4 high-yield, syllabus-aligned questions on Environmental chemistry, covering 14 individually creditable mark points. Each one shows the answer that scores full marks under our guide, which phrase earns which mark, and a common incomplete answer — so you can see the difference rather than guess at it.
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Every question and solution is reviewed by examiners versed in WAEC and JAMB, each with more than thirty years of experience.
Mark allocations are TopMarks’ own, written against the published syllabus and Chief Examiners’ reports. They are a guide to how these answers are usually credited, not an official marking scheme.
Define4 marks
Define air pollution and name three common air pollutants, giving the source of each.
TopMarks model answer
Air pollution is the contamination of the atmosphere by substances in quantities that are harmful to living organisms or to the environment. Carbon(II) oxide comes from the incomplete combustion of fuels in vehicle engines. Sulphur(IV) oxide comes from the burning of sulphur-containing fossil fuels and from the roasting of sulphide ores. Oxides of nitrogen come from vehicle exhausts and from lightning discharges.
Suggested marking guide
1Definition of air pollution
2Carbon(II) oxide and its source
3Sulphur(IV) oxide and its source
4Oxides of nitrogen and their source
Why it scores
Each pollutant is only worth its mark when the SOURCE comes with it. Naming three gases and no sources scores one mark for the definition and nothing else — the question asked for pairs, and the mark scheme is built out of pairs.
A common incomplete answer
“Air pollution is the contamination of the atmosphere with harmful substances such as carbon monoxide and sulphur(IV) oxide.”
What it costs: Three marks of four. The definition is credited, but the pollutants are given without their sources, so neither pollutant mark is complete.
Key ideas to include: contamination, carbon(II) oxide, incomplete combustion, sulphur(IV) oxide, oxides of nitrogen.
Explain4 marks
Explain how acid rain is formed and state two of its effects.
TopMarks model answer
Sulphur(IV) oxide and oxides of nitrogen released from the burning of fossil fuels dissolve in the water vapour of the atmosphere. They are oxidised and react with the water to form tetraoxosulphate(VI) acid and trioxonitrate(V) acid, which fall to the ground as acid rain. Acid rain lowers the pH of lakes and rivers, killing fish and other aquatic life. It also corrodes metal structures and dissolves the limestone and marble of buildings, and it leaches nutrients from the soil, damaging crops and forests.
Suggested marking guide
1The acidic oxides and their origin
2They dissolve in water to form acids
3Effect on aquatic life
4Effect on buildings, metals or soil
Why it scores
Half the marks are for the mechanism and half for the effects, so an answer that explains the chemistry beautifully and stops has capped itself at two out of four. When a question has two clauses joined by “and”, split your answer into two paragraphs and give each one its share.
A common incomplete answer
“Acid rain forms when gases dissolve in rain water, and it damages buildings.”
What it costs: Two marks of four. The dissolution and the effect on buildings are credited, but the gases are never named or traced to fossil fuels, and only one effect is given.
Key ideas to include: sulphur(IV) oxide, oxides of nitrogen, tetraoxosulphate(VI) acid, pH, leaching.
Explain3 marks
Explain the greenhouse effect and name two greenhouse gases.
TopMarks model answer
Short-wave radiation from the sun passes through the atmosphere and warms the earth’s surface. The earth re-radiates this energy as long-wave infra-red radiation, which greenhouse gases in the atmosphere absorb and re-emit in all directions, much of it back towards the surface rather than escaping into space. The trapped energy raises the average temperature of the earth. Carbon(IV) oxide and methane are two greenhouse gases.
The mechanism turns on a change of WAVELENGTH: short-wave radiation passes in freely, while the long-wave radiation the earth re-radiates is absorbed by greenhouse gases and re-emitted in all directions — much of it back towards the surface. Some still escapes to space, so the effect is to SLOW the loss of energy rather than to seal it in. An answer that says only “the gases trap heat” has described the outcome without the mechanism, and the first mark depends on the mechanism.
A common incomplete answer
“Greenhouse gases such as carbon dioxide and methane trap heat and stop it escaping into space.”
What it costs: One mark of three. The trapping and the two named gases are credited, but nothing explains the change from short-wave to long-wave radiation, which is why the trapping happens at all.
Key ideas to include: short-wave radiation, long-wave infra-red, re-radiate, absorb, global warming.
State3 marks
State three methods used to treat water to make it fit for drinking.
TopMarks model answer
Sedimentation and coagulation with alum, in which the suspended particles clump together and settle out. Filtration through beds of sand and gravel, which removes the remaining fine suspended matter. Chlorination, in which a controlled amount of chlorine is added to kill bacteria and other pathogens; the water may also be aerated to improve its taste.
Suggested marking guide
1Sedimentation or coagulation with alum
2Filtration
3Chlorination
Why it scores
The three methods do three different jobs: settling removes the big particles, filtration removes the fine ones, chlorination kills what is left alive. Presenting them in that order shows the examiner you understand the sequence rather than reciting a list.
A common incomplete answer
“The water is filtered to make it clean.”
What it costs: Two marks of three. Filtration is credited, but sedimentation and, more importantly, chlorination — the only step that makes the water safe to drink — are missing.
Key ideas to include: sedimentation, coagulation, alum, filtration, chlorination, aeration.
Practise Environmental chemistry on real questions
Reading a full-mark answer is the first half. Writing one under time is the other. A free account opens exam-standard practice in Chemistry with the full solution on every question.