What is the role of sulfur in the environment according to the Cambridge IGCSE Chemistry syllabus?

In the Cambridge IGCSE Chemistry syllabus, sulfur is highlighted for its role in the environment, particularly in the formation of sulfur dioxide (SO2) which contributes to air pollution and acid rain. Sulfur is also a key element in the sulfur cycle, where it moves between the atmosphere, lithosphere, and biosphere, playing a crucial role in various biological and chemical processes.

How does sulfur dioxide contribute to acid rain?

Sulfur dioxide (SO2) is a significant contributor to acid rain. When released into the atmosphere, it reacts with water vapor to form sulfurous acid (H2SO3), which can further oxidize to sulfuric acid (H2SO4). These acids then fall to the ground as acid rain, which can harm ecosystems, corrode buildings, and affect soil and water quality.

What are the industrial sources of sulfur dioxide emissions?

Industrial sources of sulfur dioxide emissions include the burning of fossil fuels such as coal and oil in power plants, and the smelting of metal ores that contain sulfur. These processes release significant amounts of SO2 into the atmosphere, contributing to air pollution and environmental issues like acid rain.

How is sulfur used in industry, and what are its environmental impacts?

Sulfur is used in various industries, including the production of sulfuric acid, which is essential for manufacturing fertilizers, chemicals, and in petroleum refining. While sulfur itself is not harmful, its conversion to sulfur dioxide during industrial processes can lead to environmental issues such as air pollution and acid rain, impacting ecosystems and human health.

What are the methods to reduce sulfur dioxide emissions in the environment?

To reduce sulfur dioxide emissions, industries can implement technologies such as flue-gas desulfurization, which removes SO2 from exhaust gases. Switching to low-sulfur fuels and using alternative energy sources like wind or solar power can also significantly reduce emissions. Additionally, regulatory measures and international agreements aim to limit SO2 emissions to protect the environment.

Why is "Saying iron is the catalyst" a common mistake in Sulfur?

Why it happens: Confusing with the Haber process. How to avoid it: Haber: iron. Contact: V_2O_5. Source: 0620/42 — recurring

Why is "Saying SO₃ is directly added to water" a common mistake in Sulfur?

Why it happens: Simplifying. How to avoid it: Direct SO_3 + H_2O produces a hazardous acid mist. Industrially, SO₃ is dissolved in concentrated H_2SO_4 first.

Why is "Choosing very high T to maximise yield" a common mistake in Sulfur?

Why it happens: Confusing rate with yield. How to avoid it: Forward reaction is EXOTHERMIC → lower T favours yield. Higher T favours rate. 450C is compromise.

Chemistry of the EnvironmentCambridge IGCSE Chemistry (0620)

Sulfur

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Sulfur — Cambridge IGCSE 0620 Chemistry Extended (2026)

Sources, properties, and the Contact process for making sulfuric acid. Plus uses of sulfuric acid in industry.

What you’ll learn

Mapped to the Cambridge IGCSE 0620 syllabus (2026-2028).

12.5 — Describe the manufacture of sulfuric acid (Contact process).
12.5 — State uses of sulfuric acid.
12.5 — Describe the role of $\text{V}_{2}\text{O}_{5}$ as a catalyst.

The Contact process — making sulfuric acid

Three stages: burn S, oxidise SO₂ to SO₃ (catalysed), absorb SO₃ in water (via oleum).

Stage 1: Burn sulfur in air. $\text{S} + \text{O}_{2} \to \text{SO}_{2}.$

This is highly exothermic and easily achieved.

