Topic 10 looks like the easy topic — no moles, no electrolysis, just air and water. That is exactly why it is dangerous: the marks are lost on precision, not difficulty. Candidates say anhydrous copper(II) sulfate proves water is pure (it does not), claim carbon dioxide is toxic (that is carbon monoxide), file SO₂ under greenhouse gases and CH₄ under acid rain, blame the ozone hole for global warming, run photosynthesis backwards, forget the word damp in the ammonia test, and halve their %N because they counted one nitrogen atom instead of two. Every one of those errors appears on real scripts every session. This guide hunts them all down, one by one, so that on the day Topic 10 is the topic where you drop nothing.
Twelve traps that cost students marks on Topic 10 questions. Every one of them appears on challenge papers regularly.
Six challenging questions broken down step by step. Try each step yourself before revealing the next.
Treatment runs from coarse to fine. First sedimentation: the water stands in large tanks so that the largest insoluble particles settle out under gravity. Then filtration through beds of sand and gravel removes the remaining insoluble solids. Neither stage touches anything dissolved — keep that thought for part (c).
Passing the water over activated carbon removes substances causing unpleasant tastes and odours. Finally chlorination: a small amount of chlorine is added to kill microbes/bacteria. This is the public-health stage — it is what prevents diseases such as cholera and typhoid spreading through the supply. Never write that chlorine "removes solids" or "cleans the water"; its job is to sterilise.
Anhydrous copper(II) sulfate turning from white to blue shows only that water is present. Sea water, lemonade and muddy puddle water all give the same result, because the test detects water molecules, not the absence of everything else. The student has answered the question "does this contain water?", not "is this pure water?" — two different questions with two different tests.
To check purity, measure the boiling point: pure water boils at exactly 100 °C (at standard atmospheric pressure), or melts at exactly 0 °C. Dissolved impurities raise the boiling point (and lower the melting point) and make the substance boil over a range of temperatures rather than sharply. So: exactly 100 °C → pure; above 100 °C or a spread-out boiling range → impure.
Water treatment removes solids and microbes but not dissolved salts (chlorides, calcium compounds, and the added chlorine itself). In the lab those dissolved substances would interfere with tests — for example, tap water can give a false positive in a chloride test with silver nitrate. Distilled water has been boiled and condensed, leaving the dissolved solids behind, so it contains no ions to interfere.
With a plentiful supply of air, the carbon in the fuel burns completely: C → CO₂. Inside an engine the air supply is limited, so combustion is incomplete and some carbon leaves as carbon monoxide, CO (and some as soot, C). One fuel, one variable — the amount of oxygen — two products. Say "limited/insufficient supply of oxygen" explicitly; "not burned properly" is not creditworthy language.
Petrol contains no nitrogen — but the air drawn into the engine is 78% N₂. At the very high temperature of the spark and burning fuel, nitrogen and oxygen from the air combine: N₂ + O₂ → 2NO. This never happens in ordinary air because N₂'s triple bond needs enormous energy to break. Two marks live here: where the nitrogen comes from, and why it reacts only in the engine.
The sulfur compounds in the fuel are oxidised when the fuel burns: S + O₂ → SO₂. Contrast this with NOₓ: sulfur comes from the fuel, nitrogen comes from the air. Examiners test exactly this contrast — and it explains the fix: sulfur can be removed from the fuel at the refinery (low-sulfur petrol and diesel), but nitrogen cannot be removed from air.
CO: toxic — binds to haemoglobin so blood cannot carry oxygen. NOₓ: causes acid rain (and photochemical smog / respiratory problems). SO₂: causes acid rain. CO₂: not toxic, but a greenhouse gas driving climate change. Keep each effect glued to its gas — the examiner marks pairings, not lists.
Over the hot platinum/rhodium catalyst, carbon monoxide is oxidised to CO₂ while nitrogen monoxide is reduced to N₂: 2CO + 2NO → 2CO₂ + N₂. Both toxic/acidic gases leave as harmless (N₂) or at least non-toxic (CO₂) products. Notice what it cannot do: it does not reduce CO₂ emissions — it makes CO₂.
Coal is mostly carbon, but it contains sulfur compounds as impurities. When the coal burns, the sulfur burns too: S + O₂ → SO₂. Two marks, two ideas: the sulfur is in the fuel (not from the air), and it is oxidised during combustion. Candidates who skip the word "impurity" often lose the first mark — the examiner wants to know you understand the sulfur was never meant to be there.
SO₂ dissolves in water in the atmosphere, and it is oxidised (by oxygen in the air), ultimately forming sulfuric acid, H₂SO₄. The rain that falls is therefore appreciably acidic (pH around 4 or below, versus about 5.6 for natural rain, which is only weakly acidic from dissolved CO₂). "SO₂ mixes with rain" is not enough — the scoring verbs are dissolves and is oxidised / forms sulfuric acid.
The cathedral is limestone: calcium carbonate. Acid rain is an acid; CaCO₃ is a carbonate; acids react with carbonates to give a salt, water and carbon dioxide. The stone is slowly eaten away / eroded. This is why the question names the building material — it is inviting you to write the acid + carbonate reaction. Steel structures corrode faster for the parallel reason: acids attack metals.
Forests: acid rain damages leaves/needles directly and releases harmful ions (e.g. aluminium) in the soil, so trees are damaged or killed. Lakes: the water becomes too acidic for fish and other aquatic life, killing them or preventing eggs hatching. One sentence each is enough — but the sentence must connect the acidity to the harm, not just say "it is bad for nature".
