Pentane, C₅H₁₂, burns in a poorly ventilated heater. Write equations for burning in plenty of air, in limited air forming carbon monoxide, and in very limited air forming carbon.

• plenty of air: C₅H₁₂ + 8O₂ → 5CO₂ + 6H₂O
• limited air: 2C₅H₁₂ + 11O₂ → 10CO + 12H₂O
• very limited: C₅H₁₂ + 3O₂ → 5C + 6H₂O

Check the oxygen atoms each time: 16 = 10 + 6; 22 = 10 + 12; 6 = 6.

Worked example 4. A school boiler burns 1200 kg of heating oil containing 0.80% sulfur by mass. All the sulfur becomes sulfur dioxide. Relative atomic masses: S = 32, O = 16. Calculate the mass of sulfur dioxide.

• mass of sulfur = 1200 × 0.80 ÷ 100 = 9.6 kg
• Mr of SO₂ = 32 + 16 + 16 = 64, so 32 kg of S gives 64 kg of SO₂
• mass of SO₂ = 9.6 × 64 ÷ 32 = 19.2 kg

Potable water (C6.3g)

Potable water is water that is safe to drink. It is not pure: it still contains low levels of dissolved salts. The method depends on the source.

Ground water and fresh surface water (aquifers, rivers, reservoirs):

1. Sedimentation: water stands so that larger solid particles settle.
2. Filtration through beds of sand and gravel removes smaller insoluble particles.
3. Sterilisation with chlorine, ozone or ultraviolet light kills microbes.

Waste water needs extra stages first: screening out large objects and grit, settling, and bacteria breaking down organic matter.

Salt water needs desalination:

• distillation: boil the water and condense the steam, leaving the salts behind;
• reverse osmosis: force water at high pressure through a membrane that lets water molecules through but holds back salt ions.

Ease of treatment. Ground water is usually easiest, as rock has already filtered it. Waste water needs more stages. Salt water is hardest because desalination needs a lot of energy. Distillation and filtration are the same separation techniques you met in C2.1; see the C2.1 separation methods page.

Worked example 5. A coastal town needs 20 000 m³ of water a day. Use these values: reverse osmosis uses 3 kWh per m³ and distillation uses 30 kWh per m³. Compare the daily energy.

• reverse osmosis: 20 000 × 3 = 60 000 kWh
• distillation: 20 000 × 30 = 600 000 kWh
• ratio = 30 ÷ 3 = 10

Distillation needs about ten times as much energy: one order of magnitude more.

Common errors

• Writing that greenhouse gases “reflect” heat. They absorb and re-emit infrared radiation.
• Linking global warming to the ozone hole.
• Saying oxides of nitrogen come from nitrogen in the fuel.
• Calling potable water “pure”.

Keep going

Next: the C6.3 revision notes and the C6.3 practice set. For balancing equations, revisit the J248 equations page.

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).