IGCSE Chemistry Topic 10: Chemistry of the Environment

10.1 Water 10.2 Fertilisers 10.3 Air Quality & Climate

Hey Tara! Welcome to Topic 10 — Chemistry of the Environment. This topic is a gift: the facts are few, the questions repeat year after year, and almost every mark comes from precise wording rather than difficult chemistry. There are three stories. One: water — how to prove a liquid contains water (two colour-change tests), why those tests can never prove it is pure, and how a treatment works step by step from river to tap. Two: fertilisers — why plants need N, P and K, and how ammonia from the Haber process becomes ammonium salts. Three: air — what clean air is made of, the five pollutants with their sources and effects, and the two global problems (acid rain and the greenhouse effect) that examiners love because students mix them up. Keep those two problems in separate boxes in your head and this topic becomes one of your most reliable scorers. Let's go!

10.1 Water ▼

The Big Idea: Detecting Water Is Not the Same as Trusting It

Chemists constantly need to answer two different questions about a colourless liquid. Question one: does this contain water? Question two: is this pure water? IGCSE gives you two colour-change tests for the first question — and then immediately tests whether you understand that neither of them can answer the second. That single distinction is worth marks almost every session.

The Two Chemical Tests for Water

Both tests use an anhydrous solid — a compound from which all the water of crystallisation has been driven off by heating. When water is added back, the solid re-hydrates and its colour changes.

Test 1: anhydrous copper(II) sulfate turns from WHITE to BLUE

CuSO₄ (white) + 5H₂O → CuSO₄·5H₂O (blue) — the water becomes water of crystallisation in the blue hydrated crystal. The change is reversible: heating the blue crystals drives the water off again and the white anhydrous powder returns. This is the classic reversible-reaction example from Topic 7.

Test 2: anhydrous cobalt(II) chloride turns from BLUE to PINK

CoCl₂ (blue) + 6H₂O → CoCl₂·6H₂O (pink) — often used as cobalt chloride paper, which is simply filter paper soaked in the anhydrous salt. Memory anchor: Copper sulfate goes white → blue; cobalt chloride goes blue → pink. The word "blue" appears in both tests but on opposite sides!

The Purity Trap — Learn This Sentence

These tests show that a liquid contains water; they do not show that it is pure water. Sea water, orange juice and dilute sulfuric acid would all turn anhydrous copper(II) sulfate blue. To show that water is pure, you measure a physical constant: pure water boils at exactly 100 °C at standard atmospheric pressure (and freezes at exactly 0 °C). Dissolved impurities raise the boiling point and lower the freezing point, and an impure sample boils over a range of temperatures rather than sharply at one value.

What Is Really in "Natural" Water?

Water from rivers, lakes and boreholes is never just H₂O. The syllabus lists the substances you must know, and — crucially — asks you to sort them into beneficial and harmful.

Substance in natural water	Where it comes from	Beneficial or harmful?
Dissolved oxygen	Dissolves from the air; produced by aquatic plants	Beneficial — essential for fish and all aquatic life, and for aerobic bacteria that break down waste
Metal compounds	Dissolved from rocks and soil; industrial discharge	Both — some provide essential minerals (e.g. calcium compounds); others, such as lead compounds, are toxic
Plastics	Litter and waste washed into waterways	Harmful — harms aquatic life, e.g. animals swallow or become trapped in it; microplastics enter food chains
Sewage	Human and animal waste entering rivers	Harmful — contains harmful microbes and is decomposed by bacteria that use up dissolved oxygen
Harmful microbes	Mainly from sewage contamination	Harmful — cause diseases such as cholera and typhoid
Nitrates	Run-off of fertilisers from farmland	Both — nutrients for plant growth, but excess causes deoxygenation of water (see below)
Phosphates	Fertiliser run-off and detergents	Both — same story as nitrates: useful nutrient, harmful in excess

Supplement

How nitrates and phosphates kill fish without touching them

Excess nitrates and phosphates washed into a lake act as fertiliser for algae, which grow explosively (an algal bloom). The algae block sunlight, plants below die, and when the algae themselves die, aerobic bacteria decompose them — using up the dissolved oxygen. The water becomes deoxygenated and fish and other aquatic life suffocate. This chain — nutrients → algal growth → death and decomposition → bacteria use up dissolved oxygen → aquatic life dies — is called eutrophication, and examiners want the chain, not just the name. Notice the irony they love to test: the pollutant is a nutrient, and the killer is bacteria respiring, not the nitrate itself poisoning the fish.

From River to Tap: Treatment of the Domestic Water Supply

Water treatment does not aim to make chemically pure water — that would need distillation on an impossible scale. It aims to make water that is safe to drink: free of solids and free of harmful microbes. Each stage has one job, and the exam wants each stage matched to its job.

The order matters and so does the purpose of each stage. Sedimentation and filtration remove insoluble solids, carbon removes tastes and odours, and chlorination kills microbes. Nothing in this plant removes dissolved salts — which is why tap water is safe but not pure.

Stage	What happens	Purpose
1. Sedimentation	Water stands in large tanks; large insoluble particles settle to the bottom under gravity	Removes larger solid particles (mud, grit)
2. Filtration	Water passes through beds of sand and gravel	Removes remaining smaller insoluble solids
3. Carbon (charcoal) beds	Water passes over activated carbon, which adsorbs dissolved organic substances	Removes unpleasant tastes and odours
4. Chlorination	A small, controlled amount of chlorine gas is added	Kills harmful microbes (sterilises the water)

Memory Trick

"Settle, Sieve, Sip, Sterilise." Sedimentation lets solids settle; filtration sieves out the rest; carbon makes it pleasant to sip (tastes and odours); chlorine sterilises (kills microbes). Four S-words in treatment order — and each S-word is the purpose the mark scheme wants.

Why Chemists Use Distilled Water, Not Tap Water

In practical chemistry, tap water is a menace. It contains dissolved chloride, calcium and other ions that would contaminate solutions and interfere with tests — imagine testing for chloride ions with silver nitrate using water that already contains chloride! Distilled water is used in practical chemistry because it contains fewer chemical impurities than tap water. That exact phrase — fewer chemical impurities — is the syllabus wording, and it earns the mark.

Worked Example 1 A student is given a colourless liquid. Describe a chemical test to show that the liquid contains water, and a further test to show whether the water is pure. State the results expected in each case. [4]

Step 1: Choose one of the two chemical tests

Either test works: add the liquid to anhydrous copper(II) sulfate, or touch it onto anhydrous cobalt(II) chloride (cobalt chloride paper). Name the solid with the word "anhydrous" — hydrated copper(II) sulfate is already blue and would show nothing.

Step 2: State the colour change precisely

Copper(II) sulfate: white → blue. Cobalt(II) chloride: blue → pink. Both the starting colour and the final colour are needed; "it turns blue" without "from white" can be ambiguous and risks the mark.

Step 3: Recognise what this test cannot do

This colour change happens with any liquid containing water — sea water, fruit juice, acid. So a different kind of test is needed for purity: a physical constant.

Step 4: The purity test

Measure the boiling point. Pure water boils at exactly 100 °C at standard atmospheric pressure. If it contains dissolved impurities it boils above 100 °C and over a range of temperatures.

Add the liquid to anhydrous copper(II) sulfate [1]; if water is present it turns from white to blue [1]. To test purity, measure the boiling point [1]; pure water boils at exactly 100 °C at atmospheric pressure — an impure sample boils above 100 °C [1]. (Anhydrous cobalt(II) chloride turning blue to pink earns the same first two marks.)

Worked Example 2 A water treatment works on the Hooghly river supplies Kolkata with drinking water. Describe the four main stages of treatment in the correct order, giving the purpose of each stage. Explain why the treated water is still not chemically pure. [6]

Step 1: Deal with the solids first — two physical stages

Sedimentation: the water stands in large tanks so that larger insoluble particles settle out under gravity. Then filtration through beds of sand and gravel removes the remaining, smaller insoluble solids. Two stages, one job each — do not merge them into "the water is filtered".

Step 2: Carbon — the taste stage

Passing the water over activated carbon removes substances causing unpleasant tastes and odours. Note the carbon does not kill microbes and does not remove all dissolved substances — keep its job small and precise.

Step 3: Chlorination — the safety stage

A small amount of chlorine is added to kill harmful microbes. This is the stage that prevents cholera and typhoid; it is the only stage whose job is biological.

Step 4: Why is it still not pure?

No stage removes dissolved substances such as calcium and chloride ions (and the added chlorine itself). Only distillation would do that. So tap water is safe to drink but not pure — it would still leave a residue when evaporated and boils slightly above 100 °C.

Sedimentation — larger insoluble particles settle out [1]; filtration through sand/gravel removes remaining insoluble solids [1]; carbon beds remove tastes and odours [1]; chlorination kills microbes [1]. The water is not chemically pure because dissolved salts/ions are not removed by any of these stages [1]; only distillation would remove them, so the water is safe rather than pure [1].

Worked Example 3 Fish are found dead in a lake surrounded by farmland in Punjab. Tests show the water contains high levels of nitrate ions but no toxic metals. Explain, as a sequence of steps, how the nitrates caused the death of the fish. [4] (Supplement)

Step 1: How did the nitrates get there?

Fertiliser run-off: rain dissolves the very soluble nitrate fertilisers on the fields and washes (leaches) them into the lake. Nitrates are nutrients, so this is over-feeding, not poisoning.

