C10 is the “chemistry in the real world” topic — how industry turns the Earth’s resources into the water, materials and fertilisers we depend on, and how it tries to do so sustainably. It is broad rather than deep, so the marks come from clear sequences (the steps of water treatment, the stages of the Haber process), precise definitions, and balanced evaluate answers.
- Everyone — sections 1–3 and 5–6: resources and sustainability, potable and waste water (including Required practical 8), life cycle assessment, and recycling.
- Higher Tier H — the biological metal-extraction methods in section 4, and explaining the Haber conditions with equilibrium in section 10.
- Triple (Chemistry only) T — sections 7–11: corrosion, alloys, ceramics/polymers/composites, the Haber process and NPK fertilisers.
Several ideas reach back into other topics: the Haber process uses the equilibrium of C6, polymers come from the alkenes of C7, and recycling metals links to the metal extraction of C4. Each is recapped where it comes up.
1Earth’s Resources & Sustainability
Everything we make starts with a natural resource — something from the Earth, oceans or atmosphere that provides warmth, shelter, food or transport. Chemistry’s job is to turn these into useful products while keeping an eye on whether they will run out.
- Natural resources — materials from the Earth, oceans and atmosphere (eg metal ores, crude oil, water, air, timber). Agriculture supplements them with food, timber, clothing and fuels.
- Finite (non-renewable) resources — ones that are being used up faster than they form, or don’t reform at all: fossil fuels and metal ores.
- Renewable resources — ones that can be replaced as fast as they’re used, on a human timescale: eg timber (trees can be replanted).
- Sustainable development — development that meets the needs of today without compromising the ability of future generations to meet their own needs.
Many natural products can be supplemented or replaced by agricultural or synthetic alternatives:
- Rubber (natural latex from trees) is replaced by synthetic polymers designed for the job.
- Fertilisers boost the yield of natural food crops (the Haber process, section 10).
You should be able to distinguish finite from renewable resources and interpret data about resources from charts, graphs and tables — including using orders of magnitude to judge how significant a figure really is.
- “Finite” and “non-renewable” mean the same thing — the terms are interchangeable.
- Renewable doesn’t mean instant. Timber is renewable but trees still take years to regrow — sustainability is about the rate of use versus replacement.
🧪 Exam-style questions
Which of these is a renewable resource? Tick (✓) one box.
What is meant by sustainable development? Tick (✓) one box.
Which natural product has been largely replaced by synthetic alternatives? Tick (✓) one box.
A metal’s known reserves are about 1012 tonnes, and it is used at about 1010 tonnes per year. Using orders of magnitude, roughly how long will the reserves last? Tick (✓) one box.
2Potable Water
Water of the right quality is essential for life. Potable water is water that is safe to drink — it has low levels of dissolved salts and microbes. Crucially, potable water is not the same as pure water.
- Pure water (chemistry) contains only H2O molecules — nothing dissolved.
- Potable water is safe to drink but contains dissolved substances (small, safe amounts of minerals and salts). So it is not chemically pure.
Making potable water from fresh water
In the UK, rain gives fresh water (low in dissolved substances) that collects in lakes, rivers and reservoirs (surface water) or underground in aquifers (ground water). To make it potable it is filtered then sterilised — filtration is the same separating technique you met in C1:
Fresh water is made potable in two key steps: filtration removes insoluble solids, then sterilisation (chlorine, ozone or UV) kills microbes.
Desalination — when fresh water is scarce
Where fresh water is limited (hot, dry regions), potable water can be made from sea water or salty water by desalination: removing the salt by distillation or by reverse osmosis (forcing water through a membrane that holds back the ions). Both need large amounts of energy, so desalination is expensive and used only when there is no better source.
Part A — analysis. Measure the pH with universal indicator, and find the dissolved solids: weigh an evaporating basin, add a known volume (eg 25 cm3) of the sample, evaporate the water, then re-weigh — the mass gained is the dissolved solid. (Don’t overheat, or you decompose the solids.)
