ChemistryICSE Class 10 · Sulphuric Acid

Concentrated Sulphuric Acid as an Oxidising and Dehydrating Agent (ICSE Class 10)

Updated 15 October 2026 · Fact-checked

Hot concentrated sulphuric acid is an oxidising agent: it oxidises carbon to carbon dioxide and sulphur to sulphur dioxide, and is itself reduced to SO₂. It is also a dehydrating agent: it removes the elements of water from sugar, leaving black carbon, and water of crystallisation from blue copper(II) sulphate, leaving a white solid.

Understand Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent

Concentrated sulphuric acid behaves very differently from the dilute acid. Three properties are in the syllabus.

1. Oxidising agent (hot, concentrated). The acid gives up oxygen and is reduced to sulphur dioxide:

Carbon: $C + 2 H_{2} S O_{4} \to C O_{2} + 2 S O_{2} + 2 H_{2} O$
Sulphur: $S + 2 H_{2} S O_{4} \to 3 S O_{2} + 2 H_{2} O$

In each, carbon or sulphur gains oxygen (is oxidised) and is the reducing agent; sulphuric acid is the oxidising agent and is reduced. Sulphur dioxide is recognised by its smell of burning sulphur, by turning acidified potassium dichromate from orange to green, and by decolourising purple acidified potassium permanganate. (Carbon dioxide does neither.)

2. Dehydrating agent. A dehydrating agent removes the elements of water (hydrogen and oxygen in the ratio 2 : 1) from a compound, or removes water of crystallisation. A drying agent only removes moisture (free water) without changing the substance. Concentrated sulphuric acid is both, but the syllabus examples are dehydration:

Sugar: $C_{12} H_{22} O_{11} co n c . H_{2} S O_{4} 12 C + 11 H_{2} O$ . White sugar turns yellow, brown and then a black, spongy, swelling mass of carbon ('charring'), with steam; much heat is given out.
Copper(II) sulphate crystals (blue vitriol): $C u S O_{4} \cdot 5 H_{2} O co n c . H_{2} S O_{4} C u S O_{4} + 5 H_{2} O$ . The blue crystals turn white (anhydrous copper(II) sulphate).

Because the acid reacts with water so vigorously, it is diluted by adding acid slowly to water with stirring, never water to acid.

3. Non-volatile nature. Sulphuric acid has a high boiling point (about 338 °C), so it is non-volatile. Heated with the salt of a volatile acid, it displaces that acid as vapour:

$N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$ (below 200 °C)
$K N O_{3} + H_{2} S O_{4} \to K H S O_{4} + H N O_{3}$ (below 200 °C)

This is how hydrogen chloride and nitric acid are prepared in the laboratory.

Which property is it? Look at the products: SO₂ formed → oxidising; water removed from a compound → dehydrating; a volatile acid driven off → non-volatile; a salt and water (or H₂) with the dilute acid → acidic.

Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent: formulas and key facts

Oxidation of carbon: $C + 2 H_{2} S O_{4} \to C O_{2} + 2 S O_{2} + 2 H_{2} O$
Oxidation of sulphur: $S + 2 H_{2} S O_{4} \to 3 S O_{2} + 2 H_{2} O$
Dehydration of sugar: $C_{12} H_{22} O_{11} \to 12 C + 11 H_{2} O$
Dehydration of blue vitriol: $C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$
Non-volatile nature: $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l; K N O_{3} + H_{2} S O_{4} \to K H S O_{4} + H N O_{3}$
Drying versus dehydrating: Drying agent removes moisture (free water) from a substance
Dehydrating agent removes the elements of water (H and O, 2 to 1) from a compound
Example of drying conc. H₂SO₄ dries HCl gas
Example of dehydrating conc. H₂SO₄ chars sugar
Spotting the property: SO₂ among the products oxidising agent
Water removed from a compound dehydrating agent
HCl or HNO₃ driven off non-volatile acid
Salt and hydrogen or water, dilute acid acidic nature

Drying agent	removes moisture (free water) from a substance
Dehydrating agent	removes the elements of water (H and O, 2 to 1) from a compound
Example of drying	conc. H₂SO₄ dries HCl gas
Example of dehydrating	conc. H₂SO₄ chars sugar

SO₂ among the products	oxidising agent
Water removed from a compound	dehydrating agent
HCl or HNO₃ driven off	non-volatile acid
Salt and hydrogen or water, dilute acid	acidic nature

How to solve Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent questions

Most questions show a reaction and ask which property of sulphuric acid it shows, or ask for the equation and observation. Use the product clues.

