Who this is for: Cambridge IGCSE Chemistry (0620) students who want sulfur — sulfur dioxide, acid rain and pollution control — to become structured explain answers instead of vague statements about “pollution”.
What query it owns: how to understand and revise sulfur in Cambridge IGCSE Chemistry.
Why this is safe: this page owns the sulfur revision-guide angle, while Tutopiya’s Sulfur subtopic page owns the learning resource and the free Sulfur quiz owns the practice.

Sulfur is a key pollution topic in the Chemistry Of The Environment unit of Cambridge IGCSE Chemistry (0620). Examiners expect you to name sources of sulfur dioxide, explain how acid rain forms, describe effects on buildings, lakes and forests, and suggest ways to reduce SO₂ emissions. This guide organises each idea with the exam phrasing that signals it.

Key takeaways

• Sulfur dioxide (SO₂) is produced when sulfur or sulfur compounds in fossil fuels burn, and from volcanic activity.
• SO₂ dissolves in rain to form sulfurous acid, which is oxidised to sulfuric acid — this is acid rain.
• Acid rain damages limestone and marble buildings, kills fish in lakes (low pH) and damages forests.
• Flue gas desulfurisation (scrubbers) uses calcium oxide or calcium hydroxide to remove SO₂ from power-station waste gases.
• Low-sulfur fuels reduce SO₂ at source — examiners often ask for both prevention and treatment.

What is sulfur in Cambridge IGCSE Chemistry?

Sulfur chemistry at IGCSE focuses on atmospheric pollution rather than the element’s industrial uses. You describe how sulfur dioxide enters the atmosphere, the sequence from gas to acid rain, environmental consequences, and methods to reduce emissions from power stations and vehicles. You may also need the test for sulfur dioxide: it decolourises acidified potassium manganate(VII).

Read the full notes on Tutopiya’s Sulfur subtopic page before attempting questions.

The core ideas you must master

Idea	What it means	How the exam uses it
SO₂ sources	Burning fossil fuels with sulfur impurities; volcanoes	”State two sources of sulfur dioxide.”
Acid rain formation	SO₂ + H₂O → sulfurous acid → oxidised to sulfuric acid	”Explain how acid rain forms.”
Effects	Buildings, lakes, forests damaged	”Describe two effects of acid rain.”
Prevention	Low-sulfur fuels, catalytic converters (vehicles)	“Suggest how to reduce SO₂ emissions.”
Flue gas treatment	Scrubbers with CaO/Ca(OH)₂ absorb SO₂	”Describe how flue gas desulfurisation works.”

How to explain acid rain formation — step by step

1. Sulfur in coal or oil burns to form sulfur dioxide: S + O₂ → SO₂ (or sulfur compounds in fuel oxidise).
2. SO₂ is released into the atmosphere from power stations, factories and volcanoes.
3. SO₂ dissolves in rain water to form sulfurous acid (H₂SO₃).
4. Sulfurous acid is oxidised in air to sulfuric acid (H₂SO₄).
5. Acid rain (pH below 5.6) falls on buildings, soil and water — causing environmental damage.

Test yourself with the free Sulfur quiz.

Sources vs effects vs prevention: which does the question want?

Situation	What to write	Typical signal words
Where SO₂ comes from	Burning fossil fuels, volcanoes	”sources of sulfur dioxide”
Acid rain mechanism	SO₂ dissolves → acids form	”explain how acid rain forms”
Environmental damage	Buildings, fish, trees	”effects of acid rain”
Reducing emissions	Low-sulfur fuel, scrubbers	”how to reduce sulfur dioxide”

Sulfur in past-paper wording: command words that matter

Command word / phrase	What the question wants	Typical sulfur stem
State / Give	Named source or product	”Give a source of sulfur dioxide.”
Explain	Acid rain sequence or effect link	”Explain how sulfur dioxide causes acid rain.”
Describe	Effects or scrubber process	”Describe two effects of acid rain on the environment.”
Suggest	Pollution reduction method	”Suggest how power stations can reduce SO₂ emissions.”
Write an equation	Combustion of sulfur	”Write an equation for sulfur burning in air.”

Worked exam-style stems (how to answer the wording)

1. “State two sources of sulfur dioxide in the atmosphere.” Burning fossil fuels (coal/oil containing sulfur) and volcanic eruptions. Reward: any two valid sources.
2. “Explain how sulfur dioxide causes acid rain.” SO₂ dissolves in rain water to form sulfurous acid, which is oxidised to sulfuric acid. Rain with pH below normal is acid rain. Reward: dissolve + acid formation + acid rain named.
3. “Describe how flue gas desulfurisation reduces sulfur dioxide emissions.” Waste gases pass through a scrubber containing calcium oxide or calcium hydroxide, which reacts with SO₂ to form a solid (calcium sulfite/sulfate) that can be removed. Reward: scrubber + reactant + SO₂ removed.

When you can recognise the wording instantly, work through the Sulfur quiz and Carbonates for acid damage to limestone.

How sulfur connects to the rest of the environment unit

Sulfur links to Air (changing composition), Carbon Dioxide And Methane (other pollutants), Carbonates (limestone damaged by acid rain) and Fuels (sulfur impurities in coal). The Cambridge IGCSE Chemistry resource hub links every environment subtopic.

Common mistakes students make

• Describing acid rain as caused by carbon dioxide alone — SO₂ (and NOₓ) are the acid-rain gases tested with sulfur.
• Missing the oxidation step from sulfurous acid to sulfuric acid in explain answers.
• Listing effects without linking to acid rain (e.g. “buildings damaged” needs acid reacting with carbonate).
• Forgetting flue gas desulfurisation when asked how power stations reduce emissions.
• Confusing the SO₂ test (decolourises acidified KMnO₄) with the CO₂ test (limewater).

When you need more support

If sulfur dioxide or acid rain questions keep losing marks, work through the Sulfur quiz, then get focused help from a Cambridge IGCSE Chemistry tutor.

Frequently asked questions

What is sulfur dioxide and where does it come from? Sulfur dioxide (SO₂) is a pollutant gas formed when sulfur in fossil fuels burns, and from volcanic activity.

How does sulfur dioxide cause acid rain? It dissolves in rain to form sulfurous acid, which is oxidised to sulfuric acid, lowering the pH of rain.

What are two effects of acid rain? Damage to limestone buildings and statues, acidification of lakes (killing fish), and damage to forests.

How do I revise sulfur effectively? Learn two sources, one acid-rain explain sequence, two effects and one prevention method, then take the Sulfur quiz.

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