Overview
What you’ll learn
- Describe tests for water and for its purity, and explain why distilled water is used in practical work.
- List what natural water may contain, which contents are beneficial or harmful, and how domestic water is treated.
- Explain the use of ammonium salts, nitrates and NPK fertilisers in agriculture.
- State the composition of clean air, the sources and effects of each pollutant, and strategies to reduce them.
- Describe photosynthesis and explain how greenhouse gases warm the atmosphere (Supplement).
Syllabus checklist (0620)
Statements are paraphrased and grouped by tier. Core is studied by everyone; Supplement is examined on the Extended papers only.
Core — every candidate
- 10.1Describe chemical tests for water using anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate.
- 10.1Describe how to test the purity of water using melting point and boiling point.
- 10.1Explain that distilled water is used in practical chemistry because it has fewer chemical impurities than tap water.
- 10.1State that natural water may contain dissolved oxygen, metal compounds, plastics, sewage, harmful microbes, nitrates from fertilisers, and phosphates from fertilisers and detergents.
- 10.1State that some of these substances are beneficial: dissolved oxygen for aquatic life, and some metal compounds as essential minerals.
- 10.1State that some are potentially harmful: toxic metal compounds, plastics harming aquatic life, sewage carrying disease-causing microbes, and nitrates and phosphates causing deoxygenation that damages aquatic life.
- 10.1Describe domestic water treatment: sedimentation and filtration to remove solids, use of carbon to remove tastes and odours, and chlorination to kill microbes.
- 10.2State that ammonium salts and nitrates are used as fertilisers.
- 10.2Describe the use of NPK fertilisers to supply nitrogen, phosphorus and potassium for improved plant growth.
- 10.3State the composition of clean, dry air: approximately 78% nitrogen, 21% oxygen, and the remainder a mixture of noble gases and carbon dioxide.
- 10.3State the sources of air pollutants: carbon dioxide from the complete combustion of carbon-containing fuels; carbon monoxide and particulates from incomplete combustion; methane from decomposing vegetation and animal digestion; oxides of nitrogen from car engines; sulfur dioxide from burning fossil fuels that contain sulfur compounds.
- 10.3State the adverse effects of these pollutants: carbon dioxide and methane raise global warming and drive climate change; carbon monoxide is toxic; particulates increase the risk of respiratory problems and cancer; oxides of nitrogen cause acid rain, photochemical smog and respiratory problems; sulfur dioxide causes acid rain.
- 10.3State and explain strategies to reduce the effects of these environmental issues: planting trees, reducing livestock farming, using less fossil fuel and using more hydrogen and renewable energy for climate change; catalytic converters, low-sulfur fuels and flue-gas desulfurisation with calcium oxide for acid rain.
- 10.3Describe photosynthesis as the reaction of carbon dioxide and water to produce glucose and oxygen, using light energy absorbed by chlorophyll, and state the word equation.
Supplement — Extended candidates only
- 10.3Describe how carbon dioxide and methane cause global warming in terms of the absorption, reflection and emission of thermal energy, and the reduced loss of thermal energy to space.
- 10.3Explain how oxides of nitrogen form in car engines and how catalytic converters remove them, for example 2CO + 2NO → 2CO₂ + N₂.
- 10.3State the symbol equation for photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
In 30 seconds
- 1Anhydrous copper(II) sulfate turns white → blue and anhydrous cobalt(II) chloride turns blue → pink in water — presence tests, not purity tests.
- 2Purity is judged from melting and boiling points: pure water boils at exactly 100 °C.
- 3Domestic water treatment: sedimentation → filtration → carbon → chlorination.
- 4NPK fertilisers supply nitrogen, phosphorus and potassium for plant growth; ammonium salts and nitrates provide the nitrogen.
- 5Clean, dry air is about 78% nitrogen and 21% oxygen, with noble gases and carbon dioxide in the remainder.
- 6Carbon dioxide and methane absorb and re-emit thermal energy, reducing heat loss to space (Supplement); photosynthesis removes CO₂: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
Notes · 1 of 4
Water in the environment and water treatment
Natural water is a solution of many things — some helpful, some harmful — and treatment makes it safe.
Natural water is never just H₂O. It may carry dissolved oxygen, metal compounds, plastics, sewage with harmful microbes, and nitrates and phosphates washed off farmland and out of detergents.
| Substance | Effect |
|---|---|
| Dissolved oxygen | Beneficial — aquatic life breathes it |
| Some metal compounds | Beneficial — provide essential minerals |
| Toxic metal compounds | Harmful — poisonous to life |
| Plastics | Harmful — injure and kill aquatic life |
| Sewage and its microbes | Harmful — spread disease |
| Nitrates and phosphates | Harmful — cause deoxygenation that kills aquatic life |
Domestic water treatment runs in stages. Sedimentation lets large solids settle, then filtration removes the remaining insoluble particles. Carbon beds adsorb dissolved substances that cause tastes and odours. Finally chlorination kills the microbes that would otherwise spread disease.