Step 2: The bloom

The nitrates fertilise algae just as effectively as wheat, so the algae grow rapidly and cover the surface, blocking light to the plants below, which die.

Step 3: The decomposition — the key step

When the algae and plants die, aerobic bacteria decompose them. Respiring bacteria use up the dissolved oxygen in the water. This is the step most students miss — the oxygen is consumed by bacteria, not by the algae shading the water.

Step 4: The deaths

The water is now deoxygenated, so fish and other aquatic organisms die from lack of oxygen — suffocation, not poisoning. That is why no toxin was found.

Nitrates washed from farmland fertilise the lake, so algae grow rapidly [1]; the algae block light and then die, and bacteria decompose the dead material [1]; the respiring bacteria use up the dissolved oxygen, deoxygenating the water [1]; fish die because there is insufficient oxygen for respiration [1].

Exam Tips for 10.1

1. Always write "anhydrous". The test needs anhydrous copper(II) sulfate or anhydrous cobalt(II) chloride. Hydrated copper(II) sulfate is already blue — no "anhydrous", no mark.

2. Give both colours. Copper(II) sulfate: white → blue. Cobalt(II) chloride: blue → pink. Learn them as pairs; examiners regularly accept only the full colour change.

3. Presence ≠ purity. The colour tests show water is present. Purity is shown by a sharp boiling point of exactly 100 °C (or freezing at exactly 0 °C) at standard pressure. Impurities raise the boiling point and lower the freezing point.

4. Match each treatment stage to its exact purpose. Sedimentation = larger solids settle; filtration = removes insoluble solids; carbon = tastes and odours; chlorine = kills microbes. Swapping the purposes of carbon and chlorine is the classic error.

5. Chlorination does not "clean" or "purify". It kills microbes / sterilises. Vague verbs lose the mark.

6. Tap water is safe, not pure. Dissolved ions pass straight through every stage of treatment. This is also why distilled water is used in practical chemistry — it contains fewer chemical impurities.

7. Know both lists. Beneficial: dissolved oxygen (aquatic life), some metal compounds (essential minerals), nitrates/phosphates in moderation (plant nutrients). Harmful: some metal compounds (toxic), microbes (disease), sewage, plastics, excess nitrates/phosphates (deoxygenation).

8. The eutrophication chain has four links. Fertiliser run-off → algae grow and die → bacteria decompose them and use up dissolved oxygen → aquatic life dies. The bacteria step is the one that carries the mark.

9. Cobalt chloride paper is the portable version. If a question shows filter paper turning from blue to pink, it is the cobalt(II) chloride test in disguise.

10. "Suggest why the water boils at 101.3 °C" means: it contains dissolved impurities, so it is not pure water. Two ideas, one sentence, both marks.

🌎 Apply It: Real-World Chemistry

From the ghats of Varanasi to a desalination plant in Chennai, every one of these situations turns on the difference between water that contains things and water that is pure.

The Namami Gange programme has built hundreds of sewage treatment plants along the Ganga. Before treatment expanded, untreated sewage from cities like Kanpur flowed directly into the river, and downstream stretches showed very low dissolved oxygen and periodic fish kills, even though sewage itself is not directly toxic to fish.

Explain why raw sewage entering a river kills fish, when sewage is not itself a poison.

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The Real Killer Is Respiration

Sewage is a feast of organic matter. Aerobic bacteria decompose it, and as they respire they use up the dissolved oxygen in the water. The more sewage, the more bacteria, the faster the oxygen disappears.

Fish Suffocate, They Are Not Poisoned

Fish extract dissolved oxygen through their gills. When the water is deoxygenated, they die of oxygen starvation. Sewage also carries harmful microbes — a second, separate danger, but to humans using the water rather than to the fish.

Chemistry Connection

Sewage and fertiliser run-off kill fish by exactly the same mechanism — bacteria consuming dissolved oxygen. One exam answer, two contexts. Notice also why dissolved oxygen tops the syllabus list of beneficial substances in water: everything else in this story hangs on it.

A bottled water company in the UK advertises "100% pure spring water". A curious student boils a sample with a data logger: it boils at 100.4 °C, and evaporating 1 litre leaves 250 mg of white solid residue. The label itself lists "calcium: 55 mg/L, magnesium: 20 mg/L".

Is the advertising chemically accurate? What do the boiling point and the residue prove?

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The Boiling Point Verdict

Pure water boils at exactly 100 °C at standard atmospheric pressure. Boiling at 100.4 °C shows dissolved impurities are present — dissolved solids raise the boiling point. The solid residue confirms it: pure water evaporates leaving nothing.

"Pure" Means Different Things

In everyday language "pure" means natural or safe; in chemistry it means a single substance. The label proudly listing its calcium and magnesium content actually proves the water is chemically impure — and those mineral ions are an example of metal compounds in water that are beneficial, providing essential minerals.

Chemistry Connection

This is the purity trap as a marketing claim. Examiners run the same trick with melting points in Topic 2 and boiling points here: a sharp, exact value means pure; a shifted or spread-out value means mixture. One principle, whole-syllabus coverage.

After the 2015 Chennai floods, relief workers distributed chlorine tablets to households whose wells had been contaminated by floodwater carrying sewage. Residents were told the water might taste slightly of chlorine but was safe to drink. Some families instead filtered their water through cloth and fine sand and assumed it was then safe.

Why were the chlorine tablets essential? Why is filtered-but-unchlorinated water still dangerous?

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What Filtration Can and Cannot Remove

Cloth and sand filtration removes insoluble solids — mud, grit, debris. The water may look perfectly clear afterwards. But microbes pass straight through: bacteria are far too small for a sand filter to trap reliably, and the microbes from sewage cause cholera and typhoid.

Chlorine Does the Job Filtration Cannot

Chlorine kills microbes — it sterilises the water chemically rather than physically straining it. Clear water is not the same as safe water. This is exactly why a treatment works uses both filtration and chlorination: different stages, different targets.

Chemistry Connection

The deadliest water is often the clearest. The 1854 London cholera outbreak traced by John Snow came from a well of sparkling clear, microbe-laden water. "Filtration removes insoluble solids; chlorination kills microbes" is not just an exam line — it is the single most life-saving sentence in this syllabus.

A chemistry lab assistant in a Singapore school prepares silver nitrate solution for chloride-ion tests. She always makes it up with distilled water. A student, in a hurry, tops up the bottle with tap water — and the whole batch turns faintly cloudy. Singapore tap water is treated and perfectly safe to drink.

Why did the tap water ruin the solution, and what does this show about the difference between drinking-quality and laboratory-quality water?

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The Cloudiness Explained

Tap water contains dissolved ions, including chloride ions (some from the chlorination stage itself). Silver nitrate reacts with chloride to form insoluble white silver chloride — the cloudiness. The tap water triggered the very test the solution was made for.

Safe Is Not the Same as Suitable

Water treatment makes water safe to drink; it deliberately leaves dissolved ions in. Laboratory work needs water with fewer chemical impurities, so distilled water is used — the dissolved ions are left behind when the water is boiled and the steam condensed.

Chemistry Connection

This scenario is the syllabus statement "distilled water is used in practical chemistry because it contains fewer chemical impurities" caught in the act. Any ion test, any titration, any "colourless solution" question silently assumes distilled water — one careless top-up shows why.

Marine biologists studying the Great Pacific Garbage Patch find seabirds with stomachs full of plastic fragments, and turtles entangled in discarded fishing nets. Meanwhile, sampling shows microplastic particles in the tissues of fish sold in markets from Tokyo to London.

Using the syllabus list of substances found in natural water, explain the harm caused by plastics in water — and why this pollutant behaves differently from nitrates or sewage.

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Direct Physical Harm

Plastics harm aquatic life directly: animals swallow fragments (filling stomachs with indigestible material so they starve) and become entangled in larger items. Microplastics pass up food chains — eventually onto human plates.

Why Plastics Are a Different Kind of Pollutant

Nitrates and sewage cause harm chemically and biologically — feeding algae and bacteria that strip the water of dissolved oxygen — and they are eventually broken down. Most plastics are not biodegradable: bacteria cannot decompose them, so they persist for centuries, breaking only into smaller and smaller fragments.

Chemistry Connection

The unreactivity that makes poly(ethene) perfect for packaging (Topic 11) is exactly what makes it disastrous in the ocean. Whenever a question asks for a "harmful substance in water", plastics are the easiest to explain: swallowed, entangling, non-biodegradable — three ready-made marking points.

Practice Questions: 10.1

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

What colour change shows that water is present when a liquid is added to anhydrous copper(II) sulfate?

A Blue to white

B White to blue

C Blue to pink

D White to pink

Anhydrous copper(II) sulfate is white; adding water forms blue hydrated CuSO₄·5H₂O. C is the cobalt(II) chloride result — the two tests are the classic swap error.

Question 2

Anhydrous cobalt(II) chloride paper is touched against a damp cloth. The colour change is

A pink to blue

B white to blue

C blue to pink

D blue to white

Anhydrous cobalt(II) chloride is blue and turns pink when hydrated: CoCl₂ + 6H₂O → CoCl₂·6H₂O. A is the reverse (what happens when you dry the paper by heating it).

Question 3

Sea water turns anhydrous copper(II) sulfate blue. What does this result prove?

