Chemistry of the Environment: Air, Water and Fertilisers

Three applied topics: what is in the air and what pollutes it, how water is treated and tested, and how fertilisers work.

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1. The composition of clean, dry air

Nitrogen ≈ 78% Oxygen ≈ 21% Argon (noble gases) ≈ 1% Carbon dioxide ≈ 0.04%

Plus water vapour, which varies.

Nitrogen is the most abundant — remember 78/21 as the headline pair.

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2. Air pollutants

Pollutant	Source	Problem
Carbon monoxide, CO	incomplete combustion of fuels	TOXIC — binds to haemoglobin, preventing oxygen transport
Sulfur dioxide, SO₂	sulfur impurities in fossil fuels	ACID RAIN
Oxides of nitrogen, NOₓ	nitrogen and oxygen reacting in hot car engines	acid rain, photochemical smog, respiratory problems
Carbon dioxide, CO₂	complete combustion, respiration	greenhouse gas → climate change
Particulates (soot)	incomplete combustion	respiratory problems, blackens buildings
Methane, CH₄	livestock, decomposition	greenhouse gas

Carbon monoxide is TOXIC; carbon dioxide is a GREENHOUSE GAS. Keep the two roles apart — this is the single most confused pair in the topic.

Carbon monoxide is colourless and odourless, which is what makes it so dangerous.

Oxides of nitrogen form INSIDE the engine, from atmospheric nitrogen and oxygen at high temperature — they are not an impurity in the fuel.

Acid rain

Sulfur dioxide (and oxides of nitrogen) dissolve in rainwater to form ACIDS, lowering the rain’s pH.

Effects:

• Kills trees and damages leaves
• Acidifies lakes, killing fish
• Corrodes limestone and marble buildings, and metal structures
• Damages soil, leaching nutrients

Reducing it: remove sulfur from fuels before burning; use flue gas desulfurisation (spraying calcium oxide/carbonate to neutralise the SO₂); use catalytic converters.

Climate change

Greenhouse gases (CO₂, methane, water vapour) absorb and re-emit INFRARED radiation, trapping energy and raising the average global temperature — the enhanced greenhouse effect.

Consequences: melting ice caps, rising sea levels, extreme weather, changing habitats.

Reducing it: burn less fossil fuel, use renewable energy, plant trees, improve efficiency.

Catalytic converters

Fitted to car exhausts, they convert harmful gases into less harmful ones, using a platinum/rhodium catalyst with a honeycomb structure for a large surface area.

2CO + 2NO → 2CO₂ + N₂

This deals with two pollutants at once — carbon monoxide is oxidised and nitrogen monoxide is reduced.

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3. Water

Water treatment

1. FILTRATION (screening and sand filters) — removes solid particles and debris 2. CHLORINATION — kills bacteria and other microorganisms

Some supplies also have fluoride added to protect teeth.

Chlorine’s job is STERILISATION — killing bacteria. It does not remove dissolved solids.

Testing for water

Anhydrous copper(II) sulfate: WHITE → BLUE in the presence of water Anhydrous cobalt(II) chloride: BLUE → PINK

These tests show water is present, not that it is pure.

Testing for PURE water: it boils at exactly 100 °C and freezes at exactly 0 °C at normal pressure. Impurities change both.

Uses of water in industry

As a solvent, a coolant, and for cleaning.

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4. Fertilisers

Fertilisers replace the nutrients that plants remove from the soil, increasing crop yields.

The three essential elements:

N — NITROGEN — for protein synthesis, giving healthy leaf growth P — PHOSPHORUS — for root growth K — POTASSIUM — for flowers and fruit, and general resistance

NPK fertilisers contain all three, and the label gives the proportions.

Common fertilisers: ammonium nitrate (NH₄NO₃), ammonium sulfate, ammonium phosphate, potassium nitrate — all chosen because they are soluble, so plants can absorb them.

How they are made: ammonia from the Haber process is either used directly or neutralised with an acid:

ammonia + nitric acid → ammonium nitrate NH₃ + HNO₃ → NH₄NO₃

Ammonium salts release ammonia when warmed with an alkali — which is the test for the ammonium ion, and also why fertiliser should not be mixed with lime.

The environmental problem — eutrophication

Excess fertiliser is washed into rivers and lakes by rain.

The sequence — give it in order:

1. Nutrients cause algae to grow rapidly (an algal bloom) 2. The algae block sunlight, so plants below die 3. DECOMPOSING BACTERIA multiply and break down the dead material 4. The bacteria USE UP the dissolved OXYGEN 5. Fish and other aquatic life die from lack of oxygen

It is the DECOMPOSING BACTERIA that remove the oxygen — not the algae themselves. This is the step most often got wrong, and it is usually a separate mark.

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5. Mistakes that cost marks

Confusing carbon monoxide with carbon dioxide and their effects.

Saying nitrogen oxides come from fuel impurities — they form in the engine.

Naming acid rain’s cause as CO₂ rather than SO₂.

Saying chlorination removes solids.

Saying the cobalt chloride test proves water is pure — it only shows water is present.

Giving the wrong element for a plant function in NPK.

Saying algae use up the oxygen in eutrophication.

Giving an incomplete eutrophication sequence.

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Frequently asked questions

What is the composition of clean air? About 78% nitrogen, 21% oxygen, 1% argon and 0.04% carbon dioxide.

Which pollutant is toxic? Carbon monoxide — it binds to haemoglobin.

What causes acid rain? Sulfur dioxide and oxides of nitrogen dissolving in rainwater.

Where do nitrogen oxides come from? Nitrogen and oxygen reacting in hot car engines.

How do catalytic converters work? A platinum catalyst converts CO and NO into CO₂ and N₂.

What are the two stages of water treatment? Filtration to remove solids and chlorination to kill bacteria.

How do I test for water? Anhydrous copper sulfate white → blue, or cobalt chloride blue → pink.

How do I test that water is PURE? It boils at 100 °C and freezes at 0 °C.

What do N, P and K do for plants? Nitrogen for leaves/protein, phosphorus for roots, potassium for flowers and fruit.

What is eutrophication? Excess fertiliser causes algal blooms; plants die; decomposing bacteria use up the oxygen; fish die.

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Quick revision checklist

• I know the composition of air
• I can name each pollutant, its source and its problem
• I know CO is toxic and CO₂ is a greenhouse gas
• I know what causes acid rain and its effects
• I can explain the greenhouse effect
• I know how catalytic converters work, with the equation
• I know the two stages of water treatment
• I know both tests for water and the test for purity
• I know what N, P and K each do
• I know how ammonium nitrate is made
• I can give the eutrophication sequence in order
• I know it is the bacteria that deplete the oxygen

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These notes cover the chemistry of the environment in the Cambridge IGCSE Chemistry (0620) syllabus and are written for Grade 9–11 / Year 10–11 students. They are based on teaching patterns observed across a large set of one-to-one IGCSE Chemistry lessons. Water treatment and fertilisers were the least-covered subtopics in that set — only three and one lesson respectively — so those sections follow the syllabus closely rather than being drawn from the corpus. Always check the current syllabus for your own exam series.