Part B — purification by distillation. Heat the sample so water boils off, then condense the vapour and collect it — the distillate is pure water. Test it: pure water boils at exactly 100 °C and leaves no residue on evaporation.
- Independent variable: the water sample tested (the source, eg fresh, sea or distilled water) — or, in Part B, the purification step (before vs after distillation).
- Dependent variables: the pH, the mass of dissolved solids left per known volume after evaporation, and whether the distillate is pure water (a sharp boiling point at exactly 100 °C with no residue).
- Control variables: the same volume of each sample, and the same method and apparatus each time (same balance, same evaporating basin, evaporate to dryness without overheating).
Simple distillation: boil the salt water, condense the steam in a cooled tube, and collect pure water. The salt is left behind in the flask.
- “Potable means pure.” No — potable water still contains dissolved substances; it’s just safe to drink.
- Confusing the two steps. Filtration removes insoluble solids; sterilisation (chlorine/ozone/UV) kills microbes. Filtering does not kill bacteria.
- Forgetting why desalination is a last resort. It uses a lot of energy, so it’s expensive — only used where fresh water is unavailable.
🧪 Exam-style questions
How does potable water differ from pure water? Tick (✓) one box.
Fresh water is passed through filter beds. Why? Tick (✓) one box.
Name one method used to sterilise water in the UK.
Show answer
Any one of: chlorine, ozone or ultraviolet (UV) light. 1 mark (These kill microbes — sterilisation is a separate step from filtration.)
Sea water is made potable by desalination. Name the substance removed, and give one reason desalination is only used when necessary. Tick (✓) one box.
After distilling a water sample, how could you show the distillate is pure water? Tick (✓) one box.
3Waste Water Treatment
Homes, farms and factories all produce waste water that must be cleaned before it returns to rivers and the sea. Sewage and agricultural waste water need organic matter and harmful microbes removed; industrial waste water may also need harmful chemicals removed.
Sedimentation splits the waste into sludge (digested without air, anaerobically) and effluent (treated with air, aerobically). The methane from the sludge is even used as an energy source.
- Screening and grit removal — take out large solids (twigs, plastic) and grit.
- Sedimentation — in a settlement tank, heavy solids sink to form sludge; the liquid on top is effluent.
- Anaerobic digestion of the sludge — bacteria break it down without air; this releases methane (used for energy) and leaves a digested solid used as fertiliser.
- Aerobic biological treatment of the effluent — air is pumped in so aerobic bacteria break down the remaining organic matter.
Be ready to compare the ease of producing potable water from different sources:
- Ground / fresh water — easiest: low in dissolved substances, so just filter and sterilise.
- Waste water — harder: lots of organic matter, microbes and toxins to remove, so many treatment stages — but it still uses less energy than desalination.
- Salt / sea water — hardest: desalination removes dissolved salt and needs the most energy.
🧪 Exam-style questions
What is the first stage of sewage treatment? Tick (✓) one box.
Sedimentation separates the waste water into two parts. What are they? Tick (✓) one box.
How are the sludge and the effluent each treated? Tick (✓) one box.
Explain why it is more difficult to produce potable water from waste water than from ground water.
Show answer
- Ground water is already low in dissolved substances, so it only needs filtering and sterilising. 1 mark
- Waste water contains a lot of organic matter, microbes (and toxins). 1 mark
- So waste water needs many more treatment stages to remove them, making it harder (and more expensive). 1 mark
4Extracting Metals from Low-grade Ores H
This section (biological methods of metal extraction) is Higher Tier only, on both Combined and Triple papers.
The Earth’s metal ores are finite, and the rich, high-grade ores are running out — copper especially. Mining low-grade ores (which contain only small amounts of metal) the traditional way means digging up, moving and disposing of huge amounts of rock. Two biological methods avoid that damage.
First, a quick recap from C4: a metal is locked inside its ore as a compound, and freeing it normally takes a chemical reduction step (heating with carbon, or electrolysis). The biological methods below are alternatives aimed at low-grade ores.
Most metals are found as compounds inside ores. Crushing and concentrating the ore is physical — the metal is only freed by a chemical reduction step, using carbon or electrolysis.
