Chemistry 0620 · syllabus 10
Cambridge IGCSE Chemistry 0620 — Chemistry of the environment topical past papers
Testing and treating water, NPK fertilisers, the composition of clean air, the sources and effects of air pollutants, and ways to limit climate change and acid rain.
- Questions
- 1,114
- Years
- 2009–2026
- With mark scheme
- 99%
- Questions 2022–2026
- 351
Anyone can browse these pages. Opening the questions and their mark schemes takes a free Momentum account.
What Chemistry of the environment covers
Anhydrous cobalt(II) chloride turns from blue to pink and anhydrous copper(II) sulfate from white to blue when water is present, but only melting and boiling points show that water is pure: 0 °C and 100 °C. Distilled water is used in practical work because it has fewer impurities than tap water. Water from natural sources can contain useful substances — dissolved oxygen for aquatic life, and metal compounds that provide essential minerals — and harmful ones: toxic metal compounds, plastics, sewage carrying harmful microbes, and nitrates and phosphates from fertilisers and detergents, which lead to deoxygenation of the water. Domestic water is treated by sedimentation and filtration to remove solids, carbon to remove tastes and odours, and chlorination to kill microbes.
Ammonium salts and nitrates are used as fertilisers, and NPK fertilisers supply nitrogen, phosphorus and potassium for plant growth. Clean, dry air is about 78% nitrogen and 21% oxygen, the rest being noble gases and carbon dioxide. Each pollutant has a source: carbon dioxide comes from the complete combustion of carbon-containing fuels; carbon monoxide and particulates from incomplete combustion; methane from decomposing vegetation and from animals' digestion; oxides of nitrogen from car engines; and sulfur dioxide from burning fossil fuels that contain sulfur compounds. Carbon dioxide and methane increase global warming, which leads to climate change; carbon monoxide is toxic; particulates increase the risk of respiratory problems and cancer; sulfur dioxide and oxides of nitrogen cause acid rain, and oxides of nitrogen also cause photochemical smog.
You also need the ways of reducing these effects: planting trees, less livestock farming, burning less fossil fuel and using more hydrogen and renewable energy against climate change; catalytic converters, low-sulfur fuels and flue gas desulfurisation with calcium oxide against acid rain. Photosynthesis, in which carbon dioxide and water form glucose and oxygen using light and chlorophyll, completes the Core content. On the Extended route you explain that oxides of nitrogen form when nitrogen and oxygen from the air react at the high temperatures in an engine, and that a catalytic converter removes them, as in 2CO + 2NO → 2CO₂ + N₂; that greenhouse gases absorb thermal energy given out by the Earth and emit it again, so less is lost to space; and you write photosynthesis as 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂.
In the syllabus
The headings this topic has in the Cambridge International 2026–2028 syllabus, as the board prints them.
- 10 Chemistry of the environment
- 10.1 Water
- 10.2 Fertilisers
- 10.3 Air quality and climate
What you should be able to do
- Describe the chemical tests for water and how melting and boiling points show whether water is pure
- Name substances found in natural water and say which are beneficial and which are harmful
- Describe the stages in treating the domestic water supply and the purpose of each
- State the composition of clean, dry air and the source and effect of each named pollutant
- State and explain ways of reducing climate change and acid rain, including catalytic converters and flue gas desulfurisation
- Explain how oxides of nitrogen form and are removed, and how greenhouse gases cause global warming (Extended)
Key ideas
- Anhydrous CoCl₂: blue to pink; anhydrous CuSO₄: white to blue, when water is present
- Clean, dry air: about 78% N₂ and 21% O₂, the rest noble gases and CO₂
- Complete combustion of carbon-containing fuels gives CO₂; incomplete combustion gives CO and particulates
- SO₂ and oxides of nitrogen cause acid rain; CO₂ and CH₄ increase global warming
- Photosynthesis: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, using light and chlorophyll
How Chemistry of the environment has been examined
From 2022 to 2026, Chemistry of the environment was on Paper 3 · Theory (Core) in 14 of 14 exam series, worth about 22% of that paper’s marks.
