What are the main gases present in the Earth's atmosphere?

The Earth's atmosphere is primarily composed of nitrogen (approximately 78%) and oxygen (about 21%). Other gases present in smaller amounts include argon (around 0.93%), carbon dioxide (approximately 0.04%), and trace amounts of other gases like neon, helium, methane, krypton, and hydrogen. Understanding these gases is essential for the Edexcel IGCSE Chemistry curriculum, particularly in the study of Inorganic Chemistry.

How does the composition of gases in the atmosphere affect life on Earth?

The composition of gases in the atmosphere is crucial for sustaining life on Earth. Oxygen is essential for respiration in most living organisms, while carbon dioxide is used by plants during photosynthesis to produce oxygen. Nitrogen is a key component of amino acids and proteins. The balance of these gases helps regulate Earth's climate and supports various life processes, a concept explored in the Edexcel IGCSE Chemistry syllabus.

Why is carbon dioxide considered a greenhouse gas?

Carbon dioxide is considered a greenhouse gas because it traps heat in the Earth's atmosphere, contributing to the greenhouse effect. This process helps maintain Earth's temperature but can lead to global warming if CO2 levels rise significantly. Understanding the role of carbon dioxide and other greenhouse gases is an important aspect of studying Gases in the Atmosphere within the Edexcel IGCSE Inorganic Chemistry curriculum.

What is the significance of the ozone layer in the atmosphere?

The ozone layer, located in the Earth's stratosphere, plays a critical role in protecting life by absorbing the majority of the sun's harmful ultraviolet (UV) radiation. This protection helps prevent skin cancer and other health issues in humans and protects ecosystems. The study of the ozone layer and its importance is a key topic in the Edexcel IGCSE Chemistry course, particularly under Gases in the Atmosphere.

How do human activities impact the composition of gases in the atmosphere?

Human activities, such as burning fossil fuels, deforestation, and industrial processes, significantly impact the composition of gases in the atmosphere. These activities increase levels of carbon dioxide, methane, and other pollutants, contributing to climate change and air pollution. Understanding these impacts is crucial for students studying Edexcel IGCSE Chemistry, as it highlights the intersection of human activity and atmospheric chemistry.

Why is "Confusing CO and CO₂ as products of combustion" a common mistake in Gases in the Atmosphere ?

Why it happens: Both contain C and O — easy to swap. How to avoid it: CO₂ (carbon DIoxide) — from COMPLETE combustion (excess O₂); NOT toxic at normal levels. CO (carbon MONoxide) — from INCOMPLETE combustion (limited O₂); HIGHLY TOXIC. Memorise: DI = dignified (safe-ish); MONO = monster (deadly). Source: 4CH1 Examiner Reports 2022-2024

Why is "Confusing the greenhouse effect with ozone layer depletion" a common mistake in Gases in the Atmosphere ?

Why it happens: Both are 'atmospheric problems' caused by human emissions. How to avoid it: GREENHOUSE EFFECT = CO₂, CH₄, H₂O absorb IR → trap heat → climate change. OZONE LAYER = O₃ in stratosphere blocks UV; depleted by CFCs → skin cancer. They are SEPARATE problems with different gases and consequences.

Why is "Writing O₂ = 78% and N₂ = 21% (swapping the values)" a common mistake in Gases in the Atmosphere ?

Why it happens: Knowing the numbers but not which gas they belong to. How to avoid it: NITROGEN is the MAJORITY gas at 78%. OXYGEN is 21% (≈ one-fifth). The reactive gas (O₂) is the MINORITY. Memorise: 'NITROGEN, NEARLY FOUR-FIFTHS; OXYGEN, ONE-FIFTH'. Source: 4CH1 Examiner Reports 2023-2024

Why is "Saying CO₂ causes acid rain" a common mistake in Gases in the Atmosphere ?

Why it happens: Knowing CO₂ is acidic in water — but its contribution is minor at natural levels. How to avoid it: ACID RAIN is caused by SO₂ (→ H₂SO₃/H₂SO₄) and NOₓ (→ HNO₃). CO₂ does slightly acidify rain (pH ~5.6) but this is the NATURAL background, not acid rain. The pollution-driven acid rain is below pH 4. Source: 4CH1 Examiner Reports 2022-2024

Why is "Reading the volume of gas while still hot in the oxygen experiment" a common mistake in Gases in the Atmosphere ?

Why it happens: Forgetting that gases expand with temperature. How to avoid it: ALWAYS allow the apparatus to COOL to room temperature before reading the final volume. Hot gas occupies more volume than cool gas — a hot reading would underestimate the % O₂ used.

Why is "Omitting water as a product of combustion" a common mistake in Gases in the Atmosphere ?

Why it happens: Focusing on CO₂ / CO as the carbon products. How to avoid it: Hydrocarbons contain BOTH C and H. Both must oxidise: C → CO₂ (or CO); H → H₂O. Always include water in the equation: CH₄ + 2O₂ → CO₂ + 2H₂O.

Inorganic ChemistryEdexcel IGCSE Chemistry (4CH1)

Gases in the Atmosphere

Work through the notes, try the practice questions, then take the quiz. The report tells you exactly what to revise next.

Previous Next

Learn this Topic

Start with the slides for the quick version, then go deeper with the full study notes.

Short Study Notes in the form of Slides

Read the notes first. If the method in a worked example clicks, you're ready for the questions.

Page 1 / 0

Detailed Notes

Full prose, callouts and a recap — built for A* mastery, not just a quick scan.

Take these study notes with you

Download a branded PDF — full prose, callouts, recap and memorise list for Gases in the Atmosphere , ready to print or save offline.

Gases in the Atmosphere — Pearson Edexcel International GCSE Chemistry 4CH1 Study Notes (2026 onwards, Spec Issue 4)

Composition of dry air (N₂ 78%, O₂ 21%, Ar 1%, CO₂ 0.04%). Practical determination of % oxygen by passing air over heated copper. Combustion (complete vs incomplete), products and energy implications. Acid rain (SO₂ + NOₓ) and the enhanced greenhouse effect (CO₂, CH₄) — causes, effects, and mitigation strategies.

What you’ll learn

Mapped to the Cambridge IGCSE 4CH1 syllabus (2026 onwards).

2.6 — Recall the approximate proportions by volume of the four most abundant gases in dry air (N₂ 78%, O₂ 21%, Ar ~1%, CO₂ ~0.04%).
2.7 — Describe how experiments using heated copper can be used to determine the approximate percentage of oxygen in air.
2.8 — Describe the combustion of elements in oxygen, including hydrogen, carbon and magnesium, and the acid-base nature of their oxides.
2.9 — Describe the formation of carbon dioxide from the thermal decomposition of metal carbonates such as calcium carbonate.
2.10 — Describe the products of complete and incomplete combustion of a hydrocarbon (such as methane).
2.11 — Explain why incomplete combustion of carbon-containing fuels may produce carbon monoxide, and explain why CO is toxic to humans.
2.12 — Describe the formation of sulfur dioxide and nitrogen oxides from burning fuels, and explain how these gases cause acid rain.
2.13 — Explain how the increased combustion of fossil fuels and the increased concentration of greenhouse gases (carbon dioxide, methane, water vapour) in the atmosphere contribute to climate change.

Composition of dry air (spec 2.6)

78% N₂, 21% O₂, ~1% Ar, ~0.04% CO₂. Plus variable H₂O and trace gases.

