Chemistry of the Environment: IGCSE Chemistry 0620

Chemistry of the Environment is short on content but long on marks-per-sentence: it supplies the 6-mark extended response more than any other topic except metals, because air pollution and climate change invite essay-style answers. Examiner reports are blunt about where candidates fail: vague green-talk (“pollution harms the planet”) instead of named pollutant, named source, named effect. Every scoring sentence in this topic has that three-part shape, which makes it one of the most coachable topics in 0620.

Water and water treatment

Two jobs here: test for water, then treat it.

Tests. Anhydrous copper(II) sulfate turns white to blue with water; anhydrous cobalt(II) chloride turns blue to pink. Either shows water is present. To show water is pure, check the boiling point: pure water boils at exactly 100°C. Distinguishing “present” from “pure” is a deliberate trap and a reliable mark.

Treatment. Domestic water treatment runs in stages, each with a purpose:

1. Sedimentation: large insoluble particles settle out.
2. Filtration: sand filters remove remaining insoluble solids.
3. Carbon (charcoal): removes tastes and odours.
4. Chlorination: chlorine kills microbes/bacteria.

State the stage and its purpose together; “filter it” without saying what filtration removes is half an answer. (S) Extended candidates also explain why distilled water, not tap water, is used in practical work: tap water contains dissolved salts that would interfere with results or leave residues. Water from natural sources also carries dissolved oxygen, which aquatic life depends on. Sewage and fertiliser run-off lower it.

Fertilisers (NPK)

NPK fertilisers supply the three elements plants need in quantity:

Element	Why plants need it
Nitrogen (N)	Making proteins; leaf growth
Phosphorus (P)	Root development
Potassium (K)	Flowering, fruiting and healthy growth

Ammonium salts and nitrates are the standard nitrogen carriers. Ammonium nitrate, NH4NO3, is the exam favourite because it carries nitrogen in both ions. This subtopic interlocks with acids, bases and salts: ammonium sulfate is made by neutralising sulfuric acid with ammonia solution, and a salt-preparation question can be dressed up as a fertiliser question. One more crossover trap: adding lime (calcium hydroxide) to soil at the same time as an ammonium fertiliser releases ammonia gas and wastes the nitrogen.

Air quality, pollutants and climate change

Clean, dry air is roughly 78% nitrogen and 21% oxygen, with small amounts of noble gases and carbon dioxide. Learn the pollutant table as source → effect pairs:

Pollutant	Source	Effect
Carbon monoxide, CO	Incomplete combustion of carbon-containing fuels	Toxic gas: binds to haemoglobin, preventing oxygen transport
Carbon dioxide, CO2	Complete combustion of fossil fuels	Greenhouse gas: climate change
Sulfur dioxide, SO2	Combustion of fossil fuels containing sulfur compounds	Acid rain
Oxides of nitrogen, NOx	Reaction of nitrogen and oxygen at high temperature in car engines	Acid rain; photochemical smog; respiratory problems
Particulates	Incomplete combustion	Increased risk of respiratory problems and cancer
Methane, CH4	Livestock farming; decomposition of vegetation	Greenhouse gas: climate change

Acid rain follows a chain you should be able to write: SO2 and NOx dissolve in rainwater to form acidic solutions, which corrode limestone buildings and metal structures, acidify lakes and damage plant life. The combustion chemistry behind CO and particulates is covered under fuels in organic chemistry.

The greenhouse mechanism, as 0620 phrases it. The Earth’s surface absorbs energy from the Sun and re-radiates it as infrared (thermal) radiation. Greenhouse gases (carbon dioxide and methane) absorb this radiation and re-radiate it in all directions, including back to the surface, so thermal energy is trapped in the atmosphere. Rising greenhouse gas levels therefore raise average global temperature: climate change. Two strategies appear in mark schemes for reducing it: cut fossil fuel combustion (renewable energy, fewer car journeys) and plant trees, because photosynthesis removes carbon dioxide:

6CO2 + 6H2O → C6H12O6 + 6O2 (in the presence of light and chlorophyll)

(S) Catalytic converters earn Extended marks: in a car exhaust, carbon monoxide and nitrogen monoxide react on the catalyst surface to give the harmless products carbon dioxide and nitrogen: 2CO + 2NO → 2CO2 + N2.

Worked exam question

Sulfur dioxide and oxides of nitrogen are released when fossil fuels burn. Explain how each pollutant forms and describe one environmental effect they share. [4]

Model answer: Sulfur dioxide forms because fossil fuels contain sulfur compounds, which burn/react with oxygen to form SO2 (1). Oxides of nitrogen form inside car engines, where the temperature is high enough for nitrogen and oxygen from the air to react (1). Both gases dissolve in rainwater to form acid rain (1), which corrodes limestone buildings and metalwork and acidifies lakes, harming aquatic life (1).

Mark-by-mark: M1 needs the sulfur to come from the fuel: “sulfur in the air burns” is a common wrong answer. M2 needs both the source of the nitrogen (the air) and the condition (high temperature in the engine); “cars produce NOx” alone is too vague. M3 is the named effect, acid rain. M4 is a specific consequence: one named damage type, precisely stated.

The mistakes that cost marks

1. Confusing carbon monoxide with carbon dioxide. CO is the toxic product of incomplete combustion; CO2 is the greenhouse gas from complete combustion. Swapping their effects is the single most common error in this topic.
2. “NOx comes from burning nitrogen in fuel.” The nitrogen comes from the air, reacting with oxygen at the high temperatures inside engines. The source-of-nitrogen mark depends on saying so.
3. Greenhouse answers about the ozone layer. Ozone depletion is a different issue and scores nothing here. The mechanism is absorption and re-radiation of infrared by CO2 and methane.
4. Water tests that claim purity. Anhydrous copper(II) sulfate turning blue shows water is present, not pure. Purity needs the boiling point at exactly 100°C.
5. Treatment stages without purposes. Listing “sedimentation, filtration, chlorination” is half marks at best. Chlorination kills microbes; carbon removes tastes and odours. Pair every stage with its job.

How to phrase it for full marks

Student wording	Mark-scheme wording
”Pollution damages the environment"	"Sulfur dioxide dissolves in rainwater forming acid rain, which corrodes limestone buildings"
"CO2 traps heat"	"Carbon dioxide absorbs infrared radiation from the Earth’s surface and re-radiates it back, trapping thermal energy"
"Carbon monoxide is poisonous"	"Carbon monoxide is toxic because it binds to haemoglobin and prevents oxygen being carried"
"Chlorine cleans the water"	"Chlorine is added to kill microbes"
"Fertilisers help plants grow"	"Nitrogen is needed to make proteins and promotes leaf growth”

The pattern: pollutant, source, effect: three nouns per sentence, no green slogans. That structure is exactly what carries a Level 3 answer in the 6-mark extended response technique.

The Malaysia note

This topic plays well for Malaysian students because the contexts are local: haze episodes from biomass burning are a textbook particulates case, palm-oil-belt fertiliser use makes NPK questions concrete, and anyone who has crossed the Klang Valley at rush hour understands NOx. The risk runs the other way: familiarity breeds vague answers, and “haze is bad for health” scores nothing without particulates and respiratory problems named. Schools in KL, Penang and JB usually teach this topic late in Year 11, close to the May/June exams, so it gets one pass and no revision. A free trial lesson with a Chemistry specialist will show you how to turn what you already know about Malaysian air into mark-scheme sentences.

Every sub-topic in Chemistry of the Environment

• Water and Water Treatment
• Fertilisers (NPK)
• Air Quality, Pollutants and Climate Change