1Read the products: SO₂ means the acid was reduced, so it acted as an oxidising agent.
2If the other reactant loses H and O in a 2 : 1 ratio (sugar) or loses water of crystallisation (CuSO₄·5H₂O), it is dehydration.
3If HCl or HNO₃ is produced from a chloride or nitrate, it is the non-volatile nature.
4Write the balanced equation (C + 2H₂SO₄; S + 2H₂SO₄ → 3SO₂).
5Give the observation (black spongy mass; blue to white; brown or choking gas that turns dichromate green).
6In redox items, name the oxidising agent (H₂SO₄) and the reducing agent (C or S).

Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent in real life

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Charring sugar safely

Suppose a teacher pours concentrated sulphuric acid on 17.1 g of table sugar (sucrose, C₁₂H₂₂O₁₁) in a fume cupboard for a demonstration. (C = 12, H = 1, O = 16)

Describe what the class sees, and calculate the maximum mass of carbon left and of water removed.

Try it first, then show the solutionSolution

Observation: sugar turns yellow, then brown, then a black spongy column of carbon rises, with steam and heat.
Molar mass of sucrose $= 342$ ; moles $= 17.1/342 = 0.05$ .
Carbon $= 12 \times 0.05 \times 12 = 7.2$ g; water $= 11 \times 0.05 \times 18 = 9.9$ g.

Answer: A black spongy mass of up to 7.2 g of carbon; 9.9 g of water removed.

Memory hook: Sugar minus water equals carbon: that is dehydration you can see.

Tricks to remember Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent

Short memory aids. Say each one aloud once, then use it on a question the same day.

SO₂ means oxidising
If sulphuric acid turns into SO₂, it gave away oxygen: oxidising agent.
Use it when: Identifying the property from an equation.
Drying takes moisture, dehydrating takes H₂O out of molecules
Dry = towel on a wet surface; dehydrate = pulling water out of the molecule itself.
Use it when: The drying versus dehydrating question.
Always add acid
'Do what you oughta, add acid to water.'
Use it when: Safe dilution.

Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent: solved examples

Ordered from board level to HOTS. Try each one before you open the solution.

Example 1: Which substance is reduced? · 1 mark · MCQBoard level

In the reaction $C + 2 H_{2} S O_{4} \to C O_{2} + 2 S O_{2} + 2 H_{2} O$ , the substance that is reduced is:
(a) carbon
(b) sulphuric acid
(c) carbon dioxide
(d) water

Try it first, then show the solutionSolution

Carbon gains oxygen and becomes CO₂: it is oxidised (the reducing agent).
Sulphuric acid loses oxygen and becomes SO₂: it is reduced, acting as the oxidising agent.

Answer: (b) sulphuric acid, which is the oxidising agent.

Example 2: Blue vitriol · 2 marks · Short answerBoard level

State what is observed when concentrated sulphuric acid is added to blue copper(II) sulphate crystals, and write the equation. Name the property shown.

Try it first, then show the solutionSolution

The blue crystals turn white.
$C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$ ; dehydrating property.

Answer: Blue to white; dehydrating agent.

Example 3: Name the property · 3 marks · Short answerHarder board level

Name the property of sulphuric acid shown in each: (a) $C + 2 H_{2} S O_{4} \to C O_{2} + 2 S O_{2} + 2 H_{2} O$ ; (b) $N a C l + H_{2} S O_{4} \to N a H S O_{4} + H C l$ ; (c) $C_{12} H_{22} O_{11} \to 12 C + 11 H_{2} O$ .

Try it first, then show the solutionSolution

(a) SO₂ is formed: oxidising agent.
(b) Volatile HCl is displaced: non-volatile nature.
(c) Elements of water removed: dehydrating agent.

Answer: (a) Oxidising (b) non-volatile (c) dehydrating.