Dry air — the well-mixed gases of the atmosphere with water vapour removed — has a remarkably stable composition near the surface:

Gas	Formula	% by volume
Nitrogen	N₂	≈ 78%
Oxygen	O₂	≈ 21%
Argon	Ar	≈ 0.93% (≈ 1%)
Carbon dioxide	CO₂	≈ 0.04% (~420 ppm and rising)
Other (Ne, He, Kr, Xe, CH₄, H₂)	—	trace amounts

Why N₂ is essentially constant. Nitrogen is by far the largest reservoir (78%). The N≡N triple bond is very strong (~945 kJ/mol), so N₂ is generally unreactive. The natural rates of nitrogen fixation (by lightning, bacteria, and the Haber process) and denitrification are tiny compared with the atmospheric reservoir, so the % stays essentially constant.

Why CO₂ varies. Carbon dioxide is only 0.04% — a small reservoir. It is involved in many fast processes:

Removed by: photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂).
Released by: respiration (reverse of photosynthesis), combustion of fossil fuels and biomass, volcanic activity.
Exchanged with: the oceans (CO₂ dissolves more in cold water).

Because the inflows and outflows are large relative to the small reservoir, CO₂ levels vary seasonally (lower in summer when photosynthesis dominates), geographically (higher near cities and fires), and historically (rising from ~280 ppm in 1800 to >420 ppm today due to human activity).

Water vapour. Real atmospheric air contains 0-4% water vapour, depending on temperature and humidity. The 'dry' composition above is air with this variable amount of H₂O removed.

Where does the air come from? Today's atmosphere evolved over billions of years. The original atmosphere (4 billion years ago) was mostly CO₂ + N₂ + water vapour from volcanic outgassing. The current ~21% O₂ was produced over ~2 billion years by photosynthetic cyanobacteria and later plants. The current N₂ is the long-standing 'leftover' that doesn't react easily.

N₂ 78%, O₂ 21%, Ar ~1%, CO₂ ~0.04% — memorise these.
N₂ constant (large unreactive reservoir, strong triple bond).
CO₂ variable (small reservoir; photosynthesis/respiration/combustion cycle).
Air also contains variable water vapour (0-4%) and trace gases.

Required practical — % oxygen in air (spec 2.7)

Pass air over red-hot Cu; the O₂ reacts (2Cu + O₂ → 2CuO); cool; measure final volume.

Method. This is a classic Edexcel required practical for determining the percentage of oxygen in air. The principle: oxygen in a sample of air is removed by reaction with hot copper, and the resulting decrease in gas volume equals the volume of oxygen.

Apparatus.

Two gas syringes connected by a hard-glass combustion tube.
The combustion tube is loosely packed with copper turnings (excess, so all the oxygen can react).
A Bunsen burner (with a heat-resistant mat).

Procedure.

Push the air sample (e.g. 100 cm³) into one syringe; close the other.
Heat the copper-filled tube strongly until the copper is RED-HOT.
Push the gas slowly back and forth from one syringe to the other through the heated copper. The oxygen reacts with the hot copper to form black copper(II) oxide:

$2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}$

Copper turns from pink-brown to BLACK. Total gas volume decreases.

Keep cycling the gas until no further change in volume occurs (all O₂ has reacted, or copper is fully oxidised).
Allow the apparatus to COOL before reading the final volume — hot gas occupies more volume than cool gas.
Read final volume of gas from the syringes.

Calculation.

$\%\ \text{O}_2 = \dfrac{V_{\text{initial}} - V_{\text{final}}}{V_{\text{initial}}} \times 100\%$

Example: 100 cm³ → 79 cm³ gives % O₂ = (100 − 79)/100 × 100 = 21% (close to the accepted value).

Sources of error and how to address them.

Error	Effect on % O₂	Mitigation
Reading volume HOT	% O₂ too low (gas volume artificially high)	Cool before reading
Not enough copper or heat	% O₂ too low (O₂ not all reacted)	Use excess copper, heat until no change
Apparatus leaks	Can go either way	Check seals; do test run with N₂
Water vapour in starting air	% O₂ varies	Use dry air

Variations of the experiment. Other valid methods that the same principle:

Burning phosphorus in a sealed jar over water. White phosphorus burns to P₂O₅ which dissolves; water rises to replace the consumed O₂ — measure how high.
Iron filings in damp cotton wool over water. Iron rusts slowly, consuming O₂; water level rises over a few days.

The heated-copper version is the most reliable and commonly examined.

Apparatus: two gas syringes + copper-filled combustion tube + Bunsen.
2Cu + O₂ → 2CuO (pink-brown → black) as O₂ is consumed.
% O₂ = (V_initial − V_final) / V_initial × 100%.
Cool BEFORE reading; use excess copper; check for leaks.

See the full worked example for gases in the atmosphere →

Combustion: complete vs incomplete (spec 2.10, 2.11)

Complete: hydrocarbon + excess O₂ → CO₂ + H₂O. Incomplete: → CO + C + H₂O (CO toxic).

General concept. Combustion is the rapid exothermic reaction of a fuel with oxygen. The PRODUCTS depend on whether the supply of oxygen is sufficient.

Complete combustion (excess oxygen present).

$\text{fuel} + \text{excess O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O} \text{ (+ heat)}$

For methane (the main component of natural gas):

$\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$

This is the IDEAL outcome of burning a fuel:

Products: CO₂ (non-toxic at normal levels) + H₂O only.
Flame: clean, hot BLUE flame.
Energy: maximum heat released per mole of fuel (~890 kJ/mol for CH₄).
Visible signs: no smoke, no soot, no smell.

Complete combustion needs PLENTY of oxygen — air-hole on the Bunsen FULLY OPEN ('roaring' blue flame), or a well-tuned boiler/burner.

Incomplete combustion (insufficient oxygen).

$\text{fuel} + \text{limited O}_2 \rightarrow \text{CO} + \text{C (soot)} + \text{H}_2\text{O} \text{ (less heat)}$

For methane (one possible balanced equation):

$2\text{CH}_4\text{(g)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{CO(g)} + 4\text{H}_2\text{O(l)}$

Or (more limited O₂):

$\text{CH}_4\text{(g)} + \text{O}_2\text{(g)} \rightarrow \text{C(s)} + 2\text{H}_2\text{O(l)}$

In practice the products are a mixture of CO, C and CO₂ depending on the air-to-fuel ratio.

Products: carbon monoxide (CO — TOXIC) + carbon soot (yellow-black smoke) + water.
Flame: cooler YELLOW/orange flame (luminous because hot soot particles glow).
Energy: LESS heat released per mole of fuel (energy is locked up in the unburnt CO and C).
Visible signs: smoky flame, yellow colour, deposits soot on cool surfaces.

Common causes of incomplete combustion: blocked air inlet, poorly-tuned burner, sooty Bunsen ('luminous' flame with air-hole closed), inadequate ventilation in a room.

Why CO is so toxic.

CO is colourless and odourless — humans cannot detect it. When inhaled, it diffuses into the bloodstream and binds to the IRON in haemoglobin (the protein in red blood cells that carries O₂). CO binds about 210× more strongly than O₂, forming carboxyhaemoglobin (HbCO).

This irreversibly BLOCKS oxygen transport — even at low concentrations (~100-200 ppm), CO causes headache, dizziness, confusion, then unconsciousness and death from asphyxiation. Many deaths each year from faulty domestic boilers, blocked flues, or running car engines in closed garages.

