HookThe lab bench that learned to feed the world
On 2 July 1909, in a cramped laboratory at Karlsruhe, Fritz Haber watched a few drops of liquid ammonia collect at the foot of a high-pressure tube — the first time anyone had forced nitrogen and hydrogen straight from the air and the gas main to combine on demand. Carl Bosch at BASF turned that bench trick into the vast Oppau plant by 1913, and the process still fixes more nitrogen than every thunderstorm and soil bacterium on Earth put together. The chemist Vaclav Smil reckons roughly half the nitrogen atoms in your body were pulled out of the air by Haber's reaction. It conjured bread from air — and, in the same decade, the explosives of the First World War.
That is C10 in a single story: this chapter is about how chemistry manages the Earth's resources — stretching what is finite, cleaning and reusing what is renewable, and paying an energy-and-pollution bill for every gram of useful material. You will meet water clean enough to drink, metals grown inside plants, a scoring system for a product's entire life, the slow rot of corrosion, the alloys and composites engineered to beat it, and the reaction that feeds billions. Every topic is the same tension: human demand keeps rising, many of the raw materials are running out, and the chemist's job is to make what we have go further.
ModelFinite, renewable and what 'sustainable' really means
Natural resources — food, timber, clothing and fuels — come from the Earth, the oceans and the air, and chemistry works alongside them: synthetic rubber stands in for the sap of rubber trees, and fertilisers push crop yields far above what unaided soil could give. The first thing an examiner wants you to do is split those resources two ways. Finite (non-renewable) resources are being used up faster than they can ever form, so in time they will run out — fossil fuels, metal ores, and the minerals behind materials such as glass. Renewable resources reform, or are deliberately replaced, at least as fast as we use them — timber from replanted forests, and fresh water topped up by the water cycle.
Sustainable development is meeting the needs of the present without stopping future generations meeting their own. Chemists chase it by improving industrial processes so they use less energy and fewer raw materials, by cutting waste, and by finding substitutes for scarce materials. Note the trap in the word 'finite': it does not mean 'nearly gone'. There is plenty of limestone in the ground — the point of the label is that the stock does not renew, so whatever we burn through now is gone from the ledger for good.
MechanismMaking water fit to drink — and cleaning it afterwards
Potable water means water that is safe to drink — and it is emphatically not the same as pure water. Potable water still carries dissolved salts and very low levels of microbes; pure water is only \(\mathrm{H_2O}\). In the UK, rain keeps reservoirs and underground aquifers topped up with fresh water that is already low in dissolved solids, so treatment is cheap: choose an appropriate source, pass the water through filter beds to strip out solids, then sterilise it to kill microbes using chlorine, ozone or ultraviolet light. Where fresh water is scarce — the Gulf states, or a ship far out at sea — you must desalinate salty or sea water by distillation or by reverse osmosis, and because both need large amounts of energy, desalination is always a last resort.
Water used and dirtied has to be cleaned before it goes back to the rivers. When the summer of 1858 turned the Thames into the open sewer of the 'Great Stink', Parliament finally funded Joseph Bazalgette's London sewers — the ancestors of modern waste water treatment. Sewage and agricultural waste water are first screened to remove grit and large solids; then sedimentation lets the heavy sludge settle out from the liquid effluent; the sludge is broken down by anaerobic bacteria (which also yields methane as a fuel), while the effluent is treated with aerobic bacteria kept alive by air bubbled through it. Rank the jobs by difficulty and you have a favourite exam answer: ground water needs the least treatment, sewage more, and sea water the most.
CaseRequired practical 8 — reading a water sample
Required practical 8 asks you to analyse and purify water, and both halves hinge on the difference between what boils away and what stays behind. To analyse, you test the pH of ground water, sea water and fresh water with a pH probe or universal indicator, and you measure the dissolved solids by evaporating a known volume of the sample to dryness and weighing the residue left in the basin. To purify, you distil the sample — boil it, then condense the steam back to a liquid — collecting water with no dissolved solids, effectively pure. The logic is symmetrical: evaporation drives off the volatile water and leaves the salts so you can weigh them, whereas distillation collects the volatile water and abandons the salts in the flask. Distillation is deliberately energy-hungry, which is exactly why we do not simply distil all our tap water.
You evaporate \(50\ \mathrm{cm^3}\) of a ground-water sample to dryness. The clean evaporating basin weighs \(45.00\ \mathrm{g}\); after every last drop has boiled off it weighs \(45.02\ \mathrm{g}\). Find the concentration of dissolved solids. First the mass of residue: \(45.02 - 45.00 = 0.02\ \mathrm{g}\). Next convert the volume into cubic decimetres, because concentration is quoted per \(\mathrm{dm^3}\): \(50\ \mathrm{cm^3} = \frac{50}{1000} = 0.050\ \mathrm{dm^3}\). Then divide mass by volume: \[\frac{0.02\ \mathrm{g}}{0.050\ \mathrm{dm^3}} = 0.4\ \mathrm{g/dm^3}\] That is typical of safe tap water. Distil the same sample and the basin comes back clean — \(0\ \mathrm{g/dm^3}\) — because distillation leaves every dissolved salt in the flask. The figure also explains the cost of desalination: sea water runs at roughly \(35\ \mathrm{g/dm^3}\), nearly a hundred times as much salt to remove for the same glass of water.
