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Detailed notes on Air and Water for Cambridge IGCSE Coordinated Science, covering key concepts, explanations, examples, and exam-focused revision points.
Nitrogen gas must be 'fixed' into a usable form before plants can absorb it. The Haber process (NH₃) and subsequent reactions make nitrogenous fertilisers. Cambridge tests the Haber process conditions, reasons for them, and problems of fertiliser overuse.
Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).
The Haber process synthesises ammonia from nitrogen and hydrogen under carefully chosen conditions — a compromise between yield and rate.
The Haber Process: the industrial synthesis of ammonia.
N₂(g) + 3H₂(g) ⇌ 2NH₃(g) ΔH = −92 kJ/mol (exothermic, REVERSIBLE)
Raw materials:
Process conditions and justification:
| Condition | Value | Justification |
|---|---|---|
| Temperature | ~450°C | Compromise: lower T → more NH₃ (exothermic reaction favours low T) but too slow. 450°C gives acceptable rate while maintaining reasonable yield |
| Pressure | ~200 atm | Higher pressure favours NH₃ (fewer moles on right side → 4 mol → 2 mol). But very high pressure is expensive and dangerous. 200 atm is a compromise |
| Catalyst | Iron (Fe) | Does not affect yield or equilibrium position — only speeds up rate. Allows reaction to reach equilibrium faster |
Process overview:
Yield at 450°C and 200 atm: ~15% conversion per pass. Recycling ensures economical production.
Ammonia is converted into nitrogen fertilisers that boost crop yields. Overuse causes eutrophication in waterways.
Making nitrogen fertilisers from ammonia:
Why fertilisers are needed:
Environmental problems — Eutrophication:
Other impacts: Nitrates in drinking water → health concerns (methaemoglobinaemia in infants). Acidification of soil.
Verbatim phrases and definitions Cambridge mark schemes credit.
Paper 4: 'Explain why a temperature of 450°C is used in the Haber process rather than 200°C or 700°C' (3 marks — forward reaction is exothermic so lower T favours more NH₃, but too slow; 450°C is a compromise giving acceptable rate and yield). 'Explain why high pressure is used in the Haber process' (2 marks — 4 mol gas → 2 mol; fewer moles of gas on right; high pressure increases proportion of NH₃). 'Explain the stages by which excess fertiliser leads to the death of fish' (4 marks — leaching, algal bloom, blocks light, plants die, decomposers, O₂ used, fish suffocate).
Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C11); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.
Step-by-step solutions to past-paper-style questions on nitrogen and fertilisers, written exactly the way a tutor would explain them at the board.
Question
State the conditions used in the Haber process for the manufacture of ammonia.
Step-by-step solution
Step 1
Equation for the Haber process:
N2(g)+3H2(g)⇌2NH3(g)
Step 2
Temperature: approximately 450 °C (compromise temperature).
Step 3
Pressure: 150–200 atm (high pressure favours more NH₃ — fewer moles on product side).
Step 4
Catalyst: iron (Fe) with promoters (e.g. Al₂O₃/K₂O).
Answer
Temperature: ~450 °C; pressure: 150–200 atm; catalyst: iron.
Examiner tip
All three conditions must be stated. 'High temperature and pressure with iron catalyst' is the minimum acceptable answer.
Question
Explain why the Haber process uses a temperature of 450 °C rather than 200 °C.
Step-by-step solution
Step 1
The forward reaction (N₂ + 3H₂ → 2NH₃) is exothermic; by Le Chatelier's principle, lower temperature shifts equilibrium to the right, giving a higher yield of NH₃.
Step 2
At 200 °C, the equilibrium yield would be higher, but the rate of reaction would be extremely slow — economically unviable.
Step 3
450 °C is a compromise: the rate of reaction is fast enough for industrial production, and the yield, while not maximum, is acceptable (~15% conversion per pass).
Step 4
Unreacted N₂ and H₂ are recycled, improving overall efficiency.
Answer
Lower T → higher yield but too slow. 450 °C is a compromise between acceptable yield and acceptable rate; unreacted gases are recycled.
Examiner tip
Must say 'compromise' and explain BOTH why not lower (rate too slow) and the impact of the exothermic reaction on equilibrium.
Question
Explain why excess use of nitrogen fertilisers can damage river ecosystems.
Step-by-step solution
Step 1
Excess nitrate ions (NO₃⁻) not absorbed by plants are leached from the soil into rivers and lakes by rain.