Why carbon (soot) is harmful. Fine carbon particles (PM2.5, PM10) lodge in lungs → respiratory disease, asthma, lung cancer. Major cause of urban air pollution from diesel vehicles. They also blacken buildings.

Prevention. Proper ventilation, regular boiler maintenance, CO detectors in homes, low-emission zones for diesel vehicles.

Complete: CH₄ + 2O₂ → CO₂ + 2H₂O; clean blue flame; max energy.
Incomplete: 2CH₄ + 3O₂ → 2CO + 4H₂O OR CH₄ + O₂ → C + 2H₂O; yellow flame.
CO is colourless, odourless, TOXIC — binds to haemoglobin, blocks O₂ transport.
Soot causes respiratory disease.
Prevention: ventilation, maintenance, CO detectors.

See the full worked example for gases in the atmosphere →

Acid rain (spec 2.12)

SO₂ (from sulfur in fuels) and NOₓ (from hot combustion) dissolve in rain → acid rain → damages buildings + lakes + forests.

What is acid rain? Rainwater with pH below ~5.6 (the natural value due to dissolved CO₂). Severe acid rain has pH 3-4 — as acidic as vinegar.

Two pollutant gases that cause acid rain.

1. Sulfur dioxide (SO₂) — from burning sulfur-containing fossil fuels. Coal contains 0.5-3% sulfur impurities (as iron pyrite, FeS₂, or sulfide minerals). Heavy oil also contains sulfur. When burnt:

$\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$

Major SO₂ sources: coal-fired power stations, ore smelters, oil refineries.

2. Nitrogen oxides (NOₓ — NO, NO₂) — from high-temperature combustion. Inside car engines, jet engines and power-station furnaces, temperatures exceed 1500 °C. At these temperatures, the previously inert N₂ in air reacts with O₂:

$\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO(g)}$

$2\text{NO(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO}_2\text{(g)}$

Major NOₓ sources: car engines, power stations.

Formation of acid rain.

In the atmosphere, SO₂ first oxidises (slowly) to SO₃, then dissolves in rain:

$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$ $\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3 \text{ (sulfurous acid)}$ $\text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4 \text{ (sulfuric acid)}$

NO₂ dissolves in rain to form a mixture of nitrous + nitric acids:

$4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3$

Acid rain falls many kilometres downwind from the polluting source, often crossing national borders ('transboundary pollution'). Scandinavia received acid rain from UK and German coal plants; eastern Canada from US plants.

Effects of acid rain.

Corrosion of limestone / marble buildings and statues. Limestone (CaCO₃) reacts with acid:

$\text{CaCO}_3 + \text{H}_2\text{SO}_4 \rightarrow \text{CaSO}_4 + \text{H}_2\text{O} + \text{CO}_2$

Historic buildings (cathedrals, statues) gradually lose surface detail.

Acidification of lakes → kills fish. Most freshwater fish die below pH 5; fish eggs fail to hatch. Famous example: lakes in Sweden and parts of Canada that became 'dead' in the 1970s-80s.
Damage to forests. Acid rain leaches Ca²⁺ and Mg²⁺ from soil (plant nutrients) and mobilises toxic Al³⁺ → root damage. Tree leaves directly damaged by the acid. Coniferous forests in Germany ('Schwarzwald' / Black Forest) were severely affected.
Corrosion of metals. Acid rain accelerates corrosion of iron and steel structures (bridges, railings, cars).

Mitigation.

Approach	Method
Reduce SO₂ from power stations	Flue-gas desulfurisation (FGD): spray flue gas with CaO/CaCO₃ slurry: CaO + SO₂ → CaSO₃ (later oxidised to CaSO₄/gypsum, sold for plasterboard). Removes >95% of SO₂.
Reduce SO₂ at source	Use low-sulfur fuel (natural gas, refined oil); remove sulfur from coal/oil before burning.
Reduce NOₓ from cars	Catalytic converters in petrol cars: 2NO + 2CO → N₂ + 2CO₂. Reduces both NOₓ and CO.
Reduce NOₓ from power stations	Selective catalytic reduction (SCR) using ammonia injected into flue gas.
Switch from fossil fuels	Renewable energy (solar, wind, hydro, nuclear) avoids the problem entirely.

Acid rain was a major issue in the 1970s-80s and is now substantially reduced in developed countries thanks to these technologies. It remains a serious problem in some industrialising regions.

Acid rain caused by SO₂ (fossil fuels with sulfur) + NOₓ (hot combustion).
Reactions: SO₂ + H₂O → H₂SO₃; NO₂ + H₂O → HNO₃ (and HNO₂).
Effects: corrodes limestone/marble; acidifies lakes (kills fish); damages forests.
Mitigation: flue-gas desulfurisation, catalytic converters, low-S fuels, renewables.

See the full worked example for gases in the atmosphere →

Greenhouse effect and climate change (spec 2.13)

GHGs (CO₂, CH₄, H₂O) absorb infrared → trap heat. Enhanced by human activities → climate change.

The natural greenhouse effect. Earth receives short-wavelength VISIBLE and UV radiation from the Sun. Much of this passes through the atmosphere and warms Earth's surface. The warm surface re-radiates this energy as longer-wavelength INFRARED radiation. Greenhouse gases in the atmosphere ABSORB this infrared and RE-EMIT it in all directions — including back toward the surface. This 'traps' heat, keeping Earth's average temperature at about 15 °C.

Without the natural greenhouse effect, Earth would be ~−18 °C — frozen and inhospitable to life. The greenhouse effect is essential.

The main natural greenhouse gases.

Gas	Source (natural)	Contribution
Water vapour (H₂O)	Evaporation from oceans, lakes, plants	Largest natural contribution
Carbon dioxide (CO₂)	Volcanic outgassing, respiration, decay	Significant; varies seasonally
Methane (CH₄)	Wetlands, anaerobic decay	Small but potent
Nitrous oxide (N₂O)	Microbial soil processes	Small

Why these molecules absorb IR. Polar diatomic and polyatomic molecules with C=O, O-H or C-H bonds have vibrational modes that absorb IR photons. Diatomic N₂ and O₂ are NON-greenhouse gases — their molecules are symmetric and don't absorb IR.

The ENHANCED greenhouse effect. Human activities have increased the atmospheric concentration of greenhouse gases above natural levels. This INCREASES the heat trapped, RAISING global temperature — the 'enhanced greenhouse effect'.

Human sources of greenhouse gases.

Combustion of fossil fuels — releases CO₂ stored over millions of years in coal, oil and gas:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Power stations, transport, heating, industry. Atmospheric CO₂ has risen from 280 ppm (1800) to >420 ppm today.

Deforestation — burning forests releases CO₂; loss of trees reduces photosynthesis.
Livestock farming — cattle and sheep produce CH₄ from enteric fermentation (digestion).
Rice paddies — anaerobic decomposition produces CH₄.
Landfills — anaerobic decomposition of biological waste produces CH₄.
Natural-gas pipelines + fracking — leaks of CH₄ ('fugitive emissions').
Cement production — CaCO₃ → CaO + CO₂ during cement manufacture.

Methane vs CO₂. Although CO₂ is far more abundant, methane is ~25-30× more potent as a greenhouse gas per molecule (better at absorbing IR). However, CH₄ has a shorter atmospheric lifetime (~12 years vs ~100+ years for CO₂).

