HookThe city that counted down to Day Zero
Through the summer of 2017 and into 2018, Cape Town did something no major city had done before: it printed a date on which it would run out of water. After three failed rainy seasons the reservoirs feeding four million people fell below 30%, then below 20%, and officials named the morning the municipal taps would be switched off — Day Zero — projecting it for April 2018. Residents would then queue at 200 guarded standpipes for a rationed 25 litres a head. Only ferocious rationing, holding each person under 50 litres a day, and the return of the winter rains, pushed Day Zero off the calendar.
C10 is the chemistry of not running out. Every material a society leans on — the water in the tap, the copper in its wiring, the aluminium in its cans — is drawn from the Earth's crust, oceans and atmosphere, and each is either genuinely renewable or a finite stock we are steadily drawing down. This section is about using those stocks sustainably: making water safe to drink, cleaning it after we have fouled it, prising metals from ores too poor to mine the old way, and honestly totting up the lifetime cost of the things we manufacture.
ModelFinite, renewable, and what 'sustainable' really means
Everything humans use comes from the Earth's crust, oceans and atmosphere, processed by chemistry into the forms we need. Some natural resources are renewable — timber, cotton, rubber — because they are replaced by the environment at least as fast as we take them. Others are finite, or non-renewable: metal ores, crude oil and natural gas formed over millions of years and are being used far faster than they re-form. Chemistry's job here is to make the finite stocks stretch further — improving extraction so poorer ores become worth using, replacing scarce natural products with synthetic alternatives, and cutting the energy each process needs.
Sustainable development is the guiding idea, and it has a definition worth learning word for word: development that meets the needs of the present without compromising the ability of future generations to meet their own needs. It is a balancing act, not a slogan — potable water, the metals in a phone and the fuel in a tanker are all resources under exactly this pressure, and every method in the rest of C10 is a way of easing it.
MechanismMaking water safe to drink
Potable water is water that is safe to drink — and, crucially, that is not the same as pure water. Chemically pure water contains only \(\mathrm{H_2O}\); potable water still contains low levels of dissolved salts and minerals, but few enough microbes and dissolved substances to be safe. Britain is lucky in having plenty of fresh water — rain feeding aquifers, rivers and reservoirs — and treats it in two steps: passing it through filter beds to remove insoluble solids, then sterilising it to kill microbes, using chlorine, ozone or ultraviolet light.
Where fresh water is scarce — the Gulf states, ships at sea, or Cape Town's emergency plants — the only option is to take the salt out of sea water by desalination, either by distillation (boil it off and condense the pure vapour) or by reverse osmosis (force it through a membrane that holds back the dissolved ions). Both work, but both need large amounts of energy, which makes desalinated water expensive — the reason it is a last resort rather than a first choice. The same distillation route, on a small scale, is how a laboratory makes the pure water it needs for accurate analysis.
DataRequired practical 13 — analysing and purifying water
This required practical has three parts, and it is as much about careful method as about chemistry. First, test the pH of samples from different sources with universal indicator or, better, a calibrated pH probe, which gives a precise numerical value rather than a colour you have to judge by eye. Second, find the mass of dissolved solids: measure out a known volume of the sample into a weighed evaporating basin, evaporate it gently to dryness, and reweigh — the increase in mass is the dissolved solid the water was carrying. Third, purify a sample by distillation and test the distillate; if it now reads neutral pH and leaves no residue on evaporation, you have shown it is pure.
The examinable errors are all about fairness and care. Use the same volume for every sample so the comparison is valid; make sure the basin is completely dry before the final weighing, because leftover water inflates the apparent mass of solids; and heat gently to stop the solution spitting and losing sample. A pH probe beats indicator paper on precision, and repeating and averaging tightens the result.
You evaporate 50 cm³ of a tap-water sample in an evaporating basin. The empty basin has a mass of 62.40 g; after evaporation the basin and dry residue together mass 62.58 g. The dissolved solid is \(62.58 - 62.40 = 0.18\ \text{g}\). Converting the volume, 50 cm³ is 0.050 dm³, so the concentration of dissolved solids is \[\frac{0.18}{0.050} = 3.6\ \text{g/dm}^3.\] Run the same procedure on sea water and the residue would be enormous by comparison — dozens of grams per dm³ — which is exactly why sea water cannot simply be filtered into drinking water and must instead be distilled or forced through a membrane.
MechanismCleaning water after we have used it
The other half of the water story is what happens to it after the tap. Sewage and agricultural waste water carry organic matter and harmful microbes that must be removed before the water is returned to rivers and the sea. Treatment runs in stages. First, screening removes grit and large solids like rags and sticks. Then sedimentation lets the water stand so that solids settle out, separating it into a settled sludge at the bottom and a watery effluent on top.
The two products are then treated in opposite ways. The sludge is broken down by anaerobic digestion — bacteria working without oxygen, which also releases biogas that can be burned for energy and leaves a residue usable as fertiliser. The effluent is treated aerobically — air is bubbled through so that bacteria with a plentiful oxygen supply digest the remaining dissolved organic matter. Industrial and agricultural waste can need extra steps to remove toxic chemicals. The pay-off worth remembering is energy: treating and reusing waste water needs far less energy than desalinating sea water, which is why a country with rivers cleans its sewage rather than boiling the ocean.
