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IGCSE Chemistry 0620 · Chapter 10 notes · ~12 min read

Chemistry of the environment

Safe drinking water, what fertilisers do (and the damage they cause), what is in the air, where pollutants come from — and the chemistry behind acid rain and climate change.

Water tests & treatmentFertilisersAir compositionPollutantsClimate changeAcid rain
RSby Riyaz Sir
mentorriyaz.in
No tag = Core (everyone)EXTENDED Extended onlyhighlight = comes up again and againTest yourself at the end

In 30 seconds

  • Tap water is made safe by screening, sedimentation, filtering and chlorination. It is not pure — distilled water is.
  • Fertilisers supply nitrogen (N), phosphorus (P) and potassium (K). Too much washes into rivers and causes eutrophication.
  • Dry air is about 78% nitrogen, 21% oxygen, 0.9% argon and 0.04% carbon dioxide.
  • CO₂ and methane cause global warming; SO₂ and nitrogen oxides cause acid rain. EXT explains how.

By the end you can

  • Describe tests for water and the difference between tap water and distilled water.
  • Describe how water is treated and the harm done by natural contaminants.
  • Describe NPK fertilisers and the problems of fertiliser run-off.
  • Give the composition of air, and the sources and effects of air pollutants.
  • Describe strategies against climate change and acid rain.
  • Explain the greenhouse effect and the role of catalytic converters. EXT

1Water

Testing for water and checking purity

  • Anhydrous copper(II) sulfate turns from white to blue when water is added. Cobalt(II) chloride paper turns from blue to pink. (See reversible reactions.)
  • These tests show that water is present — not that it is pure. Pure water melts at exactly 0 °C and boils at exactly 100 °C. Impurities change these values (see testing purity).

Distilled water — water that has been made pure by distillation. It contains no dissolved substances.

  • Distilled water is used in chemistry experiments and in car batteries, because dissolved salts would interfere.
  • Tap water has been treated to be safe to drink but still contains dissolved substances (calcium, magnesium and other salts, and chlorine).

What is in natural water?

ContaminantWhere fromEffect
dissolved oxygenthe airneeded by fish and other aquatic life (this is a useful substance)
harmful microbessewage and animal wastecause disease, e.g. cholera and typhoid
nitrates and phosphatesfertilisers, sewageeutrophication — see below
metal compounds (lead, mercury)industry, old pipespoisonous — damage the nervous system
plasticslitter, wasteharm and kill marine animals; break into microplastics

How water is made safe to drink

1 collectriver or reservoirwater2 screenremoves large objects(sticks, fish, litter)3 sedimentationsolids settle out;chemicals clumpfine particles4 filter bedssand and gravelremove small solids5 carbon filterremoves bad tastesand smells6 chlorinationchlorine killsbacteria and microbes→ safe drinking waterhow tap water is made safe to drinktap water still contains dissolved substances; distilled water is purer
The treatment of drinking water. Each stage removes something different. Chlorine kills microbes that remain.
  • Screening: metal grids remove large objects (sticks, fish, litter).
  • Sedimentation: the water is left in tanks so that solids settle. Chemicals such as aluminium sulfate make the fine particles clump together and sink.
  • Filtration: sand and gravel filter beds remove the smaller solid particles.
  • Carbon filter: activated carbon removes the bad tastes and smells.
  • Chlorination: a little chlorine is added to kill the bacteria and other microbes.
  • Distillation gives pure water but takes too much energy to treat large amounts. It is only used where fresh water is very scarce (for example, desalination in some coastal countries).
Common mistake"Chlorine makes the water pure." Chlorine only kills microbes. Tap water still contains dissolved substances.

2Fertilisers

Feeding the crops

rootsN nitrogenleaves and stems grow;makes proteinsP phosphorusroots grow wellK potassiumflowers and fruit;healthy plantNPK fertiliserse.g. NH₄NO₃ · (NH₄)₂SO₄ · KNO₃
The three main plant nutrients. Nitrogen helps leaves and stems grow, phosphorus helps roots, and potassium helps flowers and fruit and keeps the plant healthy.
  • Crops remove nitrogen, phosphorus and potassium from the soil. Fertilisers put them back, so yields are higher and a growing population can be fed.
  • NPK fertilisers contain all three elements (N, P and K).
  • Common fertilisers are ammonium salts and nitrates: ammonium nitrate NH₄NO₃, ammonium sulfate (NH₄)₂SO₄, potassium nitrate KNO₃. They are made by neutralising ammonia with an acid, for example NH₃ + HNO₃ → NH₄NO₃.
  • Plants can only absorb nutrients in a soluble form, so fertilisers must dissolve in water.
% of nitrogen = mass of N atoms in the formulaMr of the fertiliser × 100
how to compare fertilisers
Worked example

EXT Which has the greater percentage of nitrogen, ammonium nitrate NH₄NO₃ or ammonium sulfate (NH₄)₂SO₄?

