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Sulfur Dioxide and Sulfuric Acid

Sulfur dioxide: its acidified potassium manganate(VII) test, acid-rain effects, flue-gas removal and dilute sulfuric acid links.

  • SEC G3 Pure Chemistry 2027

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This lesson focuses on identifying sulfur dioxide, explaining its environmental effects and describing how it is removed from flue gases. The Contact Process is not required here.

1. Definition

A. Sulfur dioxide

Sulfur dioxide, SO₂, is a colourless, toxic gas. It is an acidic oxide, an air pollutant and a reducing agent.

B. Sulfuric acid

Sulfuric acid, H₂SO₄, is a strong acid with respect to its first proton; its second proton ionises only partially. See acid strength. In this topic, dilute sulfuric acid is used as an example of a typical acid and as the acid formed when sulfur dioxide is oxidised in moist air.

2. Key Ideas

  • Test SO₂ with acidified potassium manganate(VII): the solution changes from purple to colourless.
  • Burning sulfur-containing fuels releases SO₂.
  • SO₂ contributes to acid rain, which harms aquatic life, plants and carbonate stone.
  • Calcium carbonate removes SO₂ from flue gases before release.
  • Dilute H₂SO₄ shows the characteristic reactions of an acid and produces sulfate salts.

3. Detailed Explanations

Connect evidence to the process
  • Reagent: acidified potassium manganate(VII) solution.
  • Observation with sulfur dioxide: purple → colourless.
  • Chemical explanation: sulfur dioxide acts as a reducing agent.
  • Pollution control: calcium carbonate reacts with sulfur dioxide before the flue gases are released.

A. Source and environmental effect

Sulfur dioxide is produced when sulfur or sulfur compounds in fuels burn:

S(s) + O₂(g) → SO₂(g)

In moist air, sulfur dioxide can be oxidised and form sulfuric acid. The following overall model summarises several atmospheric steps rather than one direct collision:

2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq)

Acid rain can lower the pH of lakes and soils, harming organisms, damaging plants and reacting with carbonate stone such as limestone. Sulfur dioxide is one contributor; nitrogen oxides also contribute to acid rain. This is different from the greenhouse effect of carbon dioxide.

B. Prescribed test for sulfur dioxide

ReagentPositive observationConclusion
acidified potassium manganate(VII), KMnO₄(aq)purple solution turns colourlesssupports identification as SO₂ in the course’s gas-test context

The colour change occurs because SO₂ reduces manganate(VII) ions. Other reducing substances can also decolourise the reagent, so this result is not a unique identification of every possible unknown. In the course’s gas-test questions, use it with the stated gas candidates or source. Name the reagent and give both the initial and final colours. Practical tests use the school laboratory’s controls for toxic gases; do not inhale the gas.

Do not substitute another test

For the current gas-test table, learn acidified potassium manganate(VII): purple to colourless. Acidified dichromate and litmus bleaching are not the prescribed identification test here.

C. Removing sulfur dioxide from flue gases

Flue gases are waste gases leaving a furnace or power station. They can be brought into contact with calcium carbonate, for example in a wet limestone slurry. The carbonate reacts with the acidic oxide, removing much of the sulfur dioxide before the remaining gases enter the atmosphere.

One simplified equation is:

CaCO₃(s) + SO₂(g) → CaSO₃(s) + CO₂(g)

The calcium sulfite can be oxidised to calcium sulfate:

2CaSO₃(s) + O₂(g) → 2CaSO₄(s)

This process is called flue-gas desulfurisation. Calcium carbonate is a reactant: it is consumed and must be replenished. The equations show that sulfur moves from the gas into a calcium salt; it does not disappear. This treatment targets sulfur dioxide and does not remove all carbon dioxide. The simplified carbonate reaction itself also produces carbon dioxide.

Sulfur dioxide: atmospheric effects and control at the sourceBoth paths begin with combustion of sulfur-containing fuel producing sulfur dioxide. Without treatment, atmospheric oxidation and water can produce acids and lower rainwater pH. With calcium-carbonate treatment, sulfur dioxide reacts before release and much of the sulfur is captured in a calcium salt. Calcium carbonate is consumed; it is not a catalyst.Without SO₂ treatmentBurn sulfur-containing fuelSulfur dioxide is producedSO₂ enters the atmosphereOxidation + atmospheric waterAcids formRainwater pH can fallAcid rain can harm organismsand react with carbonate stone.With SO₂ treatmentBurn sulfur-containing fuelSulfur dioxide is producedSO₂ reacts with CaCO₃Before the gases are releasedSulfur enters a calcium saltMuch less SO₂ is releasedCaCO₃ is consumed,so it must be replenished.
Follow sulfur in two possible paths. Without treatment, emitted sulfur dioxide can contribute to acid formation in atmospheric water. With calcium-carbonate treatment, much of that sulfur is captured in a calcium salt before release. The control reduces sulfur-dioxide emissions; it does not remove every pollutant or all carbon dioxide. This causal diagram summarises processes rather than showing a single reaction step.

D. Dilute sulfuric acid as a typical acid

Dilute sulfuric acid produces H + (aq) and forms sulfate salts when its replaceable hydrogen is fully replaced. The equations below use enough metal, alkali or carbonate to form the sulfate:

Mg(s) + H₂SO₄(aq) → MgSO₄(aq) + H₂(g)

H₂SO₄(aq) + 2NaOH(aq) → Na₂SO₄(aq) + 2H₂O(l)

Na₂CO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g)

These are applications of the general acid reactions from Physical & Chemical Properties of Acids, not a separate set of sulfuric-acid rules.