Consequences of climate change.

Global warming. Average global temperatures have risen ~1.2 °C since 1880. Targeting <1.5 °C rise (Paris Agreement).
Melting polar ice caps and glaciers. Sea-level rise from melting + thermal expansion of warming oceans. Threatens coastal cities (London, New York, Tokyo, Singapore).
More extreme weather — heatwaves, droughts, wildfires, hurricanes, floods. Increasing in frequency and intensity.
Ocean acidification. CO₂ dissolves in seawater: CO₂ + H₂O → H₂CO₃. Lowers ocean pH → kills coral reefs, harms shellfish.
Biodiversity loss. Species can't adapt fast enough; range shifts, extinctions. Coral bleaching, polar species (polar bears, penguins) under threat.
Food and water insecurity. Crop failures in some regions; water shortages; mass migration.

Mitigation actions.

Action	What it does
Switch to renewable energy	Solar, wind, hydro, nuclear — generate electricity without CO₂
Energy efficiency	Better insulation, LED lighting, efficient appliances
Electric vehicles	Replace petrol/diesel cars (if charged from clean grid)
Reforestation	Plant trees to absorb CO₂ via photosynthesis
Carbon capture and storage (CCS)	Trap CO₂ from power-plant emissions and store underground
Plant-based diets	Reduce methane from livestock
Landfill methane capture	Capture and use CH₄ from waste instead of letting it escape

The challenge is GLOBAL — requires coordinated action by all countries through international agreements (UNFCCC, Paris Agreement). Net-zero CO₂ emissions by 2050 is the widely-agreed target.

Natural greenhouse effect (GHE) keeps Earth warm — ESSENTIAL for life.
Mechanism: GHGs absorb IR emitted by Earth, re-emit some back to surface.
Main GHGs: H₂O, CO₂, CH₄, N₂O.
Enhanced GHE = human activities (fossil fuels, deforestation, livestock) increase GHGs.
Consequences: warming, ice melt, sea-level rise, extreme weather, biodiversity loss.
Mitigation: renewables, EVs, reforestation, CCS, energy efficiency.

See the full worked example for gases in the atmosphere →

How it’s examined

Gases in the atmosphere appear on EVERY Paper 1 (4CH1/1C) and most Paper 2 sittings — worth 8-12 marks per session combined. Typical Paper 1 items: (a) air composition (1-3 marks); (b) % oxygen experiment with calculation (4-6 marks); (c) complete vs incomplete combustion equations (3-5 marks); (d) why CO is toxic (2-3 marks). Paper 2 items extend to: (e) acid rain formation with equations (5-6 marks); (f) greenhouse effect and climate change (5-6 marks); (g) calculate % O₂ from data (3-4 marks). Always include EQUATIONS where possible — they earn most marks.

Sources: Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4, September 2024 (valid for 2026 onwards); Pearson Edexcel 4CH1 Examiner Reports 2022-2024; 4CH1/1C May/Jun 2024 question paper and mark scheme; 4CH1/1C January 2024 question paper and mark scheme; 4CH1/1CR January 2024 question paper and mark scheme; 4CH1/2C May/Jun 2024 question paper and mark scheme; 4CH1/2C January 2024 question paper and mark scheme. Last reviewed 2026-05-12.

Master this Topic

Worked examples, formulae, definitions and the mistakes examiners flag — everything you need to push from a pass to an A*.

Take this whole topic with you

Download a branded revision sheet — worked examples, formulae, definitions and common mistakes for Gases in the Atmosphere , ready to print or save as PDF.

Step-by-step worked examples — Gases in the Atmosphere

Step-by-step solutions to past-paper-style questions on gases in the atmosphere , written exactly the way a tutor would explain them at the board.