A The sea water is pure water

B The sea water contains water

C The sea water contains dissolved salt

D The sea water is safe to drink

The test detects the presence of water and nothing else. It says nothing about purity, dissolved salts or safety — any liquid containing water gives the same result.

Question 4

Which observation shows that a sample of water is pure?

A It turns anhydrous copper(II) sulfate blue

B It is colourless and odourless

C It boils at exactly 100 °C at standard atmospheric pressure

D It has been chlorinated

Purity is shown by a sharp, exact boiling point (or freezing point). A only shows water is present; B is true of many impure samples; D makes water safe, not pure.

Question 5

A sample of water boils at 102 °C at standard atmospheric pressure. The best conclusion is that the water

A contains dissolved oxygen

B contains dissolved impurities

C has been chlorinated at the treatment works

D is pure but was heated too strongly

Dissolved impurities raise the boiling point (and lower the freezing point). Heating more strongly makes water boil faster, not hotter — the boiling temperature of a pure substance is fixed at a given pressure.

Question 6

In a water treatment works, what is the purpose of the sedimentation stage?

A To kill harmful microbes

B To remove unpleasant tastes and odours

C To allow larger insoluble particles to settle out under gravity

D To remove dissolved salts

Sedimentation is simply settling: the water stands and gravity pulls the larger solids down. A is chlorination, B is the carbon stage, and D is done by none of the stages — which is why tap water is not pure.

Question 7

Why is carbon used in water treatment?

A It kills bacteria and viruses

B It removes substances that cause unpleasant tastes and odours

C It neutralises acidic impurities

D It filters out mud and grit

The carbon stage deals with tastes and odours only. Killing microbes is chlorine's job; mud and grit are removed earlier by sedimentation and filtration. Confusing carbon with chlorine is the commonest 10.1 error.

Question 8

Chlorine is added at the end of water treatment in order to

A improve the taste of the water

B remove insoluble solids

C make the water chemically pure

D kill harmful microbes

Chlorination sterilises the water, preventing diseases such as cholera and typhoid. It slightly worsens the taste (A is backwards) and actually adds a chemical, so C is wrong too.

Question 9

Which is the correct order of stages in the treatment of the domestic water supply?

A Chlorination → filtration → sedimentation → carbon

B Sedimentation → filtration → carbon → chlorination

C Filtration → chlorination → sedimentation → carbon

D Carbon → sedimentation → chlorination → filtration

Big solids settle first, remaining solids are filtered, carbon polishes the taste, and chlorine is added last so the water leaves the works sterile. Remember: Settle, Sieve, Sip, Sterilise.

Question 10

Why is distilled water, rather than tap water, used to prepare solutions in practical chemistry?

A Distilled water is safer to drink

B Distilled water contains dissolved minerals that help reactions

C Distilled water contains fewer chemical impurities

D Distilled water boils at a lower temperature

Tap water's dissolved ions (e.g. chloride) would interfere with tests and contaminate solutions. Distillation leaves those ions behind, so distilled water has fewer chemical impurities — the exact syllabus wording.

Question 11

Which substance found in natural water is beneficial to aquatic life?

A Dissolved oxygen

B Plastics

C Harmful microbes from sewage

D Lead compounds

Fish and other aquatic organisms depend on dissolved oxygen for respiration. Plastics harm animals physically, microbes cause disease, and lead compounds are toxic metal compounds.

Question 12

Nitrates and phosphates enter rivers mainly from

A the chlorination of drinking water

B run-off of fertilisers from farmland (and phosphates from detergents)

C the dissolving of limestone rocks

D acid rain falling on forests

Fertilisers are highly soluble, so rain leaches them off fields into waterways; detergents add phosphates. Limestone contributes calcium compounds, not nitrates.

Question 13

Excess nitrates in a lake lead to the death of fish. What is the direct cause of the fish deaths?

A The nitrate ions poison the fish

B The algae sting the fish

C Bacteria decomposing dead algae use up the dissolved oxygen

D The water becomes too acidic for the fish

The chain is: nitrates → algae grow and die → aerobic bacteria decompose them, consuming dissolved oxygen → fish suffocate. The fish are never poisoned — the water is deoxygenated.

Question 14

Which pair correctly matches a harmful substance in water with its effect?

A Dissolved oxygen — causes cholera

B Harmful microbes — cause diseases such as cholera and typhoid

C Calcium compounds — entangle aquatic animals

D Plastics — provide essential minerals

Microbes from sewage cause water-borne diseases. Dissolved oxygen and calcium compounds are on the beneficial list; plastics harm animals that swallow or become trapped in them.

Question 15

Hydrated copper(II) sulfate crystals are heated gently. Which statement is correct?

A They turn from white to blue and this change cannot be reversed

B They turn from blue to white, and adding water reverses the change

C They turn from pink to blue because the water is lost

D They melt to form a blue liquid

Heating drives off the water of crystallisation: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O. Blue → white on heating, white → blue on adding water — a reversible reaction. Pink/blue belongs to cobalt chloride.

Question 16

Untreated sewage flows into a slow-moving river. Which change in the river water is expected downstream?

A The dissolved oxygen concentration increases

B The dissolved oxygen concentration decreases

C The number of microbes decreases

D The water becomes chemically pure

Bacteria decomposing the sewage respire aerobically, using up dissolved oxygen. Sewage also adds microbes, so C is backwards.

Question 17

Water leaving a treatment works is described as "safe but not pure". This is because the treatment does NOT remove

A insoluble solids

B harmful microbes

C dissolved salts and ions

D substances causing bad tastes

Sedimentation/filtration remove solids, chlorine deals with microbes, carbon with tastes — but dissolved ions pass through everything. Removing them would need distillation, which is far too expensive at scale.

Question 18

A student evaporates 100 cm³ of four water samples. Which result indicates the purest sample?

A A large white residue remains

B A small grey residue remains

C No residue remains at all

D The sample turns cobalt chloride paper pink

Pure water is only H₂O, so evaporation leaves nothing. Any residue is dissolved impurity. D is true of every sample containing water — it cannot distinguish purity.

Question 19

Which statement about the two chemical tests for water is correct?

A They only give a positive result with pure water

B They measure how much water is present

C They show water is present but not whether it is pure

D They kill any microbes in the sample

This is the single most examined sentence in 10.1: the colour changes prove the presence of water only. Purity requires the boiling point (exactly 100 °C) or freezing point (exactly 0 °C).

Question 20

Some metal compounds dissolved in natural water are described as beneficial because they

A kill harmful microbes in the water

B provide essential minerals needed for health

C increase the dissolved oxygen concentration

D make the water boil at exactly 100 °C

Compounds of calcium, magnesium and iron supply essential minerals. But the category cuts both ways — other metal compounds (e.g. of lead) are toxic. Dissolved solids raise the boiling point, so D is doubly wrong.

10.2 Fertilisers ▼

The Big Idea: Feeding Eight Billion People with Three Elements

Plants build themselves mostly out of carbon dioxide and water, but they cannot grow on those alone. They also need mineral elements from the soil — and intensive farming removes those elements faster than nature returns them. Every harvest carts nitrogen, phosphorus and potassium off the field inside the crop. Fertilisers put them back. The whole of 10.2 is one table, two salts and one gas test — but it feeds half the world: without synthetic nitrogen fertiliser made from Haber-process ammonia, roughly half the people alive today could not be fed.

NPK: The Three Essential Elements

Commercial fertilisers are labelled NPK after the chemical symbols of the three elements they supply. Each element has its own job in the plant, and the exam wants the exact matching.

Element	What it promotes in the plant	Typical compounds supplying it
N — nitrogen	Leaf growth — nitrogen is needed to make the proteins and chlorophyll of green, leafy tissue	Ammonium salts: ammonium nitrate NH₄NO₃, ammonium sulfate (NH₄)₂SO₄; also nitrates such as potassium nitrate
P — phosphorus	Root growth — strong root development and establishment	Phosphates, e.g. ammonium phosphate
K — potassium	Flower and fruit development — and general healthy growth	Potassium salts, e.g. potassium chloride, potassium sulfate, potassium nitrate

Memory Trick

Top, middle, bottom — in element order. N is for the top of the plant (leaves), P is for the bottom (roots), K is for what the plant produces (flowers and fruit). Or remember the phrase "Leafy Roots bear Fruit" for N–P–K in order. A pleasing cross-check: NH₄NO₃ feeds leaves, and "leaf" starts like "N"itrogen's job.

Ammonium Salts and Nitrates: The Nitrogen Carriers

Ammonium salts and nitrates are used as fertilisers because they are rich in nitrogen and — crucially — soluble in water, so plant roots can absorb the nitrogen as dissolved ions. Ammonium nitrate, NH₄NO₃, is a favourite exam molecule because both of its ions carry nitrogen.

The salts are made by the standard acid + base neutralisations you met in Topic 7, with ammonia solution as the base. The ammonia itself is manufactured from nitrogen and hydrogen by the Haber process (N₂ + 3H₂ ⇌ 2NH₃, iron catalyst, 450 °C, 200 atm) — the industrial bridge between the unreactive nitrogen of the air and the reactive nitrogen a plant can use.

NH₃ + HNO₃ → NH₄NO₃ (ammonium nitrate)

Ammonia + nitric acid → ammonium nitrate. A neutralisation: ammonia is the base, so no water is formed — the H⁺ simply transfers onto NH₃ to make NH₄⁺. Sold as "Kisan Khad" and similar brands across India; the world's most heavily used nitrogen fertiliser.