MechanismMining with plants and bacteria
The richest copper ores are running out, so chemists have learned to work the low-grade ores that were once dumped as waste — and they let biology do the digging. Phytomining grows plants on soil or spoil that holds low concentrations of copper compounds; as the plants grow they absorb those compounds, and they are then harvested, dried and burned. The ash is now far richer in copper compounds than the original ground, and it is processed to release the metal. Bioleaching instead uses bacteria that live on low-grade ores and produce a leachate — a solution containing metal compounds such as copper sulfate. From either the ash solution or the leachate you recover the copper by one of two familiar routes: displacement with a cheaper, more reactive scrap metal such as iron, \(\mathrm{CuSO_4 + Fe \rightarrow FeSO_4 + Cu}\), or by electrolysis.
Both methods are Higher-tier only, and both trade speed for reach. They use much less energy than digging, hauling and smelting conventional ore, and they win metal from deposits too poor to mine any other way — but they are slow, which is the disadvantage examiners expect you to name against the environmental gains.
DataLife cycle assessment, and the three Rs
A life cycle assessment (LCA) adds up the environmental impact of a product across four stages: extracting and processing the raw materials; manufacturing and packaging; using and operating it over its lifetime; and disposing of it at the end — with the energy and transport at every stage counted in. Some of those figures are genuinely objective: the mass of material used, the energy consumed, the volume of water drawn. But the impact of pollutants and waste often rests on value judgements, which is why an LCA can be selective, and why a company can wave a flattering 'carbon footprint' that quietly ignores what happens at disposal. Reading an LCA critically is itself an exam skill.
The cheapest environmental stage to attack is usually the first one, and the tools are the three Rs. Reducing, reusing and recycling all mean less raw material extracted and less energy spent. Glass bottles can be washed and reused whole, or crushed, melted and reshaped into new glass; metals are melted down and recast, sometimes needing to be separated by type first; and many plastics can be sorted and remoulded. Because these materials come from finite resources, every tonne recycled is a tonne of ore, sand or crude oil left in the ground.
Recycling an aluminium drinks can — melting it down and rolling fresh sheet — uses only about \(5\%\) of the energy needed to win the same aluminium from bauxite by electrolysis. That is a saving of \(100\% - 5\% = 95\%\) of the energy, on top of sparing the ore itself. Britain gets through several billion aluminium cans a year, so the arithmetic scales into power-station quantities: if extracting the metal for a can costs, say, \(1\) unit of energy, recycling it costs \(0.05\) units, so a billion recycled cans save about \(950\) million units. That single contrast — 5 against 100 — is why aluminium recycling is one of the clearest environmental wins in the whole chapter, and a perfect number to quote in an LCA question.
CasePainting the Forth Bridge — corrosion and how to stop it
For more than a century, 'painting the Forth Bridge' was British shorthand for a task that never ends: the great rail bridge over the Firth of Forth was repainted continuously to keep the salt sea air off its iron, until a glass-flake epoxy coating finished around 2011 finally promised twenty-five years of protection. Corrosion is the destruction of a metal by reaction with substances in its surroundings, and rusting is the specific corrosion of iron. Rusting needs both water and oxygen — take either away and iron will not rust, which is precisely why oiled tools and sealed tins survive for years. The product is hydrated iron(III) oxide. Aluminium corrodes too, but its oxide layer is tough and clings tightly, sealing the metal beneath instead of flaking off the way rust does, so aluminium behaves as if it does not corrode at all.
There are two families of defence. Barriers simply keep out air and water — paint, oil, greasing, a plastic coating, or electroplating with another metal. Sacrificial protection is cleverer: a more reactive metal, usually zinc or magnesium, is attached or coated on so that it corrodes in place of the iron. Galvanising — coating steel in zinc — does both jobs at once: the zinc is a barrier, and even if it is scratched through, it still protects sacrificially because it is more reactive than the iron underneath.
CaseAlloys, ceramics, polymers and composites
Pure metals are often too soft, because their identical atoms sit in neat layers that slide easily over one another. An alloy mixes in atoms of a different size, distorting those layers so they can no longer slip — which is why alloys are harder than the pure metal. Bronze is copper with tin; brass is copper with zinc; gold jewellery is gold alloyed with copper, silver or zinc and graded in carats, where 24-carat is pure. Steels are alloys of iron with carbon and sometimes other metals: high-carbon steel is hard but brittle, low-carbon steel is softer and easily pressed into shape, and stainless steel — iron with chromium and nickel — resists corrosion, which is why it makes cutlery and surgical tools. Aluminium alloys are prized for their low density in aircraft.