- Phytomining — plants are grown on land containing the metal. They absorb metal compounds through their roots, which become concentrated in the plant. The plants are harvested and burned, and the ash contains the metal compounds.
- Bioleaching — bacteria are used to break down low-grade ores, producing a leachate solution that contains metal ions (eg copper).
Either way you end up with a metal compound or solution, not the metal itself.
Phyto- means plant, and leaching means washing soluble compounds out into solution — the names tell you how each method works.
The metal compounds are then processed to obtain the metal. For copper, this is done by:
- Displacement — adding scrap iron to a solution of a copper compound (iron is more reactive, so it displaces the copper), or
- Electrolysis of the solution.
- Advantages — they let us use low-grade ores and mining waste, and avoid the digging, transport and rock disposal of traditional mining, so there is less environmental damage.
- Disadvantages — they are slow, and bioleaching can produce toxic substances that must be treated. The metal still has to be extracted by displacement or electrolysis.
🧪 Exam-style questions
Name what phytomining uses to extract metals.
Show answer
Plants. 1 mark (They absorb metal compounds and are then burned; the ash contains the metal compounds. Bioleaching uses bacteria.)
In bioleaching, name what produces the solution (leachate) containing metal ions.
Show answer
Bacteria. 1 mark (They break down low-grade ore to produce a leachate solution of metal ions.)
How can copper be obtained from a solution of a copper compound? Tick (✓) one box.
Give one advantage of phytomining and bioleaching over traditional mining. Tick (✓) one box.
Copper-rich ores are running out. Evaluate the use of phytomining and bioleaching to extract copper, compared with traditional mining. This is a levels-of-response question — weigh the benefits against the drawbacks and reach a justified conclusion. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): benefits and drawbacks discussed, ending in a justified conclusion.
- Level 2 (3–4): describes benefits and at least one drawback.
- Level 1 (1–2): one or two simple relevant points.
Indicative content — any of the following are credited.
Benefits:
- They can extract copper from low-grade ores (and mining waste) that would otherwise be uneconomic, conserving the dwindling high-grade ores.
- They avoid the digging, moving and disposal of huge amounts of rock, so there is far less landscape damage, noise and dust than traditional mining.
- Allow also: less energy than digging/transporting huge amounts of rock; less habitat destruction; uses waste tips/spoil heaps.
Drawbacks:
- Both methods are slow.
- Bioleaching can produce toxic, acidic leachate that harms the environment if it escapes.
- You still only get a metal compound or solution — the copper must then be extracted by displacement with scrap iron or by electrolysis, which costs energy.
- Allow also: phytomining needs land/growing time; lower copper recovery per tonne than smelting rich ore.
Do not accept: “biological methods make no waste / cause no harm at all”; “they give pure copper directly”.
Conclusion (needed for Level 3) — any justified verdict scores: as high-grade copper ores run out, these biological methods are increasingly worthwhile — they let us use resources we otherwise couldn’t, with much less environmental damage, even though they are slow and still need a final extraction step. 6 marks
5Life Cycle Assessment
To judge a product’s true environmental cost, you can’t just look at it in use — you have to follow it from cradle to grave. A life cycle assessment (LCA) works out the environmental impact at every stage of a product’s life.
An LCA follows a product clockwise through four stages — raw materials, manufacture, use and disposal — adding up the environmental impact (and the transport) at each step.
- Raw materials — extracting and processing them uses up finite resources and can damage habitats (mining, deforestation).
- Manufacture & packaging — uses energy, land for factories, and produces waste.
- Use — impact during the product’s lifetime (a car pollutes; a wooden desk barely does).
- Disposal — landfill space, and whether it can be recycled — plus transport at every stage.
- Some things are easy to quantify — the water, energy and resources used, and the waste produced, can be measured.
- Pollutant effects are not. Putting a number on “how bad” a pollutant is requires a value judgement, so two people may score it differently — LCA is not purely objective.
- Beware abbreviated LCAs. Selective LCAs can be misused to reach a pre-decided conclusion — for example in advertising, to make a product look greener than it is.