Fix 1 — before combustion: use low-sulfur fuel (or remove sulfur at the refinery / switch to natural gas). Fix 2 — after combustion: flue gas desulfurisation: pass the waste gases through calcium oxide or calcium carbonate. The chemistry: SO₂ is an acidic oxide, CaO is a base, so the SO₂ is neutralised and trapped as a solid calcium salt (calcium sulfite/sulfate) instead of leaving the chimney.
Carbon dioxide: from the complete combustion of fossil fuels (and from respiration). Methane: from the decomposition of vegetation (landfill, rice paddies) and from waste gases produced by digestion in animals (cattle). These are the two syllabus greenhouse gases — do not offer SO₂ (acid rain) or CO (toxic) here. Water vapour is a greenhouse gas too, but CO₂ and CH₄ are the expected answers.
Step one of the mechanism: energy from the Sun passes through the atmosphere and warms the Earth's surface. Step two: the warm Earth re-radiates the energy as thermal (infrared) radiation. This second sentence is where the marks start — the whole effect hinges on the fact that the outgoing radiation is a different kind (thermal/IR) from the incoming sunlight.
Greenhouse gas molecules absorb the thermal radiation, then re-emit it in all directions — including back down towards the surface — so the energy is trapped in the atmosphere rather than escaping to space. More CO₂ and CH₄ means more absorption, so the average temperature rises. Absorb → re-emit → trapped: those three verbs, in that order, are the mark scheme.
Reason 1: the greenhouse effect does not involve the ozone layer at all — it is caused by gases absorbing outgoing thermal radiation, not by anything "letting more light in". Reason 2: greenhouse gases such as CO₂ and CH₄ do not destroy ozone — ozone depletion is a different problem with different chemistry (CFCs). Mixing the two stories tells the examiner you have one mental folder where you need two.
Creditworthy strategies attack CO₂ or CH₄ directly: use alternative/renewable energy (solar, wind, hydroelectric, nuclear) instead of fossil fuels; reduce livestock farming; plant trees / stop deforestation so more CO₂ is removed by photosynthesis; improve energy efficiency / use electric vehicles. Vague answers ("be eco-friendly", "recycle more") do not name a gas-cutting mechanism and score nothing.
Photosynthesis removes CO₂ from the air: green plants use light energy to convert carbon dioxide and water into glucose and oxygen. (The oceans also dissolve CO₂, but photosynthesis is the process the syllabus wants.) Get the direction locked in now: photosynthesis = CO₂ in, O₂ out.
Count the atoms: 6 C, 12 H, 18 O on each side. The two conditions are part of the answer: light energy and chlorophyll (write them above the arrow). A correct equation with no conditions typically drops a mark; conditions with no equation drop two. This is one of only a handful of biology-flavoured equations in the chemistry syllabus, which is exactly why it is asked so often.
Respiration — every living organism releases CO₂ as it converts glucose and oxygen into energy. Combustion of carbon-containing fuels — wood, coal, oil, gas. (Decomposition of dead material counts too — microbes respiring.) For "natural processes" prefer respiration and decomposition; combustion of fossil fuels is the human-driven one.
The concentration rises because humans have unbalanced the cycle from both directions. Input up: burning huge quantities of fossil fuels releases CO₂ that had been locked away for millions of years. Output down: deforestation means less photosynthesis, so less CO₂ is removed. An answer that gives only one side usually scores 2 of 3; the third mark is for explicitly saying removal now lags release.
Do not write "plants breathe in CO₂ and breathe out O₂, animals do the opposite" — plants respire all the time, day and night, releasing CO₂ just like animals; photosynthesis (in the light) simply outpaces their respiration. And avoid "plants convert CO₂ into oxygen" — the oxygen atoms in the O₂ do not come from a magic conversion; CO₂ and H₂O are converted into glucose and oxygen. Precision here is what separates a chemistry answer from a primary-school one.
Nitrogen is needed by plants to make proteins, promoting strong leaf growth. That one clause is the mark. "To help the plants grow" merely defines a fertiliser and earns nothing — specify what the nitrogen builds.
NH₄NO₃: one N in the ammonium ion, one in the nitrate ion — 2 N. (NH₄)₂SO₄: the subscript 2 outside the bracket doubles everything inside — 2 N (and 8 H). Write "N × 2 = 28" on the page before any division. Most wrong answers to this question died right here, before the arithmetic even started.
NH₄NO₃: Mr = 14 + 4(1) + 14 + 3(16) = 80; %N = 28/80 × 100 = 35%. (NH₄)₂SO₄: Mr = 2(14 + 4) + 32 + 4(16) = 36 + 32 + 64 = 132; %N = 28/132 × 100 = 21.2%. Conclusion: ammonium nitrate is the better nitrogen source per kilogram. Show Mr, then the fraction, then the percentage — the method marks are attached to those visible steps.
Add aqueous sodium hydroxide and warm. Ammonia gas is given off, which turns damp red litmus paper blue. Four details, and each is checkable: NaOH (the alkali that displaces ammonia), warming (drives the gas out), damp (ammonia must dissolve to act as an alkali), red → blue (ammonia is the alkaline gas). Recite it as one breath: warm with NaOH; damp red litmus turns blue.
Calcium hydroxide is an alkali — the same role NaOH played in the test. Mixed with an ammonium salt in the soil it does the same chemistry: ammonia gas is released and escapes to the air, so the nitrogen is lost and the fertiliser is wasted. This is the challenge-paper move in miniature: the lab test and the farming problem are the same reaction wearing different clothes.
Pairs of questions that look nearly identical but have different answers. Spot the key distinction.
Click each node to see how the subtopics connect.
Spot the error in each student's answer. Think before revealing.
Ten Cambridge-style challenge questions. Write your answer, then reveal the model answer with mark scheme and examiner's notes.