| Paper | Questions | Share of marks | Series |
|---|---|---|---|
| Paper 1 · Multiple Choice (Core) | 123 | 10% | 14 of 14 |
| Paper 2 · Multiple Choice (Extended) | 131 | 10% | 14 of 14 |
| Paper 3 · Theory (Core) | 57 | 22% | 14 of 14 |
| Paper 4 · Theory (Extended) | 34 | 15% | 12 of 14 |
| Paper 6 · Alternative to Practical | 6 | 3% | 5 of 14 |
Momentum’s own count, over the questions in its bank for these years: a series is one exam session, every zone included, and a question filed under two chapters counts in both. Median marks per question: 1. The bank files this topic together with neighbouring ones, so these figures count every question in its chapters, including some on those topics.
Filed in Momentum under: Air and Water, Some Non-metals and Their Compounds.
A worked example
A farmer can use ammonium nitrate, NH₄NO₃, or ammonium sulfate, (NH₄)₂SO₄, as a nitrogen fertiliser. Calculate the percentage by mass of nitrogen in each, and say which other elements an NPK fertiliser must also supply.
- Mr of NH₄NO₃ = 14 + (4 × 1) + 14 + (3 × 16) = 80; it contains two nitrogen atoms, 28 g of nitrogen in every 80 g
- Percentage of nitrogen = 28 ÷ 80 × 100 = 35.0%
- Mr of (NH₄)₂SO₄ = 2 × (14 + 4) + 32 + (4 × 16) = 132; it also contains two nitrogen atoms, 28 g in every 132 g
- Percentage of nitrogen = 28 ÷ 132 × 100 = 21.2%
- Neither compound contains phosphorus or potassium, which an NPK fertiliser must supply as well, for example from a phosphate and a potassium salt
Answer: Ammonium nitrate is 35.0% nitrogen and ammonium sulfate 21.2%, so ammonium nitrate supplies more nitrogen per kilogram; an NPK fertiliser must also supply phosphorus and potassium
Written by Momentum for this page, with its own numbers; it is not taken from a past paper.
Mistakes that cost marks
- Saying carbon dioxide causes acid rain or carbon monoxide global warming — acid rain comes from sulfur dioxide and oxides of nitrogen, the greenhouse gases named are carbon dioxide and methane, and carbon monoxide is toxic
- Saying the nitrogen in exhaust oxides comes from the fuel — it comes from the air, where nitrogen and oxygen react at the high temperature inside the engine
- Claiming that anhydrous copper(II) sulfate shows water is pure — it shows only that water is present; purity is shown by melting at 0 °C and boiling at 100 °C
- Saying chlorination removes solids or 'cleans' the water — chlorine kills microbes; sedimentation and filtration remove solids, and carbon removes tastes and odours
- Describing global warming as gases trapping sunlight or damaging the ozone layer — greenhouse gases absorb thermal energy given out by the Earth and emit it again, so less escapes to space
Exam technique
- Pair each pollutant with its exact source and effect; vague answers such as 'pollution' or 'bad for health' earn nothing
- Separate complete combustion, with plenty of oxygen, giving CO₂ and H₂O, from incomplete combustion, with too little oxygen, giving CO and particulates
- When asked for a strategy, say how it works: a catalytic converter turns CO and NO into CO₂ and N₂; flue gas desulfurisation removes SO₂ with calcium oxide
- For fertiliser questions, use each compound's formula to see which of N, P and K it supplies, and compare percentages by mass when asked which supplies more nitrogen
Practising in Momentum
- Every question in a chapter as one list, from every paper and year in the bank, newest first or shuffled.
- Each question turns over to its own part of the mark scheme, where the board published one.
- Narrow by paper, year, session, zone and marks, and star the questions worth keeping.
- Pick chapters, a number of questions and a time, and the exam builder draws a mock to sit and mark.
Past-paper questions and mark schemes are the copyright of Cambridge International. Momentum shows them to signed-in learners for revision and does not publish them on these pages.