1Percentage of oxygen in air — heated copper experiment (6 marks, Paper 1)
Core• Adapted from 4CH1/1C Jan 2024 Q7• gases atmosphere, required practical, percentage oxygen, spec-2.6, spec-2.7
Question
Describe an experiment using heated copper to determine the percentage of oxygen in air. Include the apparatus, the procedure, the observation, the calculation, and TWO sources of error. (6 marks)
Step-by-step solution
1. Step 1
  Apparatus (1 M). Two gas syringes connected by a glass tube containing excess copper turnings, all sealed in line. A Bunsen burner heats the copper-filled tube strongly. Initial known volume of air (e.g. 100 cm³) is in one syringe; the other is empty.
2. Step 2
  Procedure (1 M). Push air slowly back and forth from one syringe through the heated copper to the other syringe. The copper must be RED-HOT. Continue until the volume in the syringes stops decreasing. Allow apparatus to COOL before taking the final volume reading.
3. Step 3
  Observation (1 A). Shiny pink/orange copper turns BLACK as copper(II) oxide forms. The total gas volume decreases (oxygen used up).
  $2\text{Cu(s)} + \text{O}_2\text{(g)} \rightarrow 2\text{CuO(s)}$
4. Step 4
  Calculation (1 A). % O₂ = (V_initial − V_final) / V_initial × 100%. E.g. if start with 100 cm³ and finish with 79 cm³, % O₂ = (100 − 79)/100 × 100% = 21%.
  $\%\ \text{O}_2 = \dfrac{V_{\text{initial}} - V_{\text{final}}}{V_{\text{initial}}} \times 100\%$
5. Step 5
  Error 1 (1 B). Not allowing the apparatus to cool before reading the final volume → gas occupies more volume hot → final volume reading too high → calculated % O₂ too low.
6. Step 6
  Error 2 (1 B). Not heating long enough / insufficient copper → some O₂ remains unreacted → final volume reading too high → % O₂ too low. Other valid errors: leaks in the apparatus; incomplete mixing of gas.
Answer
Two gas syringes + heated copper tube. Push air back and forth over hot Cu (2Cu + O₂ → 2CuO; black solid). Cool, read final volume. % O₂ = (V₁−V₂)/V₁ × 100. Expected ~21%. Errors: not cooling before reading; insufficient copper / heating time; leaks.
Examiner tip
Edexcel 6-mark mark scheme: 1 for apparatus (two syringes); 1 for procedure (push gas back and forth, heat strongly); 1 for observation (black CuO) + equation; 1 for calculation formula AND correct numerical answer ~21%; 2 for valid sources of error. Don't say 'oxygen disappears' — say 'oxygen reacts with copper to form copper(II) oxide'.
2Complete vs incomplete combustion of methane (5 marks, Paper 1)
Core• Adapted from 4CH1/1C May/Jun 2024 Q3• gases atmosphere, combustion, methane, incomplete combustion, spec-2.10, spec-2.11
Question
Methane (CH₄) is the main component of natural gas. Write balanced equations for the complete and incomplete combustion of methane. State TWO differences in the products and explain ONE health hazard of incomplete combustion. (5 marks)
Step-by-step solution
1. Step 1
  Complete combustion (1 A). With excess oxygen, CH₄ burns fully to carbon dioxide and water (vapour). Reaction is exothermic — releases a lot of heat per mole of methane (~890 kJ/mol).
  $\text{CH}_4\text{(g)} + 2\text{O}_2\text{(g)} \rightarrow \text{CO}_2\text{(g)} + 2\text{H}_2\text{O(l)}$
2. Step 2
  Incomplete combustion (1 A). When O₂ is LIMITED (e.g. blocked air inlet, faulty burner), methane is only partly oxidised. Products are carbon monoxide (CO) and/or carbon (soot, C) and water. Less heat is released per mole of fuel.
  $2\text{CH}_4\text{(g)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{CO(g)} + 4\text{H}_2\text{O(l)}$
3. Step 3
  Difference 1 — products (1 A). Complete: CO₂ + H₂O only. Incomplete: CO (toxic), C (soot — yellow/black smoke), and H₂O. Sometimes a mix of all three products.
4. Step 4
  Difference 2 — flame and energy (1 B). Complete: clean, hot BLUE flame. Incomplete: cooler YELLOW/orange flame (luminous due to incandescent soot), much less energy released.
5. Step 5
  Health hazard of CO (1 B). Carbon monoxide is COLOURLESS and ODOURLESS — undetectable to humans. It binds to haemoglobin in red blood cells ~210× more strongly than O₂, forming carboxyhaemoglobin. This BLOCKS the blood from carrying oxygen → headache, dizziness, unconsciousness, death. Faulty boilers in poorly ventilated rooms have caused many deaths — hence legal requirement for CO detectors.
Answer
Complete: CH₄ + 2O₂ → CO₂ + 2H₂O (blue flame, full energy). Incomplete: 2CH₄ + 3O₂ → 2CO + 4H₂O (or → C + 2H₂O — yellow smoky flame, less energy). Differences: (1) toxic CO/soot vs CO₂; (2) less heat released. CO is a colourless, odourless poison — binds to haemoglobin and prevents oxygen transport.
Examiner tip
Edexcel 5-mark mark scheme: 1 each for the two balanced equations; 1 for product differences; 1 for energy/flame difference; 1 for CO health hazard mechanism. Common error: writing 'CO + CO₂ + H₂O' as products of incomplete combustion. Stick to a single clean equation — examiners accept either 2CH₄ + 3O₂ → 2CO + 4H₂O OR CH₄ + O₂ → C + 2H₂O.
3Formation and effects of acid rain (6 marks, Paper 2)
Extended• Adapted from 4CH1/2C Jan 2024 Q8• acid rain, SO2, NOx, environment, spec-2.12
Question
Acid rain is a serious environmental problem in industrialised regions. Describe (a) two pollutant gases that cause acid rain and how they enter the atmosphere; (b) the chemical reactions that produce the acids; and (c) TWO environmental effects of acid rain. (6 marks)
Step-by-step solution
1. Step 1
  Pollutant 1 — SO₂ (1 A). Sulfur dioxide is released when fossil fuels (especially coal and heavy oil) are burned. Coal contains sulfur impurities (~0.5-3%). Sulfur burns in air to form SO₂:
  $\text{S(s)} + \text{O}_2\text{(g)} \rightarrow \text{SO}_2\text{(g)}$
2. Step 2
  Pollutant 2 — NOₓ (1 A). Nitrogen oxides (NO, NO₂) are produced in car engines and power-station furnaces. The combustion chamber gets hot enough (>1500 °C) for N₂ from the air to react with O₂:
  $\text{N}_2\text{(g)} + \text{O}_2\text{(g)} \rightarrow 2\text{NO(g)}$
3. Step 3
  SO₂ → H₂SO₃ / H₂SO₄ (1 A). SO₂ dissolves in rainwater to form sulfurous acid; further oxidation in the atmosphere gives sulfuric acid:
  $\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3;\quad 2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3;\quad \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$
4. Step 4
  NO → HNO₃ (1 A). NO oxidises to NO₂, which dissolves to form nitric acid (and nitrous):
  $2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2;\quad 4\text{NO}_2 + \text{O}_2 + 2\text{H}_2\text{O} \rightarrow 4\text{HNO}_3$
5. Step 5
  Effect 1 (1 B). Corrosion of buildings/statues. Acid rain reacts with limestone (CaCO₃) and marble: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. Stone gradually erodes; historic buildings and statues lose detail.
6. Step 6
  Effect 2 (1 B). Acidification of lakes/forests. Acid rain lowers the pH of lakes (→ kills fish — most freshwater fish die below pH 5). Damages tree leaves and roots; leaches nutrients (Ca²⁺, Mg²⁺) from soil → trees weaken and die. Whole forests in Scandinavia, Germany, North America have been damaged.
Answer
(a) SO₂ from burning sulfur-containing coal: S + O₂ → SO₂. NOₓ from high-temperature combustion in car/power station: N₂ + O₂ → 2NO. (b) SO₂ + H₂O → H₂SO₃; SO₃ + H₂O → H₂SO₄. NO₂ + H₂O → HNO₃/HNO₂. (c) Corrodes limestone/marble buildings; acidifies lakes (kills fish); damages forests; corrodes metal structures.
Examiner tip
Edexcel 6-mark mark scheme: 2 marks for the two pollutants + how they enter (1 each); 2 marks for the chemical equations (sulfurous/sulfuric and nitric); 2 marks for two valid environmental effects. Mitigation methods (FGD, catalytic converters) can be additional credit.
4Enhanced greenhouse effect and climate change (5 marks, Paper 2)
Extended• Adapted from 4CH1/2C May/Jun 2024 Q9• greenhouse effect, climate change, CO2, methane, spec-2.13
Question
Explain how the enhanced greenhouse effect leads to climate change. Name TWO greenhouse gases and ONE human activity that releases each. Suggest TWO actions that could be taken to reduce the enhanced greenhouse effect. (5 marks)
Step-by-step solution
1. Step 1
  Mechanism of greenhouse effect (1 M). Short-wavelength visible/UV radiation from the Sun passes through the atmosphere and warms the Earth. The warm Earth re-radiates this energy as longer-wavelength INFRARED. Greenhouse gases ABSORB the infrared and re-emit it in all directions — including back toward the surface. This 'traps' heat, raising Earth's average temperature above what it would otherwise be (a natural effect — without it, Earth would be ~−18 °C).
2. Step 2
  Greenhouse gas 1 — CO₂ from combustion (1 A). Carbon dioxide. Released by burning fossil fuels (coal, oil, gas) for electricity generation and transport, and by deforestation (burning forests + loss of photosynthesis). Atmospheric concentration has risen from ~280 ppm (pre-industrial) to >420 ppm today.
3. Step 3
  Greenhouse gas 2 — CH₄ (1 A). Methane. Sources include livestock farming (cattle release CH₄ from enteric fermentation), rice paddies, landfills (anaerobic decomposition of waste), and leaks from natural-gas pipelines / fracking. Per molecule, methane is 25-30× more potent than CO₂.
4. Step 4
  Climate change consequences (1 B). Enhanced trapping of infrared raises global mean temperature → melts polar ice caps and glaciers → sea-level rise → coastal flooding. More extreme weather — heatwaves, droughts, wildfires, intense storms. Shifts in ecosystems — species migrate or go extinct; coral bleaching. Crop failures in some regions; food/water insecurity.
5. Step 5
  Two mitigation actions (1 B). (i) Reduce fossil-fuel use — switch to renewable energy (solar, wind, hydro, nuclear); improve energy efficiency; use electric vehicles. (ii) Reforestation and reduced deforestation — trees absorb CO₂ via photosynthesis. Other valid answers: carbon capture and storage (CCS); plant-based diets to reduce livestock CH₄; methane capture at landfills.
Answer
Mechanism: sun → Earth surface as visible/UV → Earth re-radiates as IR → CO₂/CH₄ absorb IR → trap heat → atmosphere warms. Gases + sources: CO₂ from burning fossil fuels / deforestation; CH₄ from livestock / landfills / natural gas leaks. Effects: melting ice, sea-level rise, extreme weather, ecosystem damage. Mitigations: renewables, reforestation, CCS, energy efficiency, plant-based diets.
Examiner tip
Edexcel 5-mark mark scheme: 1 for mechanism (IR absorbed by GHGs and re-emitted); 1 each for two GHGs with valid source; 2 for two specific actions to reduce emissions. The word 'ENHANCED' is important — natural greenhouse effect makes Earth habitable; the enhanced effect is what causes climate change.
5Calculate % oxygen from experimental data (3 marks)
Core• Adapted from 4CH1/1CR May/Jun 2024 Q2• calculation, % oxygen, spec-2.7
Question
In an experiment to find the percentage of oxygen in air, the initial volume of air in the syringes was 100 cm³. After passing the gas back and forth over heated copper until no further change occurred, and then allowing it to cool, the final volume of gas was 78 cm³. (a) Calculate the percentage of oxygen in the sample of air. (b) Suggest why the value is slightly lower than the accepted value of 21%. (3 marks)
Step-by-step solution
1. Step 1
  Volume of oxygen used (1 M). Volume decrease = volume of oxygen reacted with copper.
  $V(\text{O}_2) = V_{\text{initial}} - V_{\text{final}} = 100 - 78 = 22\ \text{cm}^3$
2. Step 2
  Percentage of oxygen (1 A). % O₂ = (volume O₂ used / total initial volume) × 100%.
  $\%\ \text{O}_2 = \dfrac{22}{100} \times 100\% = 22\%$
3. Step 3
  Why slightly different from 21% (1 B). Experimental result is 22%, slightly higher than accepted 21%. Possible reasons: water vapour in the air was also absorbed; condensation of water on cooling; reading error on the syringe; some of the apparent 'air' was actually water vapour. Accept any sensible suggestion linked to measurement uncertainty or systematic error.
Answer
(a) Volume of O₂ used = 100 − 78 = 22 cm³. % O₂ = 22/100 × 100% = 22%. (b) Slightly above 21% — possibly because water vapour in the air condensed on cooling, exaggerating the volume change.
Examiner tip
Edexcel 3-mark mark scheme: 1 for volume of O₂ reacted; 1 for correct % calculation; 1 for sensible explanation of discrepancy. ECF applies. Accept reasonable values in the range 20-22% for measurement uncertainty.