Run the LCA — plastic vs paper bag
A life cycle assessment follows a product through four stages, adding up the impact at each. Compare the bags stage by stage, then weigh the figures an LCA can actually measure. A company claims the plastic bag has less impact — is it right?
The four stages of the life cycle
Transport adds impact at every stage too.
▼ The figures an LCA can measure (per use)
Show the numbers (Table 1)
| To make one bag | Plastic | Paper |
|---|---|---|
| Raw material | crude oil / gas | wood |
| Energy used (MJ) | 1.5 | 1.7 |
| Solid waste (g) | 14 | 50 |
| CO2 produced (kg) | 0.23 | 0.53 |
| Fresh water (dm3) | 255 | 4520 |
Data values are real AQA-style figures for making one bag. Reusing a bag spreads that one-off impact over more uses, so the bar shows the impact per use = table value ÷ number of uses. Lower is greener. In practice a plastic bag is hard-wearing while a paper bag tears sooner — so durability is part of the comparison too.
🧪 Exam-style questions
Which is not one of the four stages of a life cycle assessment? Tick (✓) one box.
Why is a life cycle assessment not a completely objective process? Tick (✓) one box.
Why should you be cautious about an abbreviated (selective) LCA used in an advert? Tick (✓) one box.
A shopper wants to know whether a paper bag or a plastic bag is the better environmental choice. Use the idea of a life cycle assessment to compare them, and explain why there is no single right answer. A levels-of-response question — compare across the LCA stages and reach a justified judgement. Plan, then compare with the model answer.
Show a model answer
How it is marked (levels of response):
- Level 3 (5–6): compares across the stages and recognises the verdict depends on assumptions/use, with a justified judgement.
- Level 2 (3–4): compares the two bags at one or more stages.
- Level 1 (1–2): one or two simple relevant points.
Indicative content — any of the following are credited.
What the measured data shows (in this kind of LCA the plastic bag is lower on every measured count, supporting the company):
- Energy: the paper bag needs more energy to make (e.g. 1.7 vs 1.5 MJ).
- Solid waste: the paper bag produces far more (e.g. 50 vs 14 g).
- CO2: the paper bag produces more (e.g. 0.53 vs 0.23 kg).
- Fresh water: the paper bag uses dramatically more (e.g. 4520 vs 255 dm3).
What the data leaves out (and why it’s not the whole story):
- The plastic bag is from a finite resource (crude oil/natural gas); the paper bag is from a renewable one (wood).
- The paper bag is biodegradable; the plastic bag is not.
- Some of the paper bag’s CO2 is offset by photosynthesis in the growing wood.
- Judging how bad each pollutant is needs a value judgement, so an LCA is not purely objective.
Allow also: a plastic bag is more durable, so it can be reused, spreading its impact over more uses; transport adds impact at every stage; the figures depend on how many times each bag is used. Do not accept: “plastic is always better” or “paper is always better” with no use of the data or stages.
Conclusion (needed for Level 3) — any justified verdict scores: you could agree (non-renewability matters less than the big savings in water, energy, waste and CO2), disagree (the finite raw material and persistence outweigh them), or say you can’t be certain because the data is incomplete. 6 marks
6Reducing the Use of Resources
Metals, glass, building materials, ceramics and most plastics are all made from limited raw materials, using energy from limited resources, and quarrying and mining damage the environment. The way to ease that is the familiar trio: reduce, reuse, recycle.
- Reduce — using less in the first place cuts the use of limited resources, energy and waste.
- Reuse — using a product again as it is. For example, glass bottles can be washed, sterilised and refilled.
- Recycle — processing a used material into something new:
- Metals are melted and recast into new products. Some scrap steel is even added to the iron from a blast furnace, reducing the iron ore needed.
- Glass that can’t be reused is sorted (by colour), crushed and melted to make new glass.
How much separation/sorting is needed depends on the material and the properties required of the final product.