2NH₃ + H₂SO₄ → (NH₄)₂SO₄ (ammonium sulfate)

Ammonia + sulfuric acid → ammonium sulfate. Sulfuric acid has two acidic hydrogens, so two NH₃ are needed — the balancing point examiners test. Check the formula: the ammonium ion is NH₄⁺ and sulfate is SO₄²⁻, so two ammonium ions balance one sulfate: (NH₄)₂SO₄.

The Alkali Problem — and the Test for Ammonium Ions

Here is the chemistry that ties 10.2 together: warming any ammonium salt with an alkali displaces ammonia gas. The hydroxide ion pulls the extra H⁺ off the ammonium ion:

NH₄⁺ + OH⁻ → NH₃ + H₂O

Example in full: (NH₄)₂SO₄ + 2NaOH → Na₂SO₄ + 2NH₃ + 2H₂O — warming needed. As a test: warm the unknown with aqueous sodium hydroxide; if ammonia is given off, it turns damp red litmus paper blue — the positive test for the ammonium ion, NH₄⁺. (Ammonia is the only common alkaline gas, and it also has a sharp, pungent smell.) As a warning for farmers: never mix an ammonium fertiliser with an alkali such as slaked lime, Ca(OH)₂. The nitrogen escapes to the air as ammonia gas — wasted money and a wasted field.

Supplement

Why the litmus must be damp — and why the test is definitive

Litmus only responds to ions in solution, so the ammonia gas must dissolve first: the damp paper provides the water, in which NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ makes the alkaline solution that turns litmus blue. Since ammonia is the only common gas that turns damp red litmus blue, this one observation identifies both the gas and — when it appears on warming a salt with NaOH — the ammonium ion in the salt. Notice the beautiful reversibility across this section: acids make ammonium salts from ammonia; alkalis unmake them back to ammonia.

Worked Example 1 A fertiliser bag in a Ludhiana farm store is labelled NPK 20-10-10. (a) Name the three elements the numbers refer to and state what each promotes in the crop. [3] (b) Name a compound that could supply the nitrogen and explain why ammonium compounds are suitable as fertilisers. [2]

Step 1: Decode NPK

The letters are chemical symbols: N = nitrogen, P = phosphorus, K = potassium (kalium). The numbers are the percentages of each — this bag is nitrogen-rich, so it is aimed at leafy growth.

Step 2: Match each element to its role

Nitrogen → leaf growth. Phosphorus → root growth. Potassium → flower and fruit development. Learn these as fixed pairs; examiners accept nothing vaguer than these functions.

Step 3: Name a nitrogen carrier and justify it

Ammonium nitrate NH₄NO₃ or ammonium sulfate (NH₄)₂SO₄. Suitability rests on two facts: they contain a high proportion of nitrogen, and they are water-soluble, so roots can absorb the ions from soil water.

(a) N = nitrogen — promotes leaf growth; P = phosphorus — promotes root growth; K = potassium — promotes flower and fruit development [3]. (b) e.g. ammonium nitrate [1]; ammonium salts contain nitrogen and are soluble in water, so the nitrogen can be taken up by the roots [1].

Worked Example 2 Ammonium sulfate is manufactured from ammonia. (a) Write the balanced equation for the reaction. [2] (b) State the source of the ammonia. [1] (c) Explain why the reaction is described as a neutralisation. [1]

Step 1: Identify the acid needed

The salt is a sulfate, so the acid must be sulfuric acid, H₂SO₄. (Nitrates come from nitric acid, chlorides from hydrochloric acid — the Topic 7 rule.)

Step 2: Build and balance the equation

Ammonium is NH₄⁺, sulfate is SO₄²⁻, so the salt is (NH₄)₂SO₄ — needing two ammonium ions, hence 2NH₃: 2NH₃ + H₂SO₄ → (NH₄)₂SO₄. The commonest error is forgetting the 2.

Step 3: Source and classification

The ammonia comes from the Haber process (nitrogen from the air + hydrogen, iron catalyst). The reaction is a neutralisation because ammonia is a base reacting with an acid to form a salt — unusually, with no water formed, because NH₃ accepts the H⁺ directly.

(a) 2NH₃ + H₂SO₄ → (NH₄)₂SO₄ — formula [1], balancing [1]. (b) The Haber process [1]. (c) A base (ammonia) reacts with an acid to form a salt [1].

Worked Example 3 A student is given a white solid thought to be ammonium nitrate. Describe a test to confirm the presence of the ammonium ion, giving the result expected. Explain why a farmer should not spread slaked lime and ammonium nitrate on a field at the same time. [4]

Step 1: The test procedure

Add aqueous sodium hydroxide to the solid and warm the mixture. Both details matter: NaOH is the reagent, and without warming the ammonia is not released quickly enough to detect.

Step 2: The observation

Ammonia gas is given off, detected because it turns damp red litmus paper blue (and has a pungent smell). The chemistry: NH₄⁺ + OH⁻ → NH₃ + H₂O.

Step 3: Apply the same chemistry to the farm

Slaked lime, Ca(OH)₂, is an alkali. Mixed with an ammonium fertiliser it does exactly what the NaOH did in the test tube: displaces ammonia gas, which escapes to the air. The nitrogen is lost, so the fertiliser is wasted.

Warm the solid with aqueous sodium hydroxide [1]; ammonia gas is evolved, which turns damp red litmus paper blue [1]. Slaked lime is an alkali, so it reacts with the ammonium salt and displaces ammonia gas [1]; the ammonia (and its nitrogen) escapes to the atmosphere, so the fertiliser loses its value [1].

Exam Tips for 10.2

1. Learn the NPK trio as fixed pairs. N → leaf growth. P → root growth. K → flower and fruit development. "Helps the plant grow" scores nothing — the specific function is the mark.

2. K is potassium, not "kalium counts as krypton". Watch careless symbol errors: P is phosphorus (not potassium!), K is potassium.

3. Two ammonias for sulfuric acid. 2NH₃ + H₂SO₄ → (NH₄)₂SO₄, but only one for nitric: NH₃ + HNO₃ → NH₄NO₃. The 2 is a deliberate balancing trap.

4. Get the brackets right. Ammonium sulfate is (NH₄)₂SO₄ — NH₄⁺ is +1, SO₄²⁻ is −2. Writing NH₄SO₄ or NH₄(SO₄)₂ loses the formula mark instantly.

5. The ammonium test has three required parts. (i) add NaOH(aq), (ii) warm, (iii) ammonia turns damp red litmus blue. Forgetting "warm" or "damp" costs a mark each.

6. Why soluble? Fertilisers must dissolve in soil water so roots can absorb the ions — but the same solubility explains run-off into rivers and the eutrophication story of 10.1. Examiners link the two sections constantly.

7. Ammonia is the base, salt but no water. NH₃ + acid gives just the ammonium salt; there is no water among the products because NH₃ accepts H⁺ directly. Do not bolt "+ H₂O" onto the equation from habit.

8. Haber link. If asked where the ammonia for fertiliser manufacture comes from: nitrogen from the air + hydrogen, combined in the Haber process (iron catalyst, 450 °C, 200 atm).

9. Ammonium nitrate is nitrogen twice over. NH₄NO₃ carries N in both cation and anion — a favourite "calculate the % of nitrogen" molecule in Topic 4-style crossover questions (2×14 out of 80 ≈ 35%).

10. Never store ammonium fertilisers with lime. Any alkali + ammonium salt → ammonia lost as gas. This appears both as a test-tube question and as a farming-context question — same chemistry, same answer.

🌎 Apply It: Real-World Chemistry

From the wheat fields of Punjab to a rose garden in Kent, every bag of fertiliser is applied chemistry — and occasionally applied disaster.

A Punjab wheat farmer notices his crop is pale yellow-green with thin, weak leaves, although the roots look healthy when a plant is pulled up. His neighbour's field, treated with urea and ammonium sulfate, is deep green. The local agricultural officer glances at the leaves and immediately names the missing element.

Which NPK element is deficient, and why does its absence show up in the leaves specifically?

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Reading the Symptoms

Pale, weak leaves with healthy roots point to nitrogen deficiency. Nitrogen promotes leaf growth: it is needed to build proteins and the green pigment chlorophyll. Without it, leaves are small and yellowish — and with less chlorophyll, photosynthesis slows and the whole plant starves.

The Fix

An ammonium salt or nitrate fertiliser — ammonium sulfate, ammonium nitrate — supplies soluble nitrogen the roots absorb from soil water. The neighbour's deep green field is the nitrogen-rich control experiment.

Chemistry Connection

Deficiency questions are just the NPK table read backwards: weak leaves → N; poor roots → P; poor flowers/fruit → K. Learn the table once, answer in both directions.

A gardener in Kent grows prize roses and tomatoes. Her spring feed is high in nitrogen, but from June she switches to a "tomato feed" whose NPK ratio is 4-3-8 — potassium-rich. The label promises "more blooms, bigger fruit".

Use the roles of N, P and K to explain the switch in feeding programme.

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Spring: Building the Plant

Early in the season the plant needs leaves (the food factories) and roots (water and mineral supply). High nitrogen drives leaf growth; phosphorus establishes the root system.

Summer: Flowers and Fruit

Once the plant is built, the goal changes to flower and fruit development — the job of potassium. Hence the potassium-rich 4-3-8 feed. Keeping nitrogen high all summer would give lush leaves but few tomatoes.

