Chemistry of the Environment

This guide covers Topic 10, Chemistry of the environment — subtopics 10.1 Water, 10.2 Fertilisers and 10.3 Air quality and climate — for Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series.

Where this fits in 0620/5070

This topic applies chemistry you already have — NPK elements and salts (Topic 7), combustion and redox (Topic 6) — to real-world contexts: water treatment, agriculture and air pollution. It’s also one of the more frequently updated topics in terms of exam context (new pollutants, new mitigation strategies), so understanding the underlying chemistry matters more here than memorising any one year’s specific example.

Syllabus coverage

CAMBRIDGE IGCSE CHEMISTRY 0620

Core

• Chemical tests for water: anhydrous cobalt(II) chloride and anhydrous copper(II) sulfate; testing purity by melting/boiling point (10.1)
• Explaining why distilled water, not tap water, is used in practical chemistry (10.1)
• Substances found in natural water sources, and which are beneficial vs potentially harmful (10.1)
• Treatment of domestic water supply: sedimentation/filtration, carbon treatment, chlorination (10.1)
• Stating that ammonium salts and nitrates are used as fertilisers, and describing NPK fertilisers (10.2)
• Composition of clean, dry air (10.3)
• Sources and adverse effects of named air pollutants: CO₂, CO, particulates, methane, oxides of nitrogen, SO₂ (10.3)
• Strategies to reduce climate change and acid rain (10.3)
• Photosynthesis as a word equation (10.3)

10.1 and 10.2 have no Extended-only content at all — every outcome in those two subtopics is Core, for every 0620 candidate.

Supplement / Extended

• Describing how CO₂ and methane cause global warming — absorption, reflection and emission of thermal energy, reducing heat loss to space (10.3)
• Explaining how oxides of nitrogen form in car engines and how catalytic converters remove them (10.3)
• Photosynthesis as a balanced symbol equation (10.3)

CAMBRIDGE O LEVEL CHEMISTRY 5070

5070 has no Core/Extended split — every outcome above, Core and Supplement alike, is required for every O Level candidate.

Testing and treating water

Two chemical tests confirm the presence of water (not its purity):

anhydrous copper(II) sulfate:  white  →  blue     (in the presence of water)
anhydrous cobalt(II) chloride: blue   →  pink      (in the presence of water)

Purity is tested differently — pure water has a fixed melting point (0°C) and boiling point (100°C); dissolved substances lower the melting point and raise the boiling point, so a sample melting or boiling at the “wrong” temperature is impure. This is also why distilled water, not tap water, is used in practical chemistry: tap water contains dissolved impurities that could interfere with a reaction or a measurement.

What’s actually in natural water, and whether it helps or harms:

Substance	Effect
Dissolved oxygen	Beneficial — needed for aquatic life
Some metal compounds	Can be beneficial (essential minerals) or toxic
Plastics	Harmful — harm aquatic life
Sewage	Harmful — contains microbes that cause disease
Nitrates/phosphates (from fertiliser, detergents)	Harmful — deoxygenate water, damaging aquatic life

Domestic water treatment is a three-step process, each step removing a different kind of impurity:

1. Sedimentation and filtration  — removes solids
2. Treatment with carbon         — removes tastes and odours
3. Chlorination                  — kills harmful microbes

Fertilisers

Plants need nitrogen, phosphorus and potassium for healthy growth — the NPK in NPK fertiliser. Ammonium salts and nitrates are the main nitrogen sources used. An NPK fertiliser supplies all three elements together, in a single application, to improve plant growth.

Air quality and climate

Clean, dry air is approximately 78% nitrogen, 21% oxygen, with the small remainder made up of noble gases and carbon dioxide.

Air pollutants — source and effect:

Pollutant	Source	Adverse effect
Carbon dioxide	Complete combustion of carbon-containing fuels	Global warming → climate change
Carbon monoxide	Incomplete combustion of carbon-containing fuels	Toxic gas
Particulates	Incomplete combustion of carbon-containing fuels	Respiratory problems, cancer risk
Methane	Decomposition of vegetation, animal digestion	Global warming → climate change
Oxides of nitrogen	Car engines	Acid rain, photochemical smog, respiratory problems
Sulfur dioxide	Combustion of fossil fuels containing sulfur	Acid rain

Notice that complete combustion produces CO₂, while incomplete combustion (insufficient oxygen) produces carbon monoxide and particulates instead — the same fuel, two very different pollutant profiles depending on how completely it burns.

How greenhouse gases cause warming. CO₂ and methane absorb thermal energy radiating from the Earth’s surface, then re-emit some of it back downward rather than letting it escape to space — reducing the rate of thermal energy loss to space is what drives the temperature increase.

Reducing the impact:

• Climate change: planting trees, reducing livestock farming, decreasing fossil fuel use, increasing hydrogen and renewable energy use (wind, solar).
• Acid rain: catalytic converters in vehicles, low-sulfur fuels, flue gas desulfurisation using calcium oxide.

Catalytic converters remove oxides of nitrogen by converting them, along with carbon monoxide, into less harmful gases:

2CO + 2NO → 2CO2 + N2

Photosynthesis removes CO₂ from the atmosphere, converting it (with water, using light energy, in the presence of chlorophyll) into glucose and oxygen:

word equation:    carbon dioxide + water → glucose + oxygen
symbol equation:  6CO2 + 6H2O → C6H12O6 + 6O2

Common mistakes

• Testing for water purity using the cobalt chloride/copper sulfate colour tests. Those two tests only confirm water is present — they say nothing about purity. Purity is tested by melting point and boiling point.
• Mixing up the three water-treatment steps’ purposes. Sedimentation removes solids, carbon removes taste/odour, chlorination kills microbes — each step does one job, and swapping them in an answer loses marks.
• Confusing complete and incomplete combustion products. Complete combustion (enough oxygen) gives CO₂; incomplete combustion (not enough oxygen) gives CO and/or particulates instead — know which conditions produce which.
• Explaining the greenhouse effect as gases “trapping” heat like a blanket without mentioning re-emission. The expected mechanism is absorption and re-emission of thermal energy, reducing the rate of loss to space — not a physical barrier.
• Forgetting NPK stands for specific elements, not compounds — nitrogen, phosphorus, potassium — supplied in various salts, not as the pure elements themselves.

Quick revision checklist

• Chemical tests for the presence of water, and how purity is tested differently
• Beneficial vs harmful substances found in natural water, and the three domestic treatment steps in order
• NPK fertilisers: which elements, and why plants need them
• Composition of clean air
• Source and adverse effect of each named air pollutant
• (0620 Extended, 5070 required) the greenhouse mechanism (absorption and re-emission), and how catalytic converters work
• Strategies to reduce climate change and acid rain
• Photosynthesis word equation, and (0620 Extended, 5070 required) the balanced symbol equation

Related resources

• Acids, Bases and Salts — the salt chemistry behind fertilisers and water treatment
• Redox Reactions — combustion as oxidation, relevant to air pollutant sources
• Cambridge IGCSE Chemistry hub · Cambridge O Level Chemistry hub

Written against Cambridge IGCSE Chemistry 0620 and Cambridge O Level Chemistry 5070, 2026–2028 series. Always check the current syllabus for your examination year.