Extracting a metal from its ore (C4) is very energy-intensive (and mining damages the landscape). Recycling a metal — just melting and reshaping it — uses far less energy, saves the finite ore, and cuts waste and landfill. This is why recycling aluminium (expensive to extract by electrolysis) is such a big win.
- Reuse ≠ recycle. Reuse keeps the object as it is (refilling a bottle); recycling breaks it down to make something new (melting glass or metal).
- Recycling isn’t free. Collecting, transporting and sorting materials needs energy and labour, and recycled material may be lower quality — useful points in an evaluate answer.
🧪 Exam-style questions
A glass bottle is washed and refilled. Is this reusing or recycling? Tick (✓) one box.
Give two reasons why metals should be recycled rather than extracted from their ores. Tick (✓) one box.
Give one disadvantage of recycling materials. Tick (✓) one box.
Scrap steel is added to the iron produced in a blast furnace. What does this reduce? Tick (✓) one box.
7Corrosion & Rusting T
Sections 7–11 (materials, the Haber process and fertilisers) are Chemistry only — not on the Combined Science papers.
Corrosion is the destruction of a metal by chemical reactions with substances in its environment. Rusting is the corrosion of iron specifically — and it needs both air (oxygen) and water.
Only the nail with both air and water rusts. Boiling the water and sealing it with oil removes the air; calcium chloride removes the water — and neither nail rusts.
Watch it rust over a week
Slide the day forward (or press play). Only the nail with both air and water rusts — the controls stay shiny, proving both are needed.
Day 0 — every nail is shiny steel. Move the days forward and watch.
Build your own tube
- Barrier methods — keep out air and water with a coating: painting, oiling, greasing, coating in plastic, or electroplating. (If the barrier is scratched, the iron underneath rusts again.)
- Aluminium protects itself — it forms a tough oxide layer that stops further corrosion.
- Galvanising — coating iron with zinc. The zinc is a barrier and gives sacrificial protection.
- Sacrificial protection — attach a more reactive metal (zinc or magnesium — check the C4 reactivity series). Being more reactive, it loses electrons and corrodes in preference to the iron, so the iron is protected even if the surface is scratched. (Magnesium blocks are bolted to ships’ hulls.)
- Corrosion is the general term; rusting is corrosion of iron specifically. Don’t use them interchangeably.
- Sacrificial metal must be more reactive than iron (zinc, magnesium) — it corrodes instead of the iron.
- Barrier vs sacrificial. A scratched barrier stops working; sacrificial protection keeps working even when scratched, because the reactive metal still corrodes first.
🧪 Exam-style questions
Which two substances are needed for iron to rust? Tick (✓) two boxes, then press Check.
In the rusting experiment, one tube has boiled water with a layer of oil on top. Why does the nail in this tube not rust? Tick (✓) one box.
Blocks of zinc are attached to an iron ship’s hull. Explain how this protects the iron. Tick (✓) one box.
Which of these is a barrier method of rust prevention? Tick (✓) one box.
8Alloys T
Most metals in everyday use are alloys — a metal mixed with one or more other elements. Alloys are usually harder and stronger than the pure metal, and the reason is all about how the atoms are arranged.
Two metal structures side by side — a pure metal and an alloy. Press the button to apply a force and watch whether the layers can slide over each other.
Why alloys are harder. Pure metal (left): identical ions in regular layers slide over each other easily. Alloy (right): atoms of a different size distort the layers, so they bump into each other and cannot slide — making the alloy harder and stronger.
- Bronze = copper + tin — harder than copper; used for statues, ornaments and medals.
- Brass = copper + zinc — corrosion-resistant; used for fittings and musical instruments.
- Gold alloys (with silver, copper, zinc) — for jewellery; purity in carats (24 carat = 100% gold, 18 carat = 75%).
- Steels = iron + carbon (+ other metals): high-carbon steel is strong but brittle; low-carbon steel is soft and easily shaped; stainless steel (with chromium and nickel) is hard and corrosion-resistant.
- Aluminium alloys — low density, used for aircraft.
- An alloy is a mixture, not a compound — the elements are not chemically combined.