Chemistry Connection

The numbers on every fertiliser bag are an NPK exam question in the wild. High first number = leafy growth (lawn feed); high last number = flowers and fruit (tomato feed). You can now decode a garden centre shelf.

A fertiliser plant near Kanpur produces ammonium nitrate. The site has two production units: one synthesises ammonia from nitrogen and hydrogen; the other oxidises some of that ammonia to nitric acid, then combines the two streams in a final reactor.

Write the equation for the final reactor and explain why this one factory needs no outside supply of either reactant for it.

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The Final Reaction

NH₃ + HNO₃ → NH₄NO₃ — ammonia (base) neutralises nitric acid to give ammonium nitrate. No water is formed: the H⁺ transfers straight onto the NH₃.

Why the Plant Is Self-Sufficient

Both reactants trace back to the Haber process on site: N₂ (from the air) + 3H₂ ⇌ 2NH₃. Part of the ammonia is used directly; part is oxidised to make the nitric acid. Air in, fertiliser out.

Chemistry Connection

Ammonium nitrate is nitrogen captured twice from the same air — once in the NH₄⁺ ion, once in the NO₃⁻ ion. This molecule is the reason the Haber process is often called the most important chemical reaction on Earth: it turned atmospheric nitrogen into food.

A gardener limes his acidic soil with slaked lime (calcium hydroxide) in the morning, then spreads ammonium sulfate fertiliser over the same beds in the afternoon. Within days there is a faint smell of ammonia after rain, and weeks later his cabbages are yellowing despite the expensive fertiliser.

Explain what went wrong, with an equation for the reaction responsible.

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Alkali Meets Ammonium Salt

Slaked lime is an alkali. In the damp soil it reacts with the ammonium sulfate: (NH₄)₂SO₄ + Ca(OH)₂ → CaSO₄ + 2NH₃ + 2H₂O — the same NH₄⁺ + OH⁻ → NH₃ + H₂O chemistry as the laboratory test, which is why he can smell ammonia.

The Consequence

The ammonia escapes as a gas, taking the fertiliser's nitrogen with it. The cabbages never receive it — hence yellowing, nitrogen-starved leaves. Liming and ammonium fertilising must be separated by weeks.

Chemistry Connection

One reaction, three exam guises: a test for NH₄⁺ in the lab, a preparation of ammonia gas, and a farming blunder. When you recognise them as the same equation, all three questions are already answered.

After heavy monsoon rain, agricultural scientists in Kerala measure sharply raised nitrate levels in streams draining fertilised paddy fields, and later that season parts of the Vembanad lake turn green with algae. Local fishermen report falling catches.

Why does the very property that makes nitrate fertilisers effective also make them a water-pollution risk? Connect this to what happens in the lake.

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Solubility: The Double-Edged Sword

Fertilisers work because they are very soluble — roots can only absorb dissolved ions. But the same solubility means heavy rain leaches the nitrates out of the soil and carries them into streams and lakes. The plant food becomes water pollution without a single chemical change.

What Happens in the Lake

The nitrates fertilise algae (the green bloom). When the algae die, bacteria decompose them and use up the dissolved oxygen. The deoxygenated water cannot support fish — the falling catches. This is the 10.1 eutrophication chain, triggered by 10.2 chemistry.

Chemistry Connection

This is the syllabus's favourite cross-section link: fertilisers (10.2) → nitrates in water (10.1) → deoxygenation. A six-mark question can span both sections, and the phrase that stitches them together is "soluble, so washed into rivers by rain".

Practice Questions: 10.2

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

In an NPK fertiliser, the three letters stand for

A nitrogen, phosphorus and krypton

B nitrogen, phosphorus and potassium

C neon, potassium and calcium

D nitrogen, potassium and phosphorus

N = nitrogen, P = phosphorus, K = potassium (from its Latin name kalium). D lists the right elements in the wrong order — the letters go with the symbols, and P is phosphorus, never potassium.

Question 2

Which element in a fertiliser chiefly promotes leaf growth?

A Nitrogen

B Phosphorus

C Potassium

D Calcium

N → leaf growth (proteins and chlorophyll), P → root growth, K → flower and fruit development. Learn the three fixed pairs.

Question 3

A crop has poorly developed roots. Which element is most likely deficient in the soil?

A Nitrogen

B Phosphorus

C Potassium

D Carbon

Phosphorus promotes root growth, so poor roots point to P deficiency. Deficiency questions are the NPK table read backwards.

Question 4

Potassium is included in fertilisers mainly to promote

A leaf growth

B root growth

C flower and fruit development

D resistance to insect attack

K → flowers and fruit — which is why "tomato feeds" are potassium-rich. Insect resistance is not an NPK role on this syllabus.

Question 5

Which compound is NOT used as a source of nitrogen in fertilisers?

A Ammonium nitrate

B Ammonium sulfate

C Potassium nitrate

D Potassium chloride

Potassium chloride (KCl) contains no nitrogen — it supplies potassium only. Ammonium salts carry N in the NH₄⁺ ion and nitrates in the NO₃⁻ ion; potassium nitrate usefully supplies both K and N.

Question 6

Ammonium sulfate is manufactured by the reaction

A NH₃ + H₂SO₄ → NH₄SO₄

B 2NH₃ + H₂SO₄ → (NH₄)₂SO₄

C NH₃ + HNO₃ → NH₄NO₃

D 2NH₄OH + SO₂ → (NH₄)₂SO₄ + H₂O

Two NH₃ molecules neutralise the two acidic hydrogens of sulfuric acid, giving (NH₄)₂SO₄. A has an impossible formula; C is the manufacture of ammonium nitrate.

Question 7

Ammonium nitrate is made by reacting ammonia with

A sulfuric acid

B hydrochloric acid

C nitric acid

D sodium hydroxide

Nitrates come from nitric acid: NH₃ + HNO₃ → NH₄NO₃. Sodium hydroxide would do the opposite — release ammonia from an ammonium salt.

Question 8

The ammonia used to manufacture fertilisers is produced industrially by

A the electrolysis of brine

B heating ammonium salts with alkali

C the Haber process, combining nitrogen and hydrogen

D the fractional distillation of liquid air

N₂ + 3H₂ ⇌ 2NH₃ with an iron catalyst — the Haber process. Distilling liquid air gives nitrogen itself, but plants cannot use unreactive N₂ directly; B is a laboratory preparation, not industry.

Question 9

Why must a fertiliser compound be soluble in water?

A So that it looks attractive when spread

B So that plant roots can absorb the dissolved ions from soil water

C So that it evaporates quickly after rain

D So that it reacts with the air to release nitrogen

Roots take up nutrients only as ions in solution. The same solubility is the weakness: rain leaches fertiliser into rivers, causing the problems described in 10.1.

Question 10

Warming an ammonium salt with aqueous sodium hydroxide produces

A hydrogen gas

B ammonia gas

C nitrogen gas

D nitrogen dioxide gas

NH₄⁺ + OH⁻ → NH₃ + H₂O. The alkali displaces ammonia from the ammonium salt — the basis of the test for the ammonium ion.

Question 11

Which observation confirms that a gas is ammonia?

A It relights a glowing splint

B It turns damp blue litmus paper red

C It turns damp red litmus paper blue

D It gives a squeaky pop with a lighted splint

Ammonia is the only common alkaline gas, so it turns damp red litmus blue. A is oxygen, B would indicate an acidic gas, D is hydrogen.

Question 12

Why must the red litmus paper be damp when testing for ammonia?

A To stop the paper catching fire

B The ammonia must dissolve in the water to form an alkaline solution

C Dry paper would blow away in the gas stream

D Water reacts with ammonia to form nitric acid

Indicators respond to ions in solution. The ammonia dissolves in the film of water, forming NH₄⁺ and OH⁻ ions — it is the OH⁻ that turns the litmus blue.

Question 13

A farmer mixes ammonium nitrate fertiliser with slaked lime. The result is that

A the fertiliser becomes more effective

B ammonia gas is released and nitrogen is lost from the soil

C the mixture explodes on contact

D nothing happens because both are solids

Slaked lime, Ca(OH)₂, is an alkali: alkali + ammonium salt → ammonia gas escapes, wasting the nitrogen. Same chemistry as the laboratory test for NH₄⁺.

Question 14

Which is the correct formula for ammonium sulfate?

A NH₄SO₄

B NH₄(SO₄)₂

C (NH₄)₂SO₄

D (NH₃)₂SO₄

Ammonium is NH₄⁺ (+1) and sulfate is SO₄²⁻ (−2), so two ammonium ions are needed per sulfate: (NH₄)₂SO₄. D uses ammonia NH₃ instead of the ammonium ion.

Question 15

Ammonium nitrate is a particularly effective nitrogen fertiliser because

A it is insoluble, so it stays in the soil for years

B both its cation and its anion contain nitrogen

C it also supplies phosphorus and potassium

D it neutralises acidic soils

NH₄⁺ and NO₃⁻ each carry nitrogen — about 35% N by mass. It contains no P or K, and fertilisers must be soluble to work, so A is wrong twice over.

Question 16

The reaction NH₃ + HNO₃ → NH₄NO₃ is best described as

A oxidation of ammonia

B displacement

C neutralisation of an acid by a base

D thermal decomposition

Ammonia acts as a base, accepting H⁺ from the acid to form a salt. Unusually for a neutralisation, no water is formed — the H⁺ joins the NH₃ directly.