  1. Mr(NH₄NO₃) = 14 + 4 + 14 + 48 = 80; N = 28 → 28 ÷ 80 × 100 = 35%
  2. Mr((NH₄)₂SO₄) = 132; N = 28 → 28 ÷ 132 × 100 = 21%
  3. ammonium nitrate has more nitrogen per kg

The problem of eutrophication

fertiliser washesinto a river or lake(nitrates, phosphates)algae grow veryfast: green“algal bloom”light is blocked:plants below thesurface diebacteria feed on thedead plants and useup the oxygenfish and otheranimals die fromlack of oxygenprevent it: use less fertiliser · do not spread it before rain · treat sewageeutrophication
Eutrophication. Fertiliser washes into a lake; algae grow wildly; dead plants rot; bacteria use up the oxygen; fish die.
  • Step 1: nitrates and phosphates from fertilisers wash into rivers and lakes.
  • Step 2: algae grow very fast (an algal bloom) and cover the surface.
  • Step 3: light is blocked, so water plants below die.
  • Step 4: bacteria feed on the dead plants and use up the oxygen. Fish and other animals die from lack of oxygen.
  • Prevention: use less fertiliser, do not spread it just before rain, treat sewage before release.
Common mistakeWriting that fish die because of "poison". They die from lack of dissolved oxygen.

3Air

What is in clean, dry air?

clean dry airnitrogen N₂ 78%O₂ 21%argon 0.9% · carbon dioxide 0.04%neon, helium, krypton, xenon: tiny amountsthe amount of water vapour varies with the weathernitrogen is unreactive, so most of the air does not take part in burning or rusting
Composition of clean, dry air. Nitrogen and oxygen make up about 99%. The amount of water vapour in real air varies.
GasPercentage
nitrogen78%
oxygen21%
argon0.9%
carbon dioxide0.04%
neon, helium, krypton, xenontiny amounts
  • Nitrogen is unreactive, so most of the air does not take part in burning or rusting.

Finding the percentage of oxygen

finding the percentage of oxygen in airsyringe A100 cm³ air at the startsyringe B≈ 79 cm³ air at the endhot copperpush the gas back and forth2Cu + O₂ → 2CuO (the copper turns black as it uses up the oxygen)100 − 79 = 21 cm³ of oxygen usedlet the apparatus cool to room temperature before reading the final volume
Passing air over hot copper between two gas syringes. The copper turns black as it uses up the oxygen, and the volume of gas falls by about one fifth.
  • Pass a known volume of air (100 cm³) over heated copper back and forth between two gas syringes. 2Cu + O₂ → 2CuO (the copper turns black).
  • Let the apparatus cool to room temperature before reading the final volume. About 79 cm³ remains, so about 21 cm³ was oxygen.
Worked example

120 cm³ of air is passed over hot copper until no further change. The final volume is 95 cm³. Find the percentage of oxygen in the air.

  1. volume of oxygen used = 120 − 95 = 25 cm³
  2. percentage = 25 ÷ 120 × 100 = 20.8% (about 21%)
Common mistakeReading the final volume while the apparatus is still hot. Hot gas takes up more room, so the volume that disappeared looks too small and the oxygen percentage comes out too low.

Air pollutants

PollutantMain sourceEffect
carbon monoxide COincomplete combustion of carbon fuels (car engines)toxic — stops blood carrying oxygen
sulfur dioxide SO₂burning fossil fuels that contain sulfur; volcanoesacid rain; breathing problems
nitrogen oxides NOₓcar engines (hot air) and lightningacid rain; breathing problems
unburnt hydrocarbonscar enginescontribute to smog
particulates (soot)incomplete combustion of fuelsbreathing problems
carbon dioxide CO₂burning fossil fuels, deforestationglobal warming
methane CH₄cattle, rice paddies, rotting waste in landfillglobal warming
  • Complete combustion: plenty of oxygen → CO₂ + H₂O. Incomplete combustion: not enough oxygen → carbon monoxide (and soot) + H₂O. For example: 2CH₄ + 3O₂ → 2CO + 4H₂O.
  • EXT Nitrogen oxides form in hot engines, where nitrogen and oxygen in the air react: N₂ + O₂ → 2NO (an endothermic reaction — it needs the high temperature; see bond energies).

Catalytic convertersEXTENDED

engineCO, NOx,unburnthydrocarbonsCO₂, N₂,H₂Ocatalyst: platinum, rhodiumhoneycomb: large surface area2CO + 2NO → 2CO₂ + N₂ (harmful gases become harmless ones)a catalyst speeds up the reaction without being used up
A catalytic converter. A honeycomb coated with platinum and rhodium turns harmful exhaust gases into harmless ones.
2CO + 2NO → 2CO₂ + N₂
carbon monoxide and nitrogen monoxide become carbon dioxide and nitrogen
  • The honeycomb has a large surface area. The catalyst (platinum and rhodium) speeds up the reaction without being used up.