- Carats are a fraction of 24. 24 carat = pure (100%); 18 carat = 18/24 = 75%; 12 carat = 50%.
- Don’t confuse an alloy with a composite. An alloy is a uniform mix of metals; a composite has two distinguishable materials (section 9).
🧪 Exam-style questions
Why is an alloy usually harder than the pure metal? Tick (✓) one box.
Bronze is an alloy of copper and one other metal. Name the other metal.
Show answer
Tin. 1 mark (Bronze = copper + tin. Don’t confuse it with brass = copper + zinc.)
An 18-carat gold ring has a mass of 8.0 g. Pure gold is 24 carat. Calculate the mass of gold in the ring.
Show answer
- Fraction of gold = 18 ÷ 24 = 0.75 (75%). 1 mark
- Mass of gold = 0.75 × 8.0 = 6.0 g. 1 mark
Stainless steel resists corrosion. Which elements are added to the steel to make it stainless? Tick (✓) one box.
9Ceramics, Polymers & Composites T
The materials we build with fall into a few families — ceramics, polymers and composites — and their properties come from their structure. Match the material to the job.
Ceramics
- Soda-lime glass — the common glass, made by heating sand, sodium carbonate and limestone until it melts.
- Borosilicate glass — made from sand and boron trioxide; it melts at a higher temperature than soda-lime glass (so it tolerates heat better).
- Clay ceramics (pottery, bricks) — made by shaping wet clay then firing it (heating in a furnace) to make it hard.
Polymers
A polymer’s properties depend on the monomer it’s made from and the conditions of making it. The same monomer can give different materials: low density (LD) and high density (HD) poly(ethene) are both made from ethene, but under different conditions — LD poly(ethene) (made at high pressure) is flexible for bags; HD poly(ethene) (made with a catalyst at lower temperature/pressure) is more rigid for pipes and tanks. (Ethene is an alkene, and the small monomers join by addition polymerisation, from C7.)
Think loose spaghetti versus a knotted net — and it’s why thermosoftening polymers can be recycled by melting and remoulding, but thermosetting polymers can’t.
Composites
A composite is made of two materials: a matrix (binder) surrounding reinforcement (fibres or fragments). The combination gives properties neither has alone. Examples: fibreglass, carbon fibre, steel-reinforced concrete, and wood (a natural composite of fibres in a polymer matrix).
- Thermosetting does NOT melt — its strong cross-links hold the structure together. Thermosoftening does melt because only weak intermolecular forces hold its separate chains together.
- LD and HD poly(ethene) are both made from ethene — the difference is the conditions of manufacture, not the monomer.
- Composite vs alloy. A composite has two distinguishable materials (matrix + reinforcement); an alloy is a uniform mixture of metals.
🧪 Exam-style questions
Why does a thermosetting polymer not melt when heated? Tick (✓) one box.
Name the three substances that are heated together to make soda-lime glass.
Show answer
Sand, sodium carbonate and limestone, heated until they melt. 1 mark (All three needed. Sand + boron trioxide makes borosilicate glass instead.)
Which of these is a composite material? Tick (✓) one box.
A thermosoftening polymer can be melted and remoulded. Explain why, in terms of its structure. Tick (✓) one box.
A bridge is to be built from concrete. An engineer has the data in the table below.
| Material | Compressive strength (MPa) | Tensile strength (MPa) |
|---|---|---|
| Concrete | 40 | 3 |
| Steel-reinforced concrete (a composite) | 40 | 40 |
Compare the two materials using the data, and explain why the engineer chooses steel-reinforced concrete for the bridge.
Show answer
- Both materials have the same compressive strength (40 MPa), so they resist being squashed equally well. 1 mark
- But the composite has a much higher tensile strength (40 MPa vs 3 MPa) — about 13 times higher — so it is far better at resisting being stretched/pulled apart. 1 mark
- A bridge is loaded so that parts are stretched, and plain concrete would crack at low tension; the steel reinforcement gives the tensile strength the concrete lacks, so the composite combines the best of both. 1 mark
Allow: any correct use of the figures (e.g. quoting both tensile values, or the difference 40 − 3 = 37 MPa, or that the composite is ~13× stronger in tension). Do not accept: “the composite is stronger” with no reference to the data.