Question 17

A gardener wants maximum blooms on her roses. The best fertiliser NPK ratio would be

A 25-5-5 (nitrogen-rich)

B 5-25-5 (phosphorus-rich)

C 5-5-25 (potassium-rich)

D 0-0-0 (pure organic matter)

Flowers are potassium's department, so the K-rich 5-5-25 feed is best. A nitrogen-rich feed would give lush leaves at the expense of blooms.

Question 18

Which equation correctly shows ammonia being displaced from an ammonium salt?

A NH₄Cl + HCl → NH₃ + Cl₂ + H₂

B NH₄Cl + NaOH → NaCl + NH₃ + H₂O

C NH₄Cl → N₂ + 2HCl + H₂

D NH₃ + NaOH → NaNH₂ + H₂O

An alkali (with warming) displaces ammonia: the OH⁻ removes H⁺ from NH₄⁺, giving NH₃ and water, with the sodium salt left behind. Acids (A) do the opposite job — they make ammonium salts.

Question 19

Heavy rain falls shortly after a nitrate fertiliser is applied. The most likely environmental consequence is

A the nitrate evaporates into the atmosphere

B the nitrate is leached into rivers and lakes, where it can cause deoxygenation

C the nitrate is converted to ammonia by the rain

D the soil becomes permanently infertile

Soluble nitrates are washed (leached) into waterways, where they feed algal growth; decomposing bacteria then use up the dissolved oxygen. This links directly to the 10.1 eutrophication chain.

Question 20

A white solid dissolves in water; warming the solution with sodium hydroxide gives a pungent gas that turns damp red litmus blue. The solid contains

A nitrate ions

B ammonium ions

C sulfate ions

D carbonate ions

Ammonia released on warming with alkali is the test for the ammonium ion: NH₄⁺ + OH⁻ → NH₃ + H₂O. Carbonates with acid give CO₂; the anion tests are entirely different.

10.3 Air Quality & Climate ▼

The Big Idea: One Clean Mixture, Five Pollutants, Two Global Problems

This section has a very tidy architecture. Start with what clean air is. Then learn the five pollutants as a table — each with its source and its adverse effect. Then keep the two big consequences rigorously separate: acid rain (caused by sulfur dioxide and oxides of nitrogen) and the greenhouse effect / climate change (caused by carbon dioxide and methane). Mixing those two lists is the most heavily punished error in the whole topic.

The Composition of Clean, Dry Air

Clean, dry air ≈ 78% nitrogen (N₂) + 21% oxygen (O₂) + ≈1% argon and other gases (including ≈0.04% CO₂)

"Dry" matters: water vapour is excluded because it varies from place to place. The remaining ≈1% is mostly argon, with small amounts of other noble gases and carbon dioxide. Nitrogen in the air is not a pollutant — it is the main ingredient. Only when it is forced to react (in hot engines) do its oxides become pollutants.

Clean, dry air. The examiner's favourite catch: the third biggest component is argon, not carbon dioxide — CO₂ is a trace gas at about 0.04%.

The Five Air Pollutants: Source and Effect

This table is the core of 10.3. Every row is examinable in both directions ("state the source of…" / "state an adverse effect of…").

Pollutant	Source	Adverse effect
Carbon dioxide, CO₂	Complete combustion of carbon-containing fuels	Greenhouse gas — higher levels lead to climate change
Carbon monoxide, CO	Incomplete combustion of carbon-containing fuels (limited oxygen)	Toxic gas — binds to haemoglobin in red blood cells, preventing oxygen transport
Particulates (soot / carbon particles)	Incomplete combustion of carbon-containing fuels (especially diesel)	Cause respiratory problems and increase the risk of cancer
Methane, CH₄	Digestion in livestock (cattle) and decomposition of vegetation (e.g. paddy fields, landfill)	Greenhouse gas — contributes to climate change
Oxides of nitrogen, NOₓ	Nitrogen and oxygen from the air react at the high temperature inside car engines (also lightning)	Acid rain, photochemical smog and respiratory problems
Sulfur dioxide, SO₂	Combustion of fossil fuels containing sulfur compounds (coal, some oils)	Acid rain

Memory Trick

Two lists, two problems. Greenhouse gang: CO₂ and CH₄ — both contain carbon, both trap heat. Acid gang: SO₂ and NOₓ — both are "-ide/-ox" oxides of non-carbon elements, both dissolve to make acids. And CO belongs to neither gang — it is the poison. If a gas contains carbon it warms the planet; if it is S or N oxide it acidifies the rain; if it is CO it kills quietly indoors.

Complete vs Incomplete Combustion — Why One Fuel Gives Three Pollutants

With plenty of oxygen, a hydrocarbon burns completely: the carbon becomes CO₂ and the hydrogen becomes water. With a limited supply of oxygen — inside an engine, a badly ventilated heater, a smoky flame — combustion is incomplete: some carbon only makes it to carbon monoxide (CO), and some never gets beyond solid carbon particulates (soot). One fuel, three carbon products, depending purely on oxygen supply.

Carbon monoxide's toxicity deserves its exact mechanism: CO is colourless and odourless, and it binds to the haemoglobin in red blood cells more strongly than oxygen does, so the blood can no longer carry oxygen around the body. Victims are asphyxiated without warning — which is why gas heaters must be serviced and ventilated.

Acid Rain: The SO₂ and NOₓ Story

SO₂ and NOₓ dissolve in atmospheric water and are oxidised to sulfuric and nitric acids. Note where prevention strikes: before the chimney (low-sulfur fuel), at the chimney (flue gas desulfurisation) and at the exhaust pipe (catalytic converter).

Notice the pleasing chemistry of flue gas desulfurisation (FGD): sulfur dioxide is an acidic oxide, so it is removed by scrubbing the waste gases with a base — calcium oxide or calcium carbonate. CaO + SO₂ → CaSO₃. It is a Topic 7 acid–base neutralisation happening inside a power station chimney.

Catalytic Converters: Cleaning the Exhaust

The exam equation: 2CO + 2NO → 2CO₂ + N₂. Carbon monoxide is oxidised (gains oxygen), nitrogen monoxide is reduced (loses oxygen) — each pollutant cleans up the other on the hot catalyst surface.

The Greenhouse Effect and Climate Change

Carbon dioxide and methane are greenhouse gases. The Supplement wants the mechanism stated as a chain of energy transfers — and wants it kept absolutely separate from acid rain and (a favourite wrong answer) from "the hole in the ozone layer", which is a different phenomenon entirely and not caused by CO₂.

The mechanism in one sentence: greenhouse gases absorb the thermal energy radiated from the Earth's surface and re-emit it in all directions, reducing the thermal energy lost to space — so the atmosphere warms.

Supplement

Consequences of climate change — and the strategies against it

Consequences: rising average global temperatures; melting of polar ice caps and glaciers; rising sea levels and flooding of low-lying land; more frequent extreme weather (droughts, storms, heatwaves); changing rainfall patterns damaging crops and habitats.

Strategies to reduce the effects: reduce fossil fuel combustion by switching to renewable energy (solar, wind, hydroelectric) and to hydrogen or electric transport; improve energy efficiency; plant trees (photosynthesis removes CO₂ from the atmosphere); reduce livestock farming and landfill to cut methane. In every answer, tie the strategy to the gas it reduces — "planting trees" earns its mark because trees photosynthesise, removing CO₂.

Photosynthesis: The Planet's CO₂ Removal Service

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

Word equation: carbon dioxide + water → glucose + oxygen. Conditions (both required in an answer): chlorophyll (the green pigment that traps the energy) and light energy. Photosynthesis is the reason "planting trees" is a climate strategy: growing plants remove CO₂ from the atmosphere and lock the carbon into glucose — and ultimately wood. Check the balancing by atoms: 6 C, 12 H, 18 O on each side. Respiration is this exact equation reversed — do not muddle the two directions.

Worked Example 1 A poorly ventilated gas water heater in a bathroom produces a pollutant that has killed sleeping occupants without any warning smell. (a) Name the pollutant and explain how it forms. [2] (b) Explain how it causes death. [2] (c) Explain why the same boiler, properly ventilated, produces a different carbon compound instead. [1]

Step 1: Identify the killer

Carbon monoxide, CO — colourless and odourless, hence no warning. It forms by incomplete combustion: the blocked ventilation gives a limited supply of oxygen, so the carbon in the fuel cannot be fully oxidised to CO₂.

Step 2: The toxicity mechanism — be precise

CO binds to the haemoglobin in red blood cells (more strongly than oxygen does), so the haemoglobin can no longer carry oxygen around the body. Death is by oxygen starvation of the tissues. "It is poisonous" scores nothing; the haemoglobin mechanism is the mark.

Step 3: The ventilated case

With a good air supply there is excess oxygen, so combustion is complete and the carbon is fully oxidised to carbon dioxide instead.

(a) Carbon monoxide, formed by incomplete combustion of the fuel in a limited oxygen supply [2]. (b) CO binds to haemoglobin in red blood cells, preventing the blood from transporting oxygen [2]. (c) With sufficient oxygen, combustion is complete and carbon dioxide is formed [1].