Stage 2: Oxidise SO₂ to SO₃ (the heart of the Contact process). $2\text{SO}_{2}(g) + \text{O}_{2}(g) \rightleftharpoons 2\text{SO}_{3}(g) \quad (\Delta H < 0).$

Conditions:

Temperature: $\sim 450\degree\text{C}$ . Compromise: forward exothermic so low T favours yield, but too low T → too slow.
Pressure: $\sim 2\,\text{atm}$ . Forward goes from 3 mol → 2 mol so higher P favours forward, but yield is already $\sim 95\%+$ at $2\,\text{atm}$ — no economic benefit to higher P.
Catalyst: vanadium(V) oxide ( $\text{V}_{2}\text{O}_{5}$ ). Speeds reaching equilibrium.
Excess O₂: shifts equilibrium toward more SO₃ (Le Chatelier).

Stage 3: Convert SO₃ to H₂SO₄ (in two steps for safety).

Don't pour SO₃ directly into water — it reacts violently and creates choking sulfuric acid mist.
Instead: dissolve SO₃ in concentrated H₂SO₄ first → forms OLEUM ( $\text{H}_{2}\text{S}_{2}\text{O}_{7}$ ): $\text{H}_{2}\text{SO}_{4} + \text{SO}_{3} \to \text{H}_{2}\text{S}_{2}\text{O}_{7}.$
Then dilute oleum carefully with water: $\text{H}_{2}\text{S}_{2}\text{O}_{7} + \text{H}_{2}\text{O} \to 2\text{H}_{2}\text{SO}_{4}.$

Net. Sulfur + air + water → sulfuric acid. The oleum step manages safety.

Cambridge tip. Always state ALL conditions for stage 2: temperature, pressure, catalyst.

Stage 1: $\text{S} + \text{O}_{2} \to \text{SO}_{2}$ .
Stage 2: $2\text{SO}_{2} + \text{O}_{2} \rightleftharpoons 2\text{SO}_{3}$ , $450\degree\text{C}$ , $2\,\text{atm}$ , $\text{V}_{2}\text{O}_{5}$ .
Stage 3: SO₃ → oleum → H₂SO₄.
Don't dissolve SO₃ directly in water — too violent.

Uses of sulfuric acid

Car batteries, fertilisers, detergents, textiles, paper, metal processing.

Sulfuric acid is one of the most-produced industrial chemicals — about $\sim 200$ million tonnes per year globally.

Major uses.

Use	Why H₂SO₄?
Car batteries (lead-acid)	Strong electrolyte; provides H⁺ for the redox reactions
Fertiliser manufacture	Reacts with phosphate rock to make superphosphate; with NH₃ to make ammonium sulfate
Detergent manufacture	Sulfonates organic compounds to make sulfonate detergents
Pickling steel	Removes rust before plating or further processing
Paper / pulp manufacture	Treats wood pulp
Petroleum refining	Removes impurities
Dye and pigment industry	Acid in many syntheses

Worked qualitative. Why is sulfuric acid the "king of chemicals" or "lifeblood of industry"? It's used in so many processes that a country's H₂SO₄ consumption tracks its industrial activity. Cheap to produce from sulfur, water and air.

Cambridge tip. Memorise at least THREE uses for the exam.

Used in car batteries, fertilisers, detergents.
Pickling steel, paper, refining oil.
Cheap and versatile — "king of chemicals".

How it’s examined

Sulfur and Contact process appear most years on Paper 4 (6-8 marks): write equations, justify conditions using Le Chatelier, list uses. Examiner reports flag students saying "high T favours yield" — Contact is exothermic, so the opposite is true.

Sources: Cambridge IGCSE Chemistry 0620 syllabus 2026-2028 (12.5); 0620/42 Oct/Nov 2024 — Q12 (Contact process); 0620 Examiner Reports 2022-2024. Last reviewed 2026-05-07.

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Step-by-step worked examples — Sulfur

Step-by-step solutions to past-paper-style questions on sulfur, written exactly the way a tutor would explain them at the board.

