Sulphuric Acid & Sulphur Dioxide
Sulfur dioxide: its acidified potassium manganate(VII) test, acid-rain effects, flue-gas removal and dilute sulfuric acid links.
Continue where you stopped
The core idea
On this page
Learning objectives
- describe the characteristic properties of acids as in reactions with metals, bases and carbonates to form salts
- classify oxides as acidic, basic, amphoteric or neutral based on metallic/non-metallic character.
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. 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
- Reagent: acidified potassium manganate(VII), KMnO₄(aq).
- Observation with SO₂: purple → colourless.
- Conclusion: sulfur dioxide is present and acts as a reducing agent.
- Pollution control: remove SO₂ from flue gases using calcium carbonate.
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:
2SO₂(g) + O₂(g) + 2H₂O(l) → 2H₂SO₄(aq)
Acid rain lowers the pH of lakes and soils, damages leaves and roots, and reacts with carbonate stone such as limestone.
B. Prescribed test for sulfur dioxide
| Reagent | Positive observation | Conclusion |
|---|---|---|
| acidified potassium manganate(VII), KMnO₄(aq) | purple solution turns colourless | SO₂ is present |
The colour change occurs because SO₂ reduces manganate(VII) ions. In an answer, name the reagent and give both the initial and final colours.
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
Power-station gases can be passed through calcium carbonate. The basic carbonate neutralises the acidic oxide, preventing much of the sulfur dioxide from entering 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.
D. Dilute sulfuric acid as a typical acid
Dilute sulfuric acid produces H + (aq) and forms sulfate salts. For example:
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
“Pass the gas through acidified potassium manganate(VII) solution. A change from purple to colourless identifies sulfur dioxide.”
- 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
Problem
Study the worked solution
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).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
Problem
Build the causal chain
Hints
Hint 1: combustion product
Hint 2: atmospheric chemistry
View solution step by step
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.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
Learner claim
Classify the chemical role
View solution step by step
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.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
Reaction transfer
Apply acid–carbonate products
Hints
Hint 1: reaction family
Hint 2: salt name
View solution step by step
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.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: A colourless gas changes acidified potassium manganate(VII) from purple to colourless. Separate the observation from the conclusion.
Show Answer
Observation: the purple solution becomes colourless. Conclusion: the gas may be sulfur dioxide; in the prescribed test context, this is a positive result for SO₂.
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.
8. Quiz
Test the prescribed sulfur dioxide test, acid-rain reasoning, flue-gas desulfurisation and dilute sulfuric-acid links.