4. Common Mistakes

  • Naming potassium manganate(VII) but omitting that it must be acidified.
  • Writing only “decolourises”; state purple to colourless.
  • Claiming sulfur dioxide causes global warming as its main tested effect; the syllabus link is acid rain.
  • Saying calcium carbonate is a catalyst. It reacts with and removes the acidic gas.
  • Memorising Contact Process conditions even though they are outside the required content here.

5. Exam Tips

A complete gas-test sentence

“Pass the gas through acidified potassium manganate(VII) solution. A change from purple to colourless supports identification as sulfur dioxide in the stated gas-test context.”

  • For pollution questions, link source → pollutant → effect → control.
  • If asked why calcium carbonate works, state that it is basic and neutralises acidic SO₂.
  • Keep observations separate from conclusions: “purple to colourless” is the observation; “SO₂ is present” is the conclusion.

6. Worked Examples

Modelled example 1

Identify sulfur dioxide

Core

Problem

State the reagent and positive observation used to identify sulfur dioxide gas.
Study the worked solution

  1. Select the prescribed reagent

    Method

    Use acidified potassium manganate(VII) solution.

    Reason

    This oxidising reagent provides the required sulfur-dioxide test.

    Working

    Reagent: acidified potassium manganate(VII).

  2. State the colour change

    Method

    Report purple to colourless.

    Reason

    Both starting and final colours are required as the positive observation.

    Working

    Purple solution becomes colourless.

Guided practice 2

Explain the acid-rain link

About 6 min

Problem

Explain how burning sulfur-containing fuel can lead to acid rain.

Build the causal chain

Gas released
Atmospheric consequence

Hints

Hint 1: combustion product
The relevant sulfur oxide is SO₂.
Hint 2: atmospheric chemistry
It is oxidised and dissolves in atmospheric water to form acids.
View solution step by step

  1. Release sulfur dioxide

    Method

    State that burning the fuel emits SO₂.

    Reason

    Sulfur in the fuel is oxidised during combustion.

    Working

    Sulfur-containing fuel → SO₂ emissions.

  2. Form atmospheric acids

    Method

    Describe oxidation and dissolution in atmospheric water.

    Reason

    The resulting acids lower rainwater pH.

    Working

    SO₂ emissions can lead to acid rain.

Common misconception 3

Remove a pollutant

Find and correct the mistake

Learner claim

A student says calcium carbonate is a catalyst in flue-gas desulfurisation. Correct the claim and explain its role.

Classify the chemical role

Role
Pollutant fate

View solution step by step

  1. Correct the classification

    Method

    Describe calcium carbonate as a basic reagent, not a catalyst.

    Reason

    It participates in neutralising the acidic pollutant.

    Working

    CaCO₃ is consumed in pollutant removal.

  2. Explain pollutant removal

    Method

    State that sulfur dioxide becomes a solid calcium salt.

    Reason

    Capturing sulfur in a solid prevents much of the gas from entering the atmosphere.

    Working

    Acidic SO₂ is neutralised before flue gas release.

Challenge 4

Predict acid-reaction products

Minimal support

Reaction transfer

Name the products when dilute sulfuric acid reacts with sodium carbonate, then write the balanced equation.

Apply acid–carbonate products

Products
Equation

Hints

Hint 1: reaction family
Acid + carbonate gives salt + water + carbon dioxide.
Hint 2: salt name
Sulfuric acid forms sulfate salts.
View solution step by step

  1. Name the product classes

    Method

    Use sodium sulfate, water and carbon dioxide.

    Reason

    Sulfuric acid supplies sulfate and carbonate releases CO₂.

    Working

    Products: Na₂SO₄, H₂O, CO₂.

  2. Write the equation

    Method

    Use one formula unit or molecule of each species.

    Reason

    The resulting equation balances Na, C, S, H and O.

    Working

    Na₂CO₃(s) + H₂SO₄(aq) → Na₂SO₄(aq) + H₂O(l) + CO₂(g).

7. Mind Stretchers

Mind stretcher 1: Build an evidence chainExtension

Question: The possible gases are carbon dioxide, oxygen and sulfur dioxide. A colourless sample changes acidified potassium manganate(VII) from purple to colourless. Separate the observation from the conclusion. Does this prove the identity of every possible unknown gas?

Show Answer

Observation: the purple solution becomes colourless. Of the stated candidates, this supports sulfur dioxide. The gas acts as a reducing agent. It does not uniquely identify every possible unknown because other reducing substances can cause the same colour change.

Mind stretcher 2: Control and trade-offExtension

Question: Why is flue-gas treatment more effective than repairing acid-rain damage later?

Show Answer

It removes much of the sulfur dioxide at the source before dispersion, so less acid forms over a wide area and fewer ecosystems and structures are exposed.

Try independently: A treatment plant removes most sulfur dioxide using limestone. A learner concludes that the gas leaving the plant cannot contribute to global warming. Explain the error, using the simplified treatment equation.

Show answer and reasoning

Removing sulfur dioxide reduces its acid-rain contribution. It does not show that carbon dioxide from combustion has been removed. The simplified reaction CaCO₃ + SO₂ → CaSO₃ + CO₂ also produces carbon dioxide. Sulfur-dioxide removal and carbon-dioxide reduction are different pollution-control tasks.

Practise and check

Use the Acid–Base Chemistry topic check to practise acid reactions. Continue to Environmental Chemistry for pollution applications and Qualitative Analysis for the wider set of gas tests.

Syllabus and review details

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