Worked Example 2 The marble of the Taj Mahal (calcium carbonate) is slowly being damaged by acid rain from nearby industry and traffic. (a) Name the two pollutant gases responsible for acid rain and give a source of each. [4] (b) Explain, with an equation, how acid rain attacks marble. [2] (c) State two methods used to reduce acid rain, and how each works. [2] (Extended)

Step 1: The two acid-rain gases — and only these two

Sulfur dioxide, from the combustion of fossil fuels containing sulfur compounds (coal-fired power stations, industrial furnaces). Oxides of nitrogen, formed when nitrogen and oxygen from the air react at the high temperature inside car engines. Not CO₂ — carbon dioxide makes rain only very weakly acidic and is a greenhouse gas.

Step 2: In the atmosphere

The gases dissolve in water in the clouds and are oxidised, forming sulfuric acid (H₂SO₄) and nitric acid (HNO₃), which fall as acid rain.

Step 3: Acid + carbonate

Marble is CaCO₃. Acid + carbonate → salt + water + carbon dioxide: e.g. CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂. The stone is eaten away, detail by detail.

Step 4: Reduction strategies

(i) Use low-sulfur fuels — less sulfur in, less SO₂ out. (ii) Flue gas desulfurisation: scrub power-station waste gases with calcium oxide or calcium carbonate, which neutralise the acidic SO₂. (iii) Catalytic converters remove NOₓ from exhausts. Any two, each with its mechanism.

(a) Sulfur dioxide — from burning fossil fuels containing sulfur compounds [2]; oxides of nitrogen — formed from N₂ and O₂ at high temperature in car engines [2]. (b) The acids react with the calcium carbonate: CaCO₃ + H₂SO₄ → CaSO₄ + H₂O + CO₂, dissolving the marble [2]. (c) Low-sulfur fuels reduce SO₂ emissions; flue gas desulfurisation with CaO/CaCO₃ neutralises SO₂ in chimney gases (or: catalytic converters convert NOₓ to N₂) [2].

Worked Example 3 Explain how carbon dioxide and methane in the atmosphere cause an increase in global temperatures. In your answer, name one source of each gas, describe the energy transfers involved, and state one strategy that reduces the amount of each gas. [6] (Supplement)

Step 1: Sources

CO₂: complete combustion of carbon-containing (fossil) fuels. CH₄: digestion in livestock (cattle) or decomposition of vegetation (paddy fields, landfill).

Step 2: The mechanism, as energy transfers

Radiation from the Sun reaches and warms the Earth's surface, which re-emits thermal energy. The greenhouse gases absorb this thermal energy and re-emit it in all directions — including back towards the surface — so less thermal energy escapes to space and the atmosphere warms.

Step 3: Strategies, each tied to its gas

CO₂: burn less fossil fuel by using renewable energy (wind, solar, hydroelectric), or plant trees, which remove CO₂ by photosynthesis. CH₄: reduce livestock farming or capture landfill gas. A strategy without its gas is only half an answer.

CO₂ comes from complete combustion of fossil fuels; CH₄ from livestock digestion / decomposing vegetation [2]. The gases absorb thermal energy radiated from the Earth's surface [1] and re-emit it in all directions, reducing the energy lost to space [1], so the atmosphere becomes warmer [1]. Strategies: renewable energy / planting trees (removes CO₂ by photosynthesis); reducing livestock farming (reduces CH₄) [1].

Exam Tips for 10.3

1. Learn 78 / 21 / ≈1. Clean dry air: 78% nitrogen, 21% oxygen, roughly 1% argon plus a trace (≈0.04%) of CO₂. The third most abundant gas is argon — not CO₂.

2. Keep the two gangs apart. Greenhouse/climate: CO₂ and CH₄. Acid rain: SO₂ and NOₓ. Writing "CO₂ causes acid rain" or "SO₂ causes global warming" throws away whole questions.

3. CO vs CO₂. One letter apart, totally different: CO is the toxic product of incomplete combustion; CO₂ is the greenhouse product of complete combustion. Say which combustion, every time.

4. The CO mechanism is compulsory. "CO binds to haemoglobin in red blood cells, preventing oxygen transport." The word haemoglobin carries the mark.

5. NOₓ needs its special source sentence. "Nitrogen and oxygen from the air react at the high temperature inside car engines." The fuel contains no nitrogen — the air does.

6. Nitrogen itself is not a pollutant. N₂ is 78% of clean air. Only its oxides pollute.

7. The converter equation: 2CO + 2NO → 2CO₂ + N₂. CO is oxidised, NO is reduced. And note what the converter does not do: it does not remove CO₂ — it makes it.

8. FGD is acid–base chemistry. SO₂ is an acidic oxide, removed by the bases calcium oxide or calcium carbonate in flue gas desulfurisation.

9. The greenhouse mechanism has three exam phrases. Greenhouse gases absorb thermal energy, re-emit it in all directions, and reduce the thermal energy lost to space. All three, in order, for full marks.

10. Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with chlorophyll and light. CO₂ is on the left — plants remove it. Reversing it gives respiration; muddling the direction ruins the "why plant trees?" answer.

🌎 Apply It: Real-World Chemistry

Delhi's November smog, a Drax power station chimney, a Himalayan glacier — the whole of 10.3 is playing out in the news every week.

Every November, Delhi's air quality index rockets into the "severe" band. Contributors include millions of vehicle engines, crop-stubble burning in neighbouring states, and cool, still air that traps the pollution near the ground. Hospitals report surges in asthma and bronchitis cases, and long-term studies link the haze to lung cancer.

Identify the two main pollutants in this smog that come from vehicles and burning, and connect each to the health effects reported.

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Particulates: The Visible Haze

Stubble burning and diesel engines carry out incomplete combustion, releasing particulates (tiny carbon/soot particles). Breathed deep into the lungs they cause respiratory problems and increase the risk of cancer — the two effects the syllabus names.

Oxides of Nitrogen: The Engine Gas

Inside hot vehicle engines, nitrogen and oxygen from the air combine to form oxides of nitrogen. These cause respiratory problems directly, and in sunlight they drive the formation of photochemical smog — the brown, irritating haze.

Chemistry Connection

Notice that Delhi's smog is not one pollutant but a cocktail, and each ingredient has its own syllabus row: particulates (incomplete combustion → respiratory problems, cancer) and NOₓ (hot engines → smog, respiratory problems). Exam answers must pick the right row, not shout "pollution!".

The UK's coal power stations, such as Drax in Yorkshire, were once Europe's biggest sources of sulfur dioxide, and Scandinavian countries complained that lakes in Norway and Sweden were turning acidic and losing their fish — hundreds of kilometres from the chimneys. Flue gas desulfurisation units were then fitted, spraying the exhaust gases with limestone slurry.

Explain how the SO₂ from Yorkshire damaged Norwegian lakes, and the chemistry of the limestone solution to the problem.

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The Long-Distance Damage

The coal contained sulfur compounds, so burning it released SO₂. High in the atmosphere the gas dissolved in cloud water and oxidised to sulfuric acid; winds carried the clouds across the North Sea before the acid fell as rain — acidifying lakes, killing fish, and damaging forests far from the source. Acid rain does not respect borders.

Neutralising the Chimney

Flue gas desulfurisation uses a base — calcium carbonate (limestone) or calcium oxide (lime) — to neutralise the acidic SO₂ before it leaves the chimney: CaCO₃ + SO₂ → CaSO₃ + CO₂. The product, calcium sulfite/sulfate, is even sold to make plasterboard.

Chemistry Connection

FGD is Topic 7 wearing a hard hat: acidic oxide + base → salt. When a question asks "suggest why calcium carbonate is used in the chimneys", the answer is simply that SO₂ is acidic and CaCO₃ is a base that neutralises it.

India's Bharat Stage VI vehicle standards, introduced in 2020, made catalytic converters with tighter performance compulsory. A converter contains a ceramic honeycomb coated with platinum and rhodium. Exhaust entering it contains carbon monoxide and nitrogen monoxide; the gas leaving contains carbon dioxide and nitrogen.

Write the equation for what happens inside, identify what is oxidised and what is reduced, and explain why the honeycomb shape is used.

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The Redox Swap

2CO + 2NO → 2CO₂ + N₂. The CO gains oxygen — it is oxidised; the NO loses oxygen — it is reduced. Two pollutants annihilate each other: the toxic gas and the acid-rain gas leave as carbon dioxide and the ordinary main component of air.

Why a Honeycomb?

The reaction happens on the surface of the catalyst. A honeycomb gives an enormous surface area in a small can, so more gas molecules react per second — and the expensive platinum is spread as a microscopically thin coating rather than solid metal.

Chemistry Connection

One caveat examiners adore: the converter fixes CO, NOₓ and unburnt hydrocarbons, but its output CO₂ is still a greenhouse gas. Catalytic converters solve toxic pollution and acid rain — they do nothing for climate change. Only burning less fuel does that.

Himalayan glaciers that feed the Ganga and Brahmaputra are retreating, and IPCC projections show sea level rise threatening low-lying Bangladesh and cities such as Mumbai and Kolkata. Meanwhile atmospheric CO₂ has climbed from about 0.028% before the industrial revolution to over 0.042% today, and methane from the world's billion-plus cattle and vast paddy fields is rising too.

Connect the gas measurements to the melting glaciers using the greenhouse mechanism, and give two strategies nations use to slow the process.

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From Percentages to Meltwater

CO₂ (from burning fossil fuels) and CH₄ (from livestock digestion and decomposing vegetation in paddies) both absorb the thermal energy radiated by the Earth's surface and re-emit it in all directions, reducing the energy lost to space. More of these gases → more trapped energy → rising global temperatures → glaciers and ice caps melt, and warming seas expand → sea levels rise.