Acid rain

burning fuelswith sulfurin themcar enginesand lightning(hot air)SO₂ andnitrogen oxides(NOx)dissolve inrain water:pH below 5.6kills treesand fishcorrodeslimestone andmetalsfix it: remove sulfur from fuels · flue gas desulfurisation (calcium oxidereacts with SO₂) · catalytic converters on cars · burn less fossil fuelacid rain
Where acid rain comes from, what it does and how it can be reduced.
  • Sources: SO₂ from burning fossil fuels containing sulfur; nitrogen oxides from car engines.
  • They dissolve in rain, so that it has a pH below 5.6.
  • Effects: kills trees and aquatic life (lakes become acidic), corrodes limestone buildings, statues and metals.
  • Solutions: burn low-sulfur fuel, flue gas desulfurisation (calcium oxide or calcium carbonate reacts with SO₂: CaO + SO₂ → CaSO₃), catalytic converters on cars, and burn less fossil fuel (renewable energy).
Common mistakeMixing acid rain up with the greenhouse effect. Acid rain is caused by SO₂ and NOₓ. Global warming is caused by CO₂ and CH₄.

4Climate change

The greenhouse effectEXTENDED

greenhousegasesEarth’s surfaceto space1 sunlight passesthrough and warmsthe Earth2 the Earth givesout infrared (heat)3 CO₂, CH₄ and watervapour absorb it andre-emit it in alldirections4 some returns to theEarth: it warms up
The greenhouse effect. Sunlight warms the Earth. The Earth gives out infrared. Greenhouse gases absorb some of it and re-emit it in all directions, and some returns to Earth.
  • The Sun's radiation passes through the atmosphere and warms the Earth's surface.
  • The warm Earth gives out infrared radiation (thermal energy).
  • Greenhouse gases (CO₂, CH₄ and water vapour) absorb some of this infrared and re-emit it in all directions. Some of it comes back to the Earth, which warms up further.
  • More CO₂ and CH₄ in the air → enhanced greenhouse effect → global warming and climate change: melting ice, rising sea levels, extreme weather, changed habitats.
  • CO₂ sources: burning fossil fuels, deforestation (fewer trees to take in CO₂), making cement. CH₄ sources: cattle, rice paddies and rotting waste.

What can we do?

  • Use renewable energy (wind, solar, hydroelectric) instead of fossil fuels.
  • Improve energy efficiency, and use public transport, so that less fuel is burned.
  • Plant trees (afforestation) and stop deforestation: trees take in CO₂ by photosynthesis.
  • Capture CO₂ from power stations and store it underground.

Photosynthesis

carbon dioxide + water → glucose + oxygen
word equation (needs light and chlorophyll)
EXT 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂
symbol equation
  • Photosynthesis takes CO₂ out of the air, and respiration and burning put it back. The balance keeps the amount of CO₂ steady — until humans burn fossil fuels and cut down forests. See plant nutrition.

5Test yourself

Test yourself — cover the answers first

Quick check

  1. What is the difference between tap water and distilled water?
  2. Give the order: sedimentation, chlorination, screening, filtration.
  3. What do the letters N, P and K stand for in fertilisers?
  4. Give the approximate percentages of nitrogen and oxygen in air.
  5. Name the pollutant that causes acid rain.
  6. Which gas is produced by incomplete combustion and is toxic?
  7. EXT What happens to infrared radiation in the greenhouse effect?
  8. EXT What does a catalytic converter do?

Exam-style

  1. Explain how fertilisers can cause fish to die in a lake. [4]
  2. Describe how water is treated for drinking. [4]
  3. EXT Explain how an increase in carbon dioxide can cause global warming. [3]
  4. Describe two ways to reduce acid rain. [2]
Show answers
  1. tap water contains dissolved salts and chlorine; distilled water is pure
  2. screening, sedimentation, filtration, chlorination
  3. nitrogen, phosphorus, potassium
  4. 78% nitrogen, 21% oxygen
  5. sulfur dioxide (also nitrogen oxides)
  6. carbon monoxide
  7. absorbed by greenhouse gases and re-emitted in all directions, some back to Earth
  8. converts CO and NOₓ into CO₂ and N₂
  9. nitrates and phosphates wash into the lake
    algae grow very quickly and block light so plants die
    bacteria feed on the dead plants and use up oxygen
    fish die from lack of oxygen
  10. screening to remove large objects
    sedimentation (with chemicals that clump particles)
    filtering through sand and gravel
    adding chlorine to kill bacteria
  11. the Earth emits infrared radiation
    CO₂ absorbs infrared radiation and re-emits it in all directions
    some returns to Earth, so the temperature increases
  12. remove sulfur from fuels / flue gas desulfurisation
    catalytic converters on cars / burn less fossil fuel
Practise this chapter
WaterFertilisersAir quality & climate
Next chapterOrganic chemistry →
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