Step 2
The high nitrate levels stimulate rapid growth of algae and aquatic plants (algal bloom), covering the water surface.
Step 3
The algal covering blocks sunlight; submerged aquatic plants cannot photosynthesise and die.
Step 4
Bacteria decompose the dead plant and algal material; the bacteria multiply rapidly and consume dissolved oxygen in the water (aerobic respiration).
Step 5
Dissolved oxygen levels fall; fish and other aquatic organisms suffocate and die. This whole process is called eutrophication.
Answer
Nitrates leach → algal bloom → blocks light → plants die → bacteria decompose → O₂ depleted → fish die (eutrophication).
Examiner tip
Must describe the full chain of events for full marks. 'Algal bloom → less O₂ → fish die' alone is insufficient; each step must be linked causally.
Question
State the role of nitrogen in plant growth and name a fertiliser that supplies nitrogen.
Step-by-step solution
Step 1
Nitrogen is essential for the synthesis of amino acids and proteins in plants; also required for chlorophyll and nucleic acids (DNA/RNA).
Step 2
Nitrogen promotes vegetative growth — the formation of leaves, stems, and shoots.
Step 3
A suitable nitrogenous fertiliser: ammonium nitrate (NH₄NO₃) — supplies nitrogen as both NH₄⁺ and NO₃⁻ ions, readily absorbed by plants.
Answer
Nitrogen: needed for protein/amino acid synthesis; promotes leaf/stem growth. Fertiliser: ammonium nitrate (NH₄NO₃) or urea.
The formulae you need to memorise for nitrogen and fertilisers on the Cambridge IGCSE 0654 paper, with every variable defined in plain English and a note on when to use it.
N2(g)+3H2(g)⇌2NH3(g)ΔH=−92kJ/mol
When to use
Writing or referencing the Haber process; applying Le Chatelier's principle to predict the effect of temperature and pressure changes.
Definitions to memorise and the exact keywords mark schemes credit for nitrogen and fertilisers answers — sharpened from recent examiner reports for the 2026 0654 sitting.
The industrial manufacture of ammonia from nitrogen and hydrogen: N₂ + 3H₂ ⇌ 2NH₃; conditions: ~450 °C, 150–200 atm, iron catalyst.
A substance added to soil to supply essential nutrients for plant growth. NPK fertilisers supply nitrogen (promotes leaf growth), phosphorus (promotes root development), and potassium (promotes flower/fruit development).
The process by which excess nutrients (especially nitrates) leach from land into water bodies, stimulating rapid algal growth, reducing dissolved oxygen, and causing death of aquatic organisms.
The conversion of atmospheric N₂ into compounds usable by living organisms; occurs naturally via lightning (N₂ → NO₃⁻) and nitrogen-fixing bacteria (e.g. Rhizobium in root nodules); industrially via the Haber process.
If a system at equilibrium is subjected to a change (temperature, pressure, concentration), the equilibrium shifts in the direction that partially opposes the change.
The conversion of ammonium ions (NH₄⁺) to nitrate ions (NO₃⁻) in soil, carried out by nitrifying bacteria (e.g. Nitrosomonas, Nitrobacter); makes nitrogen available to plants.
The traps other students keep falling into on nitrogen and fertilisers questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.
0654 Examiner Report 2023
Why it happens
Students confuse rate and equilibrium: higher temperature increases rate, but for an exothermic reaction, it shifts equilibrium left (less NH₃).
How to avoid it
Higher temperature → faster rate BUT lower yield (equilibrium shifts left for exothermic). 450 °C is the compromise. Remember: Le Chatelier's principle for exothermic reactions — heat is a 'product', so adding heat reduces yield.
Why it happens
Students know high pressure favours NH₃ but cannot explain why using mole counting.
How to avoid it
Left side: 4 moles of gas (1 N₂ + 3 H₂). Right side: 2 moles of gas (2 NH₃). High pressure favours the side with fewer moles of gas (right) → higher NH₃ yield.
Why it happens
Students mix up respiration and photosynthesis in the eutrophication sequence.
How to avoid it
Algae PHOTOSYNTHESISE but then DIE, and their decomposition by BACTERIA uses up O₂ (aerobic respiration by bacteria). It is the bacterial respiration, not algal photosynthesis, that depletes dissolved O₂.
The things students keep getting wrong in this sub-topic, answered.