Model Answers — Gases in the Atmosphere

High-scoring sample answers for gases in the atmosphere on the Pearson Edexcel IGCSE 4CH1 paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question
4CH1/1C-style — composition of air5 marks
State the approximate percentage composition of dry air, and explain why CO₂ is a 'variable' component while N₂ is essentially constant. (5 marks)
Model answer
Approximate composition of dry air (Edexcel-accepted values).

Gas % by volume
Nitrogen (N₂) 78%
Oxygen (O₂) 21%
Argon (Ar) ~1% (0.93%)
Carbon dioxide (CO₂) ~0.04%
Other (Ne, He, Kr, Xe, H₂, CH₄) trace

Note: 'dry air' means the water-vapour-free composition; real atmospheric air contains a variable amount of water vapour (0-4% by volume, depending on temperature and humidity).

Why N₂ is essentially constant. Nitrogen makes up 78% of the atmosphere — an enormous reservoir. The molecule N≡N is held by a strong triple bond (~945 kJ/mol bond enthalpy), so N₂ is very UNREACTIVE under normal conditions. Most natural processes either don't involve N₂, or fix it only slowly (lightning + bacterial nitrogen fixation). Therefore the rate at which N₂ is added or removed from the atmosphere is tiny compared with the existing reservoir, and the percentage stays essentially constant over millennia.

Why CO₂ is variable. Carbon dioxide is only ~0.04% (400 ppm) of air — a TINY reservoir. It is involved in many fast processes:

Photosynthesis REMOVES CO₂: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (during daylight; plants and phytoplankton).

Respiration ADDS CO₂: C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O (all animals, plants, microbes — continuously).

Combustion ADDS CO₂: burning fossil fuels (coal, oil, gas), wood and biomass.

Oceans DISSOLVE and RELEASE CO₂ as temperature changes (warm water holds less CO₂).

Because the inflows and outflows are large compared to the small atmospheric reservoir, CO₂ concentration varies:

Seasonally (Northern Hemisphere CO₂ peaks in spring, dips in autumn).

Daily (drops during the day from photosynthesis).

Geographically (higher near cities and forest fires).

Historically (rising from ~280 ppm in 1800 to >420 ppm today, due to fossil-fuel use and deforestation).

Water vapour is even more variable — typically 1-3% by volume in moist conditions, but only a few hundred ppm in arctic / desert dry air.
Why this scores
Edexcel 5-mark mark scheme: 3 for stating composition (N₂ 78%, O₂ 21%, Ar 1%, CO₂ 0.04%); 1 for N₂ being unreactive / large reservoir; 1 for CO₂ involved in photosynthesis + respiration + combustion → varies. AVOID rounding to ridiculous precision — examiners accept 78%/21%/1%/0.04%.

Gas	% by volume
Nitrogen (N₂)	78%
Oxygen (O₂)	21%
Argon (Ar)	~1% (0.93%)
Carbon dioxide (CO₂)	~0.04%
Other (Ne, He, Kr, Xe, H₂, CH₄)	trace

Question

4CH1/2C-style — fossil fuel combustion pollution6 marks

Explain how the combustion of fossil fuels in modern society causes both LOCAL air-quality problems and GLOBAL climate problems. Name the pollutants involved, write equations where appropriate, and suggest practical control measures. (6 marks)

Model answer

Fossil fuels (coal, oil, natural gas) are mostly hydrocarbons (with some impurities). When burnt, they release CO₂, H₂O and heat — but also a range of harmful by-products that cause distinct LOCAL and GLOBAL problems.

LOCAL air-quality problems.

1. Carbon monoxide (CO). Formed when incomplete combustion occurs (limited O₂):

$2\text{CH}_4 + 3\text{O}_2 \rightarrow 2\text{CO} + 4\text{H}_2\text{O}$

CO is COLOURLESS and ODOURLESS. It binds to haemoglobin >200× more strongly than O₂, blocking oxygen transport. Causes headaches, dizziness, unconsciousness, death. Hazard from faulty domestic boilers and car exhausts in enclosed spaces.

2. Sulfur dioxide (SO₂). Coal and crude-oil products contain sulfur impurities. When burnt:

$\text{S} + \text{O}_2 \rightarrow \text{SO}_2$

SO₂ irritates lungs/airways (asthma trigger) and dissolves in rain water to form acid rain.

3. Nitrogen oxides (NOₓ — NO, NO₂). Form in hot engines / furnaces, where N₂ + O₂ react:

$\text{N}_2 + \text{O}_2 \rightarrow 2\text{NO};\quad 2\text{NO} + \text{O}_2 \rightarrow 2\text{NO}_2$

NOₓ irritates the lungs, forms photochemical smog, contributes to acid rain.

4. Particulates / soot. Tiny carbon particles (PM2.5, PM10) from incomplete combustion. Lodge in lungs → respiratory disease, cancer, cardiovascular issues. Worst in cities with diesel traffic.