Example 4: Drying or dehydrating? · 3 marks · Short answerHarder board level

(a) Distinguish between a drying agent and a dehydrating agent. (b) Give one example of each action of concentrated sulphuric acid. (c) Why can concentrated sulphuric acid not be used to dry ammonia?

Try it first, then show the solutionSolution

(a) A drying agent removes moisture (free water) from a substance; a dehydrating agent removes the elements of water from a compound.
(b) Drying: it dries HCl or SO₂ gas. Dehydrating: it chars sugar.
(c) It is an acid and reacts with basic ammonia: $2 N H_{3} + H_{2} S O_{4} \to (N H_{4})_{2} S O_{4}$ .

Answer: As above.

Example 5: Gas volumes from carbon · 4 marks · Long answerHOTS / competency

0.36 g of carbon is heated with excess concentrated sulphuric acid. Calculate (a) the volumes of CO₂ and SO₂ formed at S.T.P., (b) the mass of sulphuric acid used. Suggest a test to show that SO₂ is present in the gas mixture. (C = 12, H = 1, S = 32, O = 16)

Try it first, then show the solutionSolution

Moles of C $= 0.36/12 = 0.03$ .
(a) CO₂ $= 0.03 \times 22.4 = 0.672$ L; SO₂ $= 0.06 \times 22.4 = 1.344$ L.
(b) H₂SO₄ $= 0.06 \times 98 = 5.88$ g.
Test: pass the gases through acidified potassium dichromate; it turns from orange to green (CO₂ does not do this).

Answer: (a) 0.672 L CO₂, 1.344 L SO₂ (b) 5.88 g; dichromate turns green.

Example 6: Sulphur and the acid · 3 marks · Short answerHOTS / competency

0.8 g of sulphur is heated with excess concentrated sulphuric acid. (a) Write the equation. (b) Calculate the volume of SO₂ at S.T.P. and the mass of acid used. (c) Why is all the SO₂ not counted as coming from the sulphur? (S = 32, H = 1, O = 16)

Try it first, then show the solutionSolution

(a) $S + 2 H_{2} S O_{4} \to 3 S O_{2} + 2 H_{2} O$ .
(b) Moles of S $= 0.025$ ; SO₂ $= 0.075$ mol $= 1.68$ L; H₂SO₄ $= 0.05$ mol $= 4.9$ g.
(c) Of the 3 SO₂, one comes from the oxidised sulphur and two from the reduced sulphuric acid.

Answer: (a) As above. (b) 1.68 L; 4.9 g. (c) Two-thirds of the SO₂ comes from the acid.

Where students lose marks in Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent

The slips examiners see most often, and what to write instead.

Mistake: Choosing SO₂ as the oxidising agent
In 2024 (Q1(vii)) candidates struggled with oxidising-agent ideas and chose sulphur dioxide.
Do this: The oxidising agent is the reactant that gives oxygen: sulphuric acid.
Usually costs: 1 mark.
Mistake: Swapping oxidising and reducing roles
In 2025 (Q1(v)) candidates confused oxidising and reducing agents.
Do this: Substance oxidised = reducing agent (S or C); substance reduced = oxidising agent (H₂SO₄).
Usually costs: 1 mark.
Mistake: Unbalanced carbon equation
In 2025 (Q5(iii)) many failed to balance the reaction of concentrated sulphuric acid with carbon.
Wrong working
C + H₂SO₄ → CO₂ + SO₂ + H₂O
Right working
C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O
Do this: Two molecules of acid per carbon atom.
Usually costs: 1 mark.
Mistake: Drying agent and dehydrating agent confused
The 2025 report lists 'drying agent versus dehydrating agent' among confused concepts, and in 2024 (Q3(i)) candidates wrote only 'concentrated' instead of the full corrected sentence.
Do this: Learn both definitions with an example, and write complete sentences in 'rewrite' questions.
Usually costs: 1 mark.

Exam tips

Always write 'concentrated' (and 'hot' for oxidising reactions); the dilute acid behaves only as an acid.
Learn the SO₂ tests: orange dichromate to green; purple permanganate decolourised; smell of burning sulphur.
For the sugar reaction, describe the colour sequence and the spongy black mass.
Non-volatile nature is shown only by reactions that release a volatile acid (HCl, HNO₃).