10The Haber Process T H
The Haber process manufactures ammonia (NH3), the starting point for nitrogen fertilisers. It’s a classic reversible reaction run at carefully chosen compromise conditions — and explaining why those conditions are used is Higher-tier gold dust.
N₂ comes from the air; H₂ from natural gas (methane).
- Raw materials: nitrogen from the air; hydrogen from natural gas (methane).
- Conditions: iron catalyst, about 450 °C, about 200 atmospheres.
- The reaction is reversible: N2(g) + 3H2(g) ⇌ 2NH3(g).
- Separation: on cooling, the ammonia liquefies (it has a higher boiling point) and is removed; the unreacted N2 and H2 are recycled.
The forward reaction is exothermic and goes from 4 gas molecules to 2. So:
- Temperature (450 °C). A lower temperature would give a higher yield (equilibrium shifts to the exothermic forward reaction) but the rate would be too slow. 450 °C is a compromise — a reasonable yield produced at a reasonable rate.
- Pressure (200 atm). A higher pressure gives a higher yield (equilibrium shifts to the side with fewer gas molecules) but very high pressures are expensive and dangerous. 200 atm is a compromise between yield and cost/safety.
- Iron catalyst. Speeds up the reaction so equilibrium is reached faster, but does not change the position of equilibrium (the yield). It lets a good rate be achieved at the lower, cheaper temperature.
Find the compromise — rate vs yield
Change the temperature and pressure and watch the yield of ammonia and the rate of reaction respond. There is no setting that wins on both — that is why the industrial conditions are a compromise.
N2(g) + 3H2(g) ⇌ 2NH3(g) · forward is exothermic, 4 gas molecules → 2
- Forgetting it’s reversible. Use the ⇌ sign — not all the N2 and H2 turn into ammonia in one pass, which is why the gases are recycled.
- Why ammonia is removed by cooling. Ammonia has a higher boiling point than N2 and H2, so it condenses to a liquid while they stay gases.
- The catalyst doesn’t change the yield — it only makes equilibrium arrive faster.
🧪 Exam-style questions
Name the source of the nitrogen and the source of the hydrogen used in the Haber process.
Show answer
- Nitrogen comes from the air. 1 mark
- Hydrogen comes from natural gas (methane). 1 mark
Which conditions are used in the Haber process? Tick (✓) one box.
The reaction mixture is cooled so that only the ammonia condenses. Why does only the ammonia condense? Tick (✓) one box.
What happens to the unreacted nitrogen and hydrogen? Tick (✓) one box.
A lower temperature would increase the yield of ammonia. Explain why a temperature of about 450 °C is used in the Haber process instead.
Show answer
- The forward reaction is exothermic, so a lower temperature shifts the equilibrium to the right, giving a higher yield of ammonia. 1 mark
- But at a lower temperature the rate of reaction is too slow, so equilibrium is reached too slowly to be economic. 1 mark
- So 450 °C is a compromise — a reasonable yield obtained at a reasonable rate. 1 mark
What is the effect of the iron catalyst on the yield of ammonia? Tick (✓) one box.
11NPK Fertilisers T
The ammonia from the Haber process feeds into fertilisers — the products that keep farm soils productive. NPK fertilisers supply the three elements plants need most: nitrogen (N), phosphorus (P) and potassium (K).
- They contain compounds of nitrogen, phosphorus and potassium, used to improve agricultural productivity.
- An NPK fertiliser is a formulation — a mixture of salts blended to give the right percentage of each element.
- The compounds must be water-soluble so plants can absorb the ions through their roots: ammonium (NH4+) and nitrate (NO3–) for nitrogen, phosphate (PO43–) for phosphorus, and potassium ions (K+).
Ammonia is the key building block:
- Ammonia is an alkali, so it neutralises acids to make ammonium salts (C4). With nitric acid it makes the important fertiliser ammonium nitrate:
NH3 + HNO3 → NH4NO3
- Ammonia can also be oxidised to make nitric acid (the source of the nitrate).