Notes · 2 of 4
Testing for water and for its purity
Two anhydrous salts prove water is present; melting and boiling points prove it is pure.
| Test | Result with water |
|---|---|
| Anhydrous copper(II) sulfate (white) | Turns blue |
| Anhydrous cobalt(II) chloride (blue) | Turns pink |
Both tests show that a liquid contains water — they do not show that the water is pure. A colour change in salty water would be just as positive. Purity comes from physical data instead: a pure substance melts and boils at single fixed temperatures, so pure water boils at exactly 100 °C (at standard pressure). A sample that boils over a range, or above or below 100 °C, contains dissolved impurities.
This is also why distilled water is used in practical chemistry: distillation removes the dissolved chemicals that tap water carries, giving water with far fewer chemical impurities that would otherwise interfere with tests and reactions.
Notes · 3 of 4
Fertilisers and plant nutrition
Ammonium salts and nitrates feed crops; NPK supplies the three key elements.
Plants need nitrogen, phosphorus and potassium for healthy growth, with nitrogen built into their proteins. Intensive farming strips these elements from the soil, so farmers replace them with fertilisers: ammonium salts (such as ammonium nitrate and ammonium sulfate) and nitrates supply nitrogen directly.
NPK fertilisers are blends that supply all three elements at once — N for nitrogen, P for phosphorus, K for potassium — giving balanced, improved plant growth in one application.
- Nitrogen (from ammonium salts and nitrates) — leafy growth and proteins.
- Phosphorus — healthy roots.
- Potassium — disease resistance and overall vigour.
Notes · 4 of 4
Air quality, climate and photosynthesis
Know the air’s composition, where every pollutant comes from, and how to fix them.
Clean, dry air is approximately 78% nitrogen and 21% oxygen; the remaining 1% is a mixture of noble gases with a trace of carbon dioxide.
| Pollutant | Source | Adverse effect |
|---|---|---|
| Carbon dioxide | Complete combustion of carbon-containing fuels | Global warming and climate change |
| Carbon monoxide | Incomplete combustion | Toxic |
| Particulates | Incomplete combustion | Respiratory problems; increased risk of some cancers |
| Methane | Decomposing vegetation; animal digestion | Global warming and climate change |
| Oxides of nitrogen | Car engines | Acid rain, photochemical smog, respiratory problems |
| Sulfur dioxide | Fossil fuels containing sulfur compounds | Acid rain |
Strategies target each problem. Against climate change: plant more trees, reduce livestock farming, burn less fossil fuel, and switch to hydrogen and renewable energy. Against acid rain: fit catalytic converters to engines, use low-sulfur fuels, and clean power-station waste gases by flue-gas desulfurisation with calcium oxide.
Supplement: greenhouse gases warm the planet because molecules of CO₂ and methane absorb thermal energy radiated by the Earth’s surface, reflect some of it back towards the Earth, and re-emit it in all directions — so less thermal energy escapes to space and the average temperature rises. Supplement: nitrogen and oxygen, unreactive at room temperature, combine inside hot car engines; the oxides of nitrogen formed are later removed by catalytic converters, for example 2CO + 2NO → 2CO₂ + N₂.
Photosynthesis runs the cycle the other way. Green plants use light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen:
Revise
Key terms
The words examiners expect you to use precisely.
- Chlorination
- Adding chlorine to water to kill harmful microbes during treatment.
- Sedimentation
- Letting suspended solids settle out of water under gravity before filtration.
- Anhydrous copper(II) sulfate
- The white salt that turns blue in water — a chemical test for the presence of water.
- Anhydrous cobalt(II) chloride
- The blue salt that turns pink in water — a second chemical test for water.
- NPK fertiliser
- A fertiliser formulated to supply the three essential plant elements: nitrogen, phosphorus and potassium.
- Global warming
- The rise in average temperature caused by greenhouse gases reducing the escape of thermal energy to space.
Show 5 more key termsShow fewer key terms
- Acid rain
- Rain made acidic by dissolved oxides of nitrogen and sulfur dioxide from human activities.
- Catalytic converter
- A device in vehicle exhausts that converts toxic gases such as carbon monoxide and nitrogen oxides into less harmful ones.
- Flue-gas desulfurisation
- Removing sulfur dioxide from power-station waste gases, for example by reaction with calcium oxide.
- Photosynthesis
- The reaction of carbon dioxide with water to make glucose and oxygen, powered by light energy absorbed by chlorophyll.
- Incomplete combustion
- Burning with a limited oxygen supply, producing carbon monoxide and particulates instead of only carbon dioxide.
Revise
Common mistakes
Where students most often lose marks in this topic — and what to do instead.