Practice questions on Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
The gas formed when sulphur is heated with concentrated sulphuric acid turns paper soaked in acidified potassium dichromate from orange to green. The gas is:
(a) CO₂
(b) SO₂
(c) H₂S
(d) O₂
Hint
The acid is reduced; which gas is a reducing agent that smells of burning sulphur?
Answer
(b) SO₂.
Q2 · 1 mark · Very short answerBoard level
What is a dehydrating agent?
Hint
Elements of water.
Answer
A substance that removes the elements of water (hydrogen and oxygen in the ratio 2 : 1) from a compound.
Q3 · 2 marks · Short answerBoard level
Write the equation for the action of hot concentrated sulphuric acid on carbon and name the reducing agent.
Hint
Carbon gains oxygen.
Answer
$C + 2 H_{2} S O_{4} \to C O_{2} + 2 S O_{2} + 2 H_{2} O$ ; carbon is the reducing agent.
Q4 · 1 mark · Very short answerHarder board level
Why is water never added to concentrated sulphuric acid?
Hint
Heat.
Answer
So much heat is released that the water boils and spatters acid out; always add acid slowly to water.
Q5 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Concentrated sulphuric acid turns sugar black.
Reason (R): Concentrated sulphuric acid removes hydrogen and oxygen from sugar in the ratio 2 : 1.
(a) Both A and R are true and R is the correct explanation of A.
(b) Both A and R are true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true.
Hint
Dehydration leaves carbon.
Answer
(a).
Q6 · 2 marks · Short answerHarder board level
A student writes: 'Dilute sulphuric acid turns blue copper(II) sulphate crystals white.' Correct the statement, name the property shown and write the equation.
Hint
Which form of the acid removes water of crystallisation?
Answer
'Concentrated sulphuric acid turns blue copper(II) sulphate crystals white.' Property: dehydrating agent. $C u S O_{4} \cdot 5 H_{2} O \to C u S O_{4} + 5 H_{2} O$ .
Q7 · 3 marks · Short answerHarder board level
4.99 g of blue copper(II) sulphate crystals are completely dehydrated by concentrated sulphuric acid. What mass of water is removed and what mass of white solid remains? (Cu = 63.5, S = 32, O = 16, H = 1)
Hint
M of CuSO₄·5H₂O = 249.5.
Answer
Moles $= 4.99/249.5 = 0.02$ . Water $= 0.1$ mol $= 1.8$ g. CuSO₄ $= 0.02 \times 159.5 = 3.19$ g.
Q8 · 3 marks · Short answerHOTS / competency
Suppose a gas mixture from heating carbon with concentrated sulphuric acid is passed first through acidified potassium permanganate and then through lime water. Predict what happens in each and explain.
Hint
Two gases, two tests.
Answer
The permanganate is decolourised (purple to colourless) because SO₂ is a reducing gas and is absorbed. The remaining CO₂ then turns lime water milky (CaCO₃ forms). (SO₂ is removed first so it cannot interfere with the lime-water test.)

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.Product of carbon with hot conc. H₂SO₄?Show answer
CO₂, SO₂ and water.
2.Colour change of CuSO₄·5H₂O with conc. H₂SO₄?Show answer
Blue to white.
3.Boiling point of sulphuric acid (approx.)?Show answer
About 338 °C.
4.Moles of SO₂ from 1 mol S with conc. H₂SO₄?Show answer
3.
5.Which property gives HCl from NaCl?Show answer
Non-volatile nature.

Concentrated Sulphuric Acid: Oxidising and Dehydrating Agent: frequently asked questions

Why is concentrated sulphuric acid called an oxidising agent?

When hot it gives oxygen to substances such as carbon and sulphur and is itself reduced to sulphur dioxide.

What is the difference between a drying agent and a dehydrating agent?

A drying agent removes free moisture; a dehydrating agent removes the elements of water from a compound. Concentrated H₂SO₄ can act as both.

What happens when concentrated sulphuric acid is added to sugar?

The sugar is dehydrated: it turns yellow, brown and then into a black, spongy mass of carbon, with steam and much heat.

Why is sulphuric acid used to prepare HCl and HNO₃?

It is non-volatile, so it displaces the more volatile acids from their salts on heating.