CaseGetting copper from ore too poor to smelt (Higher tier)
The rich copper ores are running out, and the ores that remain are often too low-grade for the traditional route of digging, crushing and roasting to be worth the energy. Two biological methods let us mine them anyway. In phytomining, plants are grown on low-grade ore or contaminated land; they absorb copper compounds through their roots, and when the plants are harvested and burned, the ash is left rich in copper compounds. In bioleaching, bacteria are used on low-grade ore to produce a leachate solution containing dissolved copper compounds, with no digging or roasting at all.
From either the ash or the leachate, the copper is extracted in one of two ways: by displacement, adding cheaper scrap iron, which is more reactive and pushes the copper out of solution, or by electrolysis. The trade-off is the examinable point. These methods can exploit ores that conventional mining could never afford, and they use less energy and cause far less landscape damage and air pollution than digging out and smelting vast tonnages of rock. Their weakness is speed: phytomining in particular is slow, tied to how fast plants grow.
Extracting copper from a bioleaching solution uses a displacement reaction, because iron is more reactive than copper: \[\mathrm{Fe + CuSO_4 \rightarrow FeSO_4 + Cu}\] The relative masses let you cost it. Using \(A_r\) values of 56 for iron and 64 for copper, 56 g of iron displaces 64 g of copper — so, scaling up, about 5.6 kg of scrap iron would displace roughly \(\frac{64}{56}\times 5.6 = 6.4\ \text{kg}\) of copper. Using cheap scrap iron to win a more valuable metal is the whole economic logic of the process, and it is why displacement, not smelting, finishes the job.
MechanismLife cycle assessment — the whole story of a product
A life cycle assessment (LCA) judges the environmental impact of a product across its entire life, in four stages: extracting and processing the raw materials; manufacturing and packaging; using the product over its lifetime; and disposal at the end — including the transport and energy needed at every step. Each stage is scored for its use of resources, energy and water, and for the waste and pollutants it produces.
The catch, and the reason LCAs are a favourite for evaluation questions, is that they are not fully objective. Measuring energy or water used is straightforward, but assigning a single number to the harm caused by a given pollutant needs a value judgement, so different assessors can reach different conclusions from the same product. Worse, selective LCAs that quote only the flattering stages are used in advertising to make a product look greener than it is. Reading an LCA critically — asking which stages and which impacts it actually counted — is the skill being tested.
Compare a single-use plastic bag with a cotton tote. On carbon emissions alone, one 2018 study by the Danish Environmental Protection Agency estimated a cotton tote must be reused well over a hundred times to beat a lightweight plastic bag, because growing and processing cotton is energy-hungry. Widen the lens to include water use and land use and the break-even leaps into the thousands of reuses. The 'greener' bag therefore depends entirely on which impacts you choose to count and how many times the tote is actually used — a concrete illustration of why an LCA is a judgement, not a single objective verdict.
CaseReduce, reuse, recycle
Because so many resources are finite, the most direct way to make them last is to use less of them, in a rough order of preference. Reduce comes first: using less material and less energy in the first place — thinner drinks cans, lighter packaging. Reuse comes next: using the same product again without reprocessing it, such as refilling glass milk bottles or refurbishing electronics. Recycle is last but still powerful: processing used material into new — metals melted down and recast, glass crushed and remelted, some plastics reprocessed.
The environmental case rests on energy and extraction. Recycling metals uses a fraction of the energy of winning them from ore — recycling aluminium takes only about 5% of the energy needed to extract it from bauxite by electrolysis, a saving of roughly 95% — and it cuts the quarrying and mining that scar landscapes, consume energy and generate waste rock. Recycling is not free: collecting, sorting and reprocessing all cost energy, and the quality of some materials (plastics especially) falls with each cycle. But for finite metals in particular, the resource and energy savings are large enough that recycling is central to using resources sustainably.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Be precise about potable water — it is safe to drink, not pure. State the two treatment steps in order (filter, then sterilise) and name a sterilising method (chlorine, ozone or UV). Keep the cheap route for fresh water clearly separate from the energy-hungry distillation and reverse osmosis used only when the source is sea water, because 'why is desalination a last resort?' is a favourite AO1 recall question answered in one word: energy.
On Required practical 13, quote the method and the numbers: evaporate a measured volume, weigh the residue, divide to get a concentration in g/dm³, and know that a wet basin makes the dissolved-solids figure too high. For the Higher-tier metals content, do not blur phytomining and bioleaching — plants versus bacteria — and remember copper is then won by displacement with scrap iron or by electrolysis. Every 'evaluate' answer needs a pro AND a con: low-grade ores usable, but slow.
The 6-mark extended responses on life cycle assessment and reduce-reuse-recycle are AO3 evaluation, and the marks reward balance over a verdict. Name the specific stage or impact (raw materials, manufacture, use, disposal), flag that parts of an LCA are subjective and can be presented selectively, and weigh the energy and resource savings of recycling against its collection and sorting costs — rather than simply declaring one option the winner.