1The Contact process equations
Extended• Adapted from 0620/42 May/Jun 2024 Q16• Contact
Question
State the four main reactions in the Contact process for making sulfuric acid.
Step-by-step solution
1. Step 1
  Burn sulfur.
  $\text{S} + \text{O}_{2} \to \text{SO}_{2}$
2. Step 2
  Catalysed conversion (V₂O₅ catalyst at $\sim 450\degree\text{C}$ ).
  $2\text{SO}_{2} + \text{O}_{2} \rightleftharpoons 2\text{SO}_{3}$
3. Step 3
  Absorb in concentrated $\text{H}_{2}\text{SO}_{4}$ → oleum.
  $\text{SO}_{3} + \text{H}_{2}\text{SO}_{4} \to \text{H}_{2}\text{S}_{2}\text{O}_{7}$
4. Step 4
  Dilute oleum with water.
  $\text{H}_{2}\text{S}_{2}\text{O}_{7} + \text{H}_{2}\text{O} \to 2\text{H}_{2}\text{SO}_{4}$
Answer
S → SO₂ → SO₃ (V₂O₅) → oleum → H₂SO₄.
2Conditions for SO₂ → SO₃
Extended• conditions
Question
State the conditions used and explain why each is a compromise.
Step-by-step solution
1. Step 1
  Temperature $\sim 450\degree\text{C}$ : COMPROMISE between yield (favoured by lower T because exothermic) and rate (favoured by higher T).
2. Step 2
  Pressure $1\text{-}2\,\text{atm}$ : extra pressure not worth the cost — already $>95\%$ yield.
3. Step 3
  $\text{V}_{2}\text{O}_{5}$ catalyst: speeds up the reaction without affecting yield.
Answer
$\sim 450\degree\text{C}$ , 1-2 atm, V₂O₅. Compromise between yield and rate.
3Uses of sulfuric acid
Core• uses
Question
List three uses of sulfuric acid.
Step-by-step solution
1. Step 1
  Manufacture of fertilisers (e.g. ammonium sulfate).
2. Step 2
  Detergents and dyes.
3. Step 3
  Lead-acid car batteries.
Answer
Fertilisers, detergents/dyes, lead-acid batteries.

Key Definitions and Keywords — Sulfur

Definitions to memorise and the exact keywords mark schemes credit for sulfur answers — sharpened from recent examiner reports for the 2026 0620 sitting.

Contact process
Examiner keyword
Industrial process for making sulfuric acid: $\text{S} \to \text{SO}_{2} \to \text{SO}_{3}$ (V₂O₅) → H₂SO₄.
Oleum
Examiner keyword
$\text{H}_{2}\text{S}_{2}\text{O}_{7}$ — intermediate formed when SO₃ dissolves in concentrated H₂SO₄.

Common Mistakes and Misconceptions — Sulfur

The traps other students keep falling into on sulfur questions — taken from recent Cambridge IGCSE 0620 examiner reports and mark schemes — and how to avoid them.

✕Saying iron is the catalyst
0620/42 — recurring
Why it happens
Confusing with the Haber process.
How to avoid it
Haber: iron. Contact: $\text{V}_{2}\text{O}_{5}$ .
✕Saying SO₃ is directly added to water
Why it happens
Simplifying.
How to avoid it
Direct $\text{SO}_{3}$ + $\text{H}_{2}\text{O}$ produces a hazardous acid mist. Industrially, SO₃ is dissolved in concentrated $\text{H}_{2}\text{SO}_{4}$ first.
✕Choosing very high T to maximise yield
Why it happens
Confusing rate with yield.
How to avoid it
Forward reaction is EXOTHERMIC → lower T favours yield. Higher T favours rate. $\sim 450\degree\text{C}$ is compromise.

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Sulfur — frequently asked questions

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Sulfur

Short Study Notes in the form of Slides

Detailed Study Notes

What you’ll learn

How it’s examined

1The Contact process equations

2Conditions for SO₂ → SO₃

3Uses of sulfuric acid

Contact process

Oleum

✕Saying iron is the catalyst

✕Saying SO₃ is directly added to water

✕Choosing very high T to maximise yield

Practice questions

Past paper style quiz

Assess your understanding

Sulfur — frequently asked questions