- Lab vs industrial: in the lab, a chemist makes a pure batch by titrating ammonia with acid (C4); industrially it’s a large-scale, continuous integrated process producing huge quantities.
Potassium chloride, potassium sulfate and phosphate rock are obtained by mining. Phosphate rock is insoluble, so it can’t be used directly — it’s treated with acid to make soluble salts:
| Treat phosphate rock with… | Products |
|---|---|
| nitric acid | phosphoric acid + calcium nitrate |
| sulfuric acid | single superphosphate (calcium phosphate + calcium sulfate) |
| phosphoric acid | triple superphosphate (calcium phosphate) |
- An NPK fertiliser is a formulation (a mixture) — not a single compound.
- Fertiliser compounds must be soluble, or the plant can’t take them up — which is exactly why insoluble phosphate rock has to be reacted with acid first.
- Lab vs industry: the lab method (titration) makes small, pure batches; industry runs a continuous, large-scale process.
🧪 Exam-style questions
State the three elements that the letters N, P and K stand for in an NPK fertiliser.
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Nitrogen (N), phosphorus (P) and potassium (K) — the three elements plants need most. 1 mark
Ammonia reacts with nitric acid to make an important fertiliser. Name this fertiliser.
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Ammonium nitrate. 1 mark (NH3 + HNO3 → NH4NO3, a key nitrogen fertiliser.)
Why must the compounds in a fertiliser be soluble in water? Tick (✓) one box.
Phosphate rock is reacted with sulfuric acid. What is the product called? Tick (✓) one box.
★Capstone: Finite or Renewable?
One question runs through the whole topic: will a resource run out? A finite (non-renewable) resource is used up faster than it forms; a renewable one can be replaced as fast as we use it. Sort all eight to bookend the topic.
Drag each resource into a box — or tap it to step through the boxes. Then press Check.
- Resources & sustainability — natural resources come from the Earth, oceans and atmosphere. Finite = fossil fuels and metal ores; renewable = things that regrow (timber). Sustainable development meets today’s needs without compromising future generations.
- Potable water — safe to drink, but not pure (it has dissolved substances). UK fresh water: choose a source → filter (remove insoluble solids) → sterilise (chlorine, ozone or UV). Sea water needs desalination (distillation or reverse osmosis) — lots of energy.
- Waste water — screening & grit removal → sedimentation (sludge sinks, effluent floats) → anaerobic digestion of sludge → aerobic treatment of effluent. Ground water is easiest to make potable; salt water hardest.
- Low-grade ores H — phytomining (plants → burn to ash) and bioleaching (bacteria → leachate); the copper is then obtained by displacement (scrap iron) or electrolysis.
- Life cycle assessment — four stages: raw materials → manufacture → use → disposal (plus transport). Energy/water/waste can be measured; pollutant harm needs a value judgement, so an LCA is not purely objective. Reduce, reuse, recycle to save finite resources and energy.
- Corrosion T — rusting needs air and water. Prevent it with a barrier (paint, oil, plastic, electroplating) or sacrificial protection (a more reactive metal like zinc/magnesium corrodes instead).
- Materials T — alloys (bronze = Cu+Sn, brass = Cu+Zn, steels, gold carats) are harder than pure metals. Thermosoftening polymers melt (weak forces between chains); thermosetting do not (cross-links). Composites = matrix + reinforcement.
- Haber process T — N2 (air) + H2 (natural gas), iron catalyst, ~450 °C, ~200 atm: N2 + 3H2 ⇌ 2NH3. Conditions are a compromise between yield and rate/cost. NPK fertilisers are formulations supplying nitrogen, phosphorus and potassium.
That completes C10 — and the whole of AQA GCSE Chemistry. From a barrel of crude oil and a river of fresh water to the alloys in a bridge and the fertiliser on a field, this topic is chemistry put to work, with sustainability as the thread running through it. For Paper 2 revision, pair it with C9 (the environmental impact of using these resources) and C6 (the equilibrium behind the Haber process).