5. Acid rain. Combination of SO₂ + NOₓ + atmospheric water:

$\text{SO}_2 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_3;\quad \text{SO}_3 + \text{H}_2\text{O} \rightarrow \text{H}_2\text{SO}_4$

Damages limestone buildings (CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂), acidifies lakes (kills fish), damages forests.

GLOBAL climate problems.

Enhanced greenhouse effect from CO₂. Even complete combustion releases CO₂:

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

Atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to >420 ppm. CO₂ absorbs the infrared radiation re-emitted by Earth, trapping heat → global temperature rises → ice cap melting, sea-level rise, extreme weather, ecosystem disruption.

Control measures.

Pollutant	Control measure
CO + soot	Ensure complete combustion (excess air); regular boiler maintenance; install CO alarms; switch to cleaner fuels (natural gas, hydrogen)
SO₂	Flue-gas desulfurisation (FGD): SO₂ + CaO → CaSO₃ — removes >95% of SO₂ in power-station chimneys
NOₓ	Catalytic converters in cars: 2NO + 2CO → N₂ + 2CO₂. Selective catalytic reduction in power stations
Particulates	Diesel particulate filters (DPFs); low-emission zones in cities
CO₂	Switch to renewable energy (solar, wind, hydro); reforestation; carbon capture and storage (CCS); electric vehicles

Why this scores

Edexcel 6-mark mark scheme: 2 marks for local pollutants (any 2 from CO/SO₂/NOₓ/soot with health link); 2 marks for global problem (CO₂ + greenhouse effect mechanism); 2 marks for control measures (any 2 from catalytic converter, FGD, renewables, EVs). High-frequency Paper 2 topic — broad knowledge needed.

Question

4CH1/1C-style — atmospheric corrosion of iron5 marks

Air contains O₂ and water vapour. Explain how this leads to the corrosion of iron, and describe TWO methods used to protect iron objects from corrosion. Write an equation where appropriate. (5 marks)

Model answer

Iron rusts (corrodes) when exposed to the combination of both oxygen and water in the air. This is the same combination that the atmosphere provides under normal conditions — hence iron objects rust steadily outdoors.

Mechanism. The overall reaction is:

$4\text{Fe(s)} + 3\text{O}_2\text{(g)} + x\text{H}_2\text{O(l)} \rightarrow 2\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O(s)}$

Rust is hydrated iron(III) oxide (a flaky, orange-brown solid). The reaction is electrochemical:

Iron is OXIDISED: Fe → Fe²⁺ + 2e⁻ (later oxidised further to Fe³⁺).
Oxygen is REDUCED (in the presence of water): O₂ + 2H₂O + 4e⁻ → 4OH⁻.
The Fe²⁺ and OH⁻ combine to form Fe(OH)₂, which further oxidises in air to give hydrated Fe₂O₃ (rust).

If EITHER oxygen OR water is absent, rusting does NOT occur. This is demonstrated in the classic three-test-tube experiment:

Tube	Contents	Result
1	Iron nail + tap water + air	Rusts (both present)
2	Iron nail + boiled water + oil layer	No rust (water but no O₂)
3	Iron nail + anhydrous CaCl₂ (drying agent)	No rust (O₂ but no water)

This proves BOTH water AND oxygen are required.

Method 1 — Barrier methods (paint, plastic, oil, grease). A physical barrier prevents O₂ and H₂O reaching the iron surface. Examples: car bodies are painted; tools coated with oil/grease; outdoor steel furniture has a powder-coated plastic finish. SIMPLE and CHEAP, but if the coating is SCRATCHED, rusting starts in the exposed area and can spread under the coating.

Method 2 — Sacrificial protection. A piece of a MORE REACTIVE metal (zinc or magnesium) is attached to the iron object. Because the more reactive metal is preferentially oxidised:

Zn → Zn²⁺ + 2e⁻ (zinc rusts instead of iron)
The iron stays intact even when exposed to air and water.

Used for ship hulls (zinc 'blocks' bolted to the keel), oil pipelines (magnesium anodes), and underground iron pipes.

Method 3 — Galvanising (a combination of both above methods). Iron/steel is coated with a layer of zinc (often dipped in molten Zn). The zinc serves as (a) a barrier — preventing O₂/H₂O from reaching the iron, AND (b) sacrificial protection — even if scratched, the zinc preferentially corrodes. Used for galvanised nails, roofing sheets, and chain-link fencing.

Method 4 — Electroplating with chromium or tin (mostly a barrier method). 'Tin cans' are tin-plated steel; the tin coating is unreactive and food-safe.

Why this scores

Edexcel 5-mark mark scheme: 1 for stating BOTH water and oxygen needed; 1 for product is hydrated iron(III) oxide / equation; 1 each for two distinct prevention methods. Sacrificial protection vs galvanising: sacrificial uses a separate piece; galvanising = whole surface coating.

Question
4CH1/2C-style — carbon cycle disruption5 marks
Explain how photosynthesis and respiration both contribute to keeping the percentages of CO₂ and O₂ in the atmosphere relatively stable, and explain how human activities have disrupted this balance. (5 marks)
Model answer
The natural carbon cycle: photosynthesis and respiration.

In the natural carbon cycle, CO₂ is removed from the atmosphere by photosynthesis and returned by respiration. These two processes have opposite stoichiometries:

Photosynthesis (plants, algae, cyanobacteria, daylight):

$6\text{CO}_2\text{(g)} + 6\text{H}_2\text{O(l)} \xrightarrow{\text{light, chlorophyll}} \text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} + 6\text{O}_2\text{(g)}$

CO₂ is fixed into carbohydrates (glucose, then cellulose, starch). O₂ is released as a by-product. This is the ONLY major natural process that produces O₂.

Respiration (all living organisms, 24 hours/day):

$\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} + 6\text{O}_2\text{(g)} \rightarrow 6\text{CO}_2\text{(g)} + 6\text{H}_2\text{O(l)} + \text{energy}$

This 'undoes' photosynthesis — CO₂ is released; O₂ is consumed.

Pre-industrial balance. Globally, the rate of photosynthesis was roughly EQUAL to the rate of respiration + natural decay. The atmospheric concentrations of CO₂ (~280 ppm) and O₂ (~21%) remained approximately constant over many thousands of years (with small seasonal oscillations).

Why this kept the air stable. Any local excess of CO₂ would stimulate plant growth → more photosynthesis → CO₂ falls. Any local depletion of CO₂ would slow photosynthesis → CO₂ rises. This natural negative feedback maintained equilibrium.

Human disruption.

1. Burning fossil fuels (since ~1750, accelerating since ~1950). Coal, oil and natural gas are 'fossilised' carbon that was removed from the atmosphere by photosynthesis MILLIONS of years ago and trapped underground. When burnt, they release this stored carbon as CO₂ back into the atmosphere — much faster than photosynthesis can re-absorb it.

$\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$

2. Deforestation. Cutting down forests for agriculture/timber reduces the global capacity to photosynthesise. Often the trees are burned, releasing even more CO₂. The Amazon, Indonesian rainforests, and African forests have all been heavily reduced.

3. Cement production. Calcium carbonate is heated to produce calcium oxide for cement, releasing CO₂:

$\text{CaCO}_3 \rightarrow \text{CaO} + \text{CO}_2$

Result. Atmospheric CO₂ has risen from ~280 ppm (pre-industrial) to >420 ppm today — a >50% increase. This is the dominant driver of the enhanced greenhouse effect and climate change.