The Counter-Strategies

Cut CO₂ production: replace fossil fuel power with renewables (India's huge solar programme) and electrify transport. Remove CO₂ already there: plant trees, which take in CO₂ by photosynthesis. Cut methane: reduce livestock numbers, manage paddy water, capture landfill gas.

Chemistry Connection

Note the scale trap: CO₂ is only 0.04% of the air, yet it drives global climate. "Small percentage" never means "small effect" — the same logic as trace CO being lethal. Examiners reward answers that treat the numbers, not the vibes.

A reforestation charity plants millions of native trees across degraded land in Karnataka, advertising each tree as "a machine that eats carbon dioxide". A skeptical donor asks exactly how a tree removes CO₂ from the air and where the carbon actually goes.

Answer the donor with the photosynthesis equation, its conditions, and the fate of the carbon.

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The Equation and Its Conditions

6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂ (carbon dioxide + water → glucose + oxygen), requiring light energy absorbed by chlorophyll. Six molecules of CO₂ are pulled from the air for every glucose made.

Where the Carbon Goes

The glucose is the tree's building material and fuel: converted to cellulose and other compounds, the carbon is locked into wood for the tree's lifetime. A growing forest is therefore a carbon store — though burning or rotting the wood releases the CO₂ again, which is why protecting forests matters as much as planting them.

Chemistry Connection

"Plant trees" only earns its exam mark with the mechanism attached: photosynthesis removes CO₂ from the atmosphere. And keep the direction straight — photosynthesis consumes CO₂; respiration and combustion produce it. One equation, run forwards or backwards, is the entire carbon story.

Practice Questions: 10.3

20 multiple choice questions. Click an option to check your answer.

Your Score 0 / 20

Question 1

The approximate composition of clean, dry air is

A 78% oxygen, 21% nitrogen, 1% other gases

B 78% nitrogen, 21% oxygen, about 1% argon and other gases

C 78% nitrogen, 21% carbon dioxide, 1% oxygen

D 70% nitrogen, 20% oxygen, 10% water vapour

78% N₂, 21% O₂, and the rest mostly argon with ≈0.04% CO₂. A swaps the two main gases; water vapour is excluded because the definition is dry air.

Question 2

After nitrogen and oxygen, the most abundant gas in clean dry air is

A carbon dioxide

B argon

C methane

D hydrogen

Argon makes up nearly all of the remaining ≈1%. CO₂ is only about 0.04% — choosing it is the classic error this question exists to catch.

Question 3

Carbon monoxide is formed when carbon-containing fuels undergo

A complete combustion in excess oxygen

B incomplete combustion in a limited supply of oxygen

C thermal decomposition without air

D reaction with nitrogen at high temperature

Limited oxygen → incomplete combustion → CO (and particulates). Excess oxygen gives complete combustion and CO₂. The phrase "limited supply of oxygen" is the mark-carrying wording.

Question 4

Carbon monoxide is toxic because it

A damages the lining of the lungs directly

B dissolves in the blood to form an acid

C binds to haemoglobin, preventing the blood from carrying oxygen

D is a greenhouse gas

CO attaches to haemoglobin in red blood cells more strongly than oxygen does, so oxygen transport fails. It is doubly dangerous for being colourless and odourless.

Question 5

Which pair are both greenhouse gases contributing to climate change?

A Sulfur dioxide and carbon dioxide

B Carbon monoxide and nitrogen

C Carbon dioxide and methane

D Oxides of nitrogen and sulfur dioxide

The greenhouse pair is CO₂ and CH₄. D is the acid rain pair — keeping these two lists separate is the most important discipline in 10.3.

Question 6

A source of atmospheric methane is

A complete combustion of petrol

B digestion in cattle and decomposition of vegetation

C car engines running at high temperature

D flue gas desulfurisation

Methane comes from livestock digestion and decomposing vegetation (paddy fields, landfill). C is the source of NOₓ, A of CO₂.

Question 7

Oxides of nitrogen form in car engines because

A petrol contains nitrogen compounds that burn

B nitrogen and oxygen from the air react at the high engine temperature

C the catalytic converter produces them

D nitrogen reacts with the fuel to form ammonia

The nitrogen comes from the air drawn into the engine, not the fuel. Only at the engine's very high temperature will the normally unreactive N₂ combine with O₂.

Question 8

Which two pollutants are responsible for acid rain?

A Carbon dioxide and methane

B Carbon monoxide and particulates

C Sulfur dioxide and oxides of nitrogen

D Methane and carbon monoxide

SO₂ and NOₓ dissolve in atmospheric water and oxidise to sulfuric and nitric acids. A is the greenhouse pair — the mirror image of Question 5's trap.

Question 9

Sulfur dioxide enters the atmosphere mainly from

A the combustion of fossil fuels containing sulfur compounds

B the digestion of food by livestock

C reaction of sulfur with nitrogen in engines

D photosynthesis in green plants

Coal and some oils contain sulfur compounds; burning them oxidises the sulfur to SO₂. Hence the two remedies: low-sulfur fuels and flue gas desulfurisation.

Question 10

Particulates in the air are linked to

A acid rain and building damage

B respiratory problems and an increased risk of cancer

C global warming only

D the water becoming deoxygenated

The soot particles from incomplete combustion lodge in the lungs: respiratory problems and cancer are the two syllabus effects.

Question 11

The equation for the reaction inside a catalytic converter is

A 2CO + 2NO → 2CO₂ + 2N₂

B 2CO + 2NO → 2CO₂ + N₂

C CO + NO → CO₂ + N₂

D 2CO₂ + N₂ → 2CO + 2NO

Count atoms: 2C, 2N, 4O on each side — only B balances. A has too much nitrogen, C too little oxygen on the left, and D runs backwards, manufacturing pollutants.

Question 12

In the catalytic converter reaction 2CO + 2NO → 2CO₂ + N₂, the nitrogen monoxide is

A oxidised, because it gains oxygen

B reduced, because it loses oxygen

C neutralised, because it is acidic

D unchanged, because it is a catalyst

NO loses its oxygen (to the CO) and becomes N₂ — reduction. Simultaneously CO gains oxygen and is oxidised. The catalyst is the platinum/rhodium surface, not the gases.

Question 13

Flue gas desulfurisation removes sulfur dioxide from power station emissions using

A sodium chloride or potassium chloride

B calcium oxide or calcium carbonate

C platinum and rhodium catalysts

D concentrated sulfuric acid

SO₂ is an acidic oxide, so it is neutralised by the bases CaO (lime) or CaCO₃ (limestone). C belongs to catalytic converters — a different strategy for a different gas.

Question 14

Which statement correctly describes how greenhouse gases warm the atmosphere?

A They react with oxygen, releasing heat energy

B They destroy the ozone layer, letting more sunlight in

C They absorb thermal energy radiated from the Earth and re-emit it in all directions, reducing the energy lost to space

D They reflect the Sun's rays back into space

C is the full mechanism in the syllabus wording. B confuses the greenhouse effect with the ozone hole — a different problem with different chemistry. D would cool the planet.

Question 15

Which is a likely consequence of climate change?

A The proportion of argon in the air will fall sharply

B Melting ice caps, rising sea levels and more extreme weather

C Rain becoming strongly acidic everywhere

D Carbon monoxide levels rising in bedrooms

Warming melts ice caps and glaciers and expands the oceans, raising sea levels, with more frequent extreme weather. C describes acid rain — caused by different gases entirely.

Question 16

Planting trees helps reduce climate change because trees

A release oxygen, which destroys methane

B remove carbon dioxide from the atmosphere by photosynthesis

C absorb sulfur dioxide through their roots

D provide shade that cools the ground

Photosynthesis (6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂) consumes CO₂ and locks the carbon into glucose and wood. The mark is for naming the process and the gas removed.

Question 17

Which is the correctly balanced equation for photosynthesis?

A C₆H₁₂O₆ + 6O₂ → 6CO₂ + 6H₂O

B 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂

C 6CO₂ + 6H₂O → C₆H₁₂O₆ + 3O₂

D CO₂ + H₂O → C₆H₁₂O₆ + O₂

B is balanced (6C, 12H, 18O each side) and runs in the photosynthesis direction: CO₂ consumed, using light energy and chlorophyll. A is respiration — the same equation reversed.

Question 18

Photosynthesis requires which two conditions?

A High temperature and an iron catalyst

B Darkness and moisture

C Chlorophyll and light energy

D Oxygen and warmth

Chlorophyll traps the light energy that drives the reaction. Both must be stated; A describes the Haber process conditions, a mischievous distractor.

Question 19

Photochemical smog and respiratory problems in cities are particularly associated with

A argon from the air

B carbon dioxide from respiration

C oxides of nitrogen from vehicle engines

D methane from cattle

NOₓ has three listed effects: acid rain, photochemical smog and respiratory problems. Smog needs sunlight acting on NOₓ — hence "photochemical".

Question 20

Which strategy reduces the pollutant it is paired with?

A Catalytic converters — carbon dioxide

B Flue gas desulfurisation — methane

C Using low-sulfur fuels — sulfur dioxide

D Planting trees — carbon monoxide

Less sulfur in the fuel means less SO₂ on burning. Catalytic converters produce CO₂ (A); FGD targets SO₂, not methane; trees absorb CO₂, not CO.