Beyond metals sit three more material families. Ceramics such as soda-lime glass (made by heating sand, sodium carbonate and limestone) and clay ceramics (wet clay shaped, then fired in a furnace) are hard and heat-resistant but brittle. Polymers split into thermosoftening — tangled chains held together by weak intermolecular forces, so they melt and can be remoulded again and again — and thermosetting — chains locked together by covalent cross-links, so they char rather than melt. Composites combine a reinforcement with a matrix or binder: glass or carbon fibres set in resin, or stone aggregate set in cement to make concrete. Picking the right material is always a properties-versus-cost decision, which loops straight back to life cycle assessment.
CaseThe Haber process and NPK fertilisers
Now the reaction the chapter opened on. The Haber process is reversible: \(\mathrm{N_2 + 3H_2 \rightleftharpoons 2NH_3}\), with the forward reaction exothermic. The nitrogen comes free from the air; the hydrogen from reacting methane (natural gas) with steam. The conditions are a deliberate compromise — the classic Higher-tier Le Chatelier question. A pressure of about 200 atmospheres pushes the equilibrium towards ammonia, because four molecules of gas become two, so more pressure means more yield — but high-pressure plant is costly and dangerous, so 200 is a compromise. The forward reaction is exothermic, so a low temperature would raise the yield — yet it would also make the reaction hopelessly slow, so about 450°C is a compromise that sacrifices some yield for a workable rate. An iron catalyst speeds the reaction without altering the yield. The ammonia is cooled until it liquefies and is drawn off, and the unreacted nitrogen and hydrogen are recycled back through the reactor.
Ammonia is the feedstock for NPK fertilisers, which replace the three elements crops strip from soil: nitrogen, phosphorus and potassium. Ammonia is converted into ammonium salts and, via nitric acid, into ammonium nitrate; potassium chloride and potassium sulfate are mined and used directly; and phosphate rock is mined but its phosphate is insoluble, so it is treated with acids — nitric acid gives phosphoric acid and calcium nitrate, sulfuric acid gives single superphosphate, and phosphoric acid gives triple superphosphate. The scale is the giveaway in the exam: a school lab makes a pure ammonium salt in small batches by titration and crystallisation, whereas industry runs continuously, by the tonne.
How much ammonia could a plant make from \(1400\ \mathrm{kg}\) of nitrogen, and why does it collect far less per pass? The equation is \(\mathrm{N_2 + 3H_2 \rightarrow 2NH_3}\). Relative formula masses: \(M_r(\mathrm{N_2}) = 2 \times 14 = 28\) and \(M_r(\mathrm{NH_3}) = 14 + (3 \times 1) = 17\). One \(\mathrm{N_2}\) makes two \(\mathrm{NH_3}\), so by mass \(28\) units of nitrogen give \(2 \times 17 = 34\) units of ammonia. Scaling up, \(1400\ \mathrm{kg}\) of nitrogen is \(1400 \div 28 = 50\) lots of that ratio, so the theoretical yield is \[50 \times 34 = 1700\ \mathrm{kg}\ \text{of ammonia.}\] In practice a single pass converts only about a quarter of the gas, so perhaps \(0.28 \times 1700 \approx 476\ \mathrm{kg}\) actually condenses out each time — which is exactly why the leftover nitrogen and hydrogen are recycled through the reactor over and over until almost all of it is converted.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
This is a 'state the conditions and justify them' chapter, and the Haber process is where that pays best. Do not merely list 450°C and 200 atmospheres — say why each is a compromise: high pressure favours ammonia because the forward reaction has fewer gas molecules, low temperature would favour it because the reaction is exothermic, and the chosen values trade some yield for a workable rate, while the iron catalyst and the recycling of unreacted gas rescue the economics. Naming the compromise is where the marks sit.
Keep your definitions crisp on the pairs examiners love to blur: potable versus pure water, finite versus renewable, phytomining versus bioleaching, thermosoftening versus thermosetting, and barrier versus sacrificial protection. A single sharp contrast for each beats a vague paragraph. For rusting, always name both conditions — water and oxygen — because 'remove either one and it will not rust' is the idea behind every prevention method, from paint to galvanising.
On evaluate questions — life cycle assessment, desalination, recycling, or choosing a material — the top band comes from balance and from spotting subjectivity: quote the objective numbers (energy, mass, the 5% aluminium-recycling figure) but flag that pollutant impacts rest on value judgements and that an LCA can be selectively quoted in marketing. In any calculation, show the residue mass or the reacting-mass ratio explicitly and put the unit on your answer — \(\mathrm{g/dm^3}\), kg or tonnes — because a bare number drops the final mark.