Restoring the balance. Reforestation, switching to renewable energy, carbon capture and storage (CCS), and energy efficiency can slow the rate of CO₂ release and accelerate its removal.
Why this scores
Edexcel 5-mark mark scheme: 1 for photosynthesis equation (or worded); 1 for respiration equation (or worded); 1 for stating they balance naturally; 1 for fossil fuel combustion releases stored carbon; 1 for deforestation reduces photosynthesis. Mark scheme accepts 'increased CO₂' OR 'climate change' as the consequence — but link them for the highest band.

Key Definitions and Keywords — Gases in the Atmosphere

Definitions to memorise and the exact keywords mark schemes credit for gases in the atmosphere answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE 4CH1 sitting.

Atmosphere
Examiner keyword
The layer of gas surrounding Earth, held in place by gravity. Approximate composition (by volume): N₂ 78%, O₂ 21%, Ar 0.93%, CO₂ 0.04%, plus variable H₂O, trace He, Ne, Kr, CH₄.
Combustion
Examiner keyword
The rapid exothermic reaction of a fuel with oxygen. Complete combustion: fuel + excess O₂ → CO₂ + H₂O. Incomplete combustion (limited O₂): fuel + O₂ → CO + C(soot) + H₂O.
Complete combustion
Examiner keyword
Combustion in EXCESS oxygen — produces CO₂ and H₂O only. Clean blue flame, maximum energy release. CH₄ + 2O₂ → CO₂ + 2H₂O.
Incomplete combustion
Examiner keyword
Combustion with INSUFFICIENT oxygen — produces a mixture of CO and/or C (soot) and H₂O. Yellow smoky flame, less energy released, produces TOXIC carbon monoxide.
Carbon monoxide (CO)
Examiner keyword
Colourless, odourless, very toxic gas. Produced by incomplete combustion. Binds to haemoglobin >200× more strongly than O₂, preventing oxygen transport. Causes headache, unconsciousness and death.
Acid rain
Examiner keyword
Rainwater with pH less than ~5.6 (the natural value due to dissolved CO₂). Caused by SO₂ (from burning sulfur-containing fossil fuels) and NOₓ (from hot combustion in engines and furnaces) dissolving to give H₂SO₃, H₂SO₄, HNO₃.
Greenhouse effect
Examiner keyword
Natural process: short-wavelength solar radiation passes through the atmosphere and warms Earth. Earth re-radiates longer-wavelength infrared, which is absorbed and re-emitted by greenhouse gases (CO₂, CH₄, H₂O), trapping heat and keeping Earth warm.
Enhanced greenhouse effect
Examiner keyword
The INCREASE in the greenhouse effect caused by human activities (burning fossil fuels, deforestation, livestock farming) raising the concentration of greenhouse gases. Leads to global warming and climate change.
Greenhouse gas
Examiner keyword
A gas that absorbs infrared radiation and re-emits it, trapping heat in the atmosphere. The main greenhouse gases on Earth are H₂O (water vapour), CO₂ (carbon dioxide), CH₄ (methane), and N₂O (nitrous oxide).
Climate change
Examiner keyword
Long-term changes to Earth's average temperature, weather patterns and ecosystems caused by the enhanced greenhouse effect. Effects: melting ice caps, sea-level rise, extreme weather, ecosystem and biodiversity disruption.

Common Mistakes and Misconceptions — Gases in the Atmosphere

The traps other students keep falling into on gases in the atmosphere questions — taken from recent Pearson Edexcel IGCSE 4CH1 examiner reports and mark schemes — and how to avoid them.

✕Confusing CO and CO₂ as products of combustion
4CH1 Examiner Reports 2022-2024
Why it happens
Both contain C and O — easy to swap.
How to avoid it
CO₂ (carbon DIoxide) — from COMPLETE combustion (excess O₂); NOT toxic at normal levels. CO (carbon MONoxide) — from INCOMPLETE combustion (limited O₂); HIGHLY TOXIC. Memorise: DI = dignified (safe-ish); MONO = monster (deadly).
✕Confusing the greenhouse effect with ozone layer depletion
Why it happens
Both are 'atmospheric problems' caused by human emissions.
How to avoid it
GREENHOUSE EFFECT = CO₂, CH₄, H₂O absorb IR → trap heat → climate change. OZONE LAYER = O₃ in stratosphere blocks UV; depleted by CFCs → skin cancer. They are SEPARATE problems with different gases and consequences.
✕Writing O₂ = 78% and N₂ = 21% (swapping the values)
4CH1 Examiner Reports 2023-2024
Why it happens
Knowing the numbers but not which gas they belong to.
How to avoid it
NITROGEN is the MAJORITY gas at 78%. OXYGEN is 21% (≈ one-fifth). The reactive gas (O₂) is the MINORITY. Memorise: 'NITROGEN, NEARLY FOUR-FIFTHS; OXYGEN, ONE-FIFTH'.
✕Saying CO₂ causes acid rain
4CH1 Examiner Reports 2022-2024
Why it happens
Knowing CO₂ is acidic in water — but its contribution is minor at natural levels.
How to avoid it
ACID RAIN is caused by SO₂ (→ H₂SO₃/H₂SO₄) and NOₓ (→ HNO₃). CO₂ does slightly acidify rain (pH ~5.6) but this is the NATURAL background, not acid rain. The pollution-driven acid rain is below pH 4.
✕Reading the volume of gas while still hot in the oxygen experiment
Why it happens
Forgetting that gases expand with temperature.
How to avoid it
ALWAYS allow the apparatus to COOL to room temperature before reading the final volume. Hot gas occupies more volume than cool gas — a hot reading would underestimate the % O₂ used.
✕Omitting water as a product of combustion
Why it happens
Focusing on CO₂ / CO as the carbon products.
How to avoid it
Hydrocarbons contain BOTH C and H. Both must oxidise: C → CO₂ (or CO); H → H₂O. Always include water in the equation: CH₄ + 2O₂ → CO₂ + 2H₂O.

Practice questions

Exam-style questions with step-by-step worked solutions. Try one before checking the method.

Past paper style quiz

Get a report showing which sub-topics you've nailed and which ones still need work.

Video lesson

Short walkthrough of the concepts students most often get stuck on.

Gases in the Atmosphere — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Gases in the Atmosphere

Short Study Notes in the form of Slides

Detailed Notes

What you’ll learn

How it’s examined

1Percentage of oxygen in air — heated copper experiment (6 marks, Paper 1)

2Complete vs incomplete combustion of methane (5 marks, Paper 1)

3Formation and effects of acid rain (6 marks, Paper 2)

4Enhanced greenhouse effect and climate change (5 marks, Paper 2)

5Calculate % oxygen from experimental data (3 marks)

Atmosphere

Combustion

Complete combustion

Incomplete combustion

Carbon monoxide (CO)

Acid rain

Greenhouse effect

Enhanced greenhouse effect

Greenhouse gas

Climate change

✕Confusing CO and CO₂ as products of combustion

✕Confusing the greenhouse effect with ozone layer depletion

✕Writing O₂ = 78% and N₂ = 21% (swapping the values)

✕Saying CO₂ causes acid rain

✕Reading the volume of gas while still hot in the oxygen experiment

✕Omitting water as a product of combustion

Practice questions

Past paper style quiz

Assess your understanding

Video lesson

Gases in the Atmosphere — frequently asked questions