Mistake: Claiming the copper sulfate test proves water is pure.
The colour change only detects water; dissolved salts do not stop it.
Instead: Purity is shown by sharp, fixed melting and boiling points, not by the chemical test.
Mistake: Quoting air composition as 21% nitrogen and 78% oxygen.
The two figures are swapped.
Instead: About 78% nitrogen, 21% oxygen, remainder noble gases and carbon dioxide.
Show 8 more mistakesShow fewer mistakes
Mistake: Saying carbon monoxide comes from complete combustion.
Complete combustion gives carbon dioxide; only incomplete combustion makes CO.
Instead: Limited oxygen supply produces carbon monoxide and particulates.
Mistake: Listing sulfur dioxide as a greenhouse gas.
SO₂ causes acid rain, not global warming.
Instead: The greenhouse gases in this topic are carbon dioxide and methane.
Mistake: Saying chlorination removes solids or tastes.
Chlorine kills microbes; it does not filter anything.
Instead: Sedimentation and filtration remove solids; carbon removes tastes and odours; chlorination kills microbes.
Mistake: Forgetting light and chlorophyll in the photosynthesis equation.
The equation alone hides the energy source of the reaction.
Instead: Photosynthesis needs light energy absorbed by chlorophyll to convert carbon dioxide and water into glucose and oxygen.
Mistake: Listing methane as a product of burning fuels.
Combustion of fuels releases carbon dioxide, carbon monoxide and particulates — methane is not a burning product.
Instead: Methane comes from decomposing vegetation and from animal digestion, such as cattle.
Mistake: Mixing up where catalytic converters and flue-gas desulfurisation are used.
They are fitted to different kinds of plant, and each targets different gases.
Instead: Catalytic converters treat car exhausts; calcium oxide desulfurisation treats power-station flue gas.
Mistake: Claiming any sample that reaches 100 °C must be pure water.
Impure water passes through 100 °C on its way up; what matters is whether the temperature holds sharp and steady.
Instead: Purity is shown by a single, constant boiling temperature at standard pressure (and a sharp melting point).
Mistake: Writing photosynthesis as carbon dioxide + water → glucose and leaving out oxygen.
Oxygen is the second product of the reaction, and omitting it unbalances the chemistry.
Instead: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, with light energy absorbed by chlorophyll.
Check yourself
Quick quiz
18 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
- Question 2: not answered
- Question 3: not answered
State the composition of air
Clean, dry air contains approximately
Ready for more practice on Environment?
Generate a fresh 5-question adaptive drill grounded in this topic’s 0620 key points. Free with your study account.
Practise
Topical questions
10 exam-style questions with model answers and mark schemes.
A set of structured and calculation questions matched to this topic, in three tiers, each with a model answer, mark scheme and examiner tip.
Practise topical questionsExam practice
Practice paper
A 30-mark paper set out like a Cambridge paper, with a separate mark scheme. Give yourself 40 min.
Question paper
Topic 10 Practice Paper
30 marks · 40 min · 6 pages
10 multiple-choice questions and structured questions with answer lines. Topic 10: Chemistry of the environment.
Mark scheme
PROTopic 10 Practice Paper — mark scheme
Answers · Marking points · 3 pages
Model answers with a mark for each creditworthy point — for marking at home or in class.
Which paper covers this topic?
Paper 1 / 2
Multiple choice (45 min, 40 marks)
Core and Extended versions
Paper 3 / 4
Theory, short and structured (1 h 15, 80 marks)
Core and Extended versions
Paper 5 / 6
Practical test or Alternative to Practical (40 marks)
Experimental skills
Want a full paper? Try the Paper 2-style multiple choice, Paper 4-style structured or Paper 6-style practical sets.
Questions
Frequently asked questions
How do you test for water, and how do you test whether it is pure?
Presence: anhydrous copper(II) sulfate turns white to blue, or anhydrous cobalt(II) chloride turns blue to pink. Purity: a pure substance melts and boils at sharp, fixed temperatures — pure water boils at exactly 100 °C.
What are the stages of domestic water treatment?
Sedimentation settles the large solids, filtration removes the smaller insoluble particles, carbon removes tastes and odours, and chlorination kills harmful microbes.
What is in clean, dry air?
Approximately 78% nitrogen, 21% oxygen, and the remaining 1% a mixture of noble gases with a small amount of carbon dioxide.
Which gases cause global warming and how?
Carbon dioxide and methane. Their molecules absorb thermal energy rising from the surface, reflect part of it back and re-emit it, so less heat escapes to space and the planet warms.
What do NPK fertilisers contain and why are they used?
They blend compounds of nitrogen, phosphorus and potassium — the three elements crops need most. Harvesting removes these minerals from the soil, so farmers replace them to keep plants growing well.
Up next
Topic 11: Organic chemistry