Study of Compounds — Sulphuric Acid

Q: Name the catalyst used in the Contact process and state the temperature and pressure used in the converter.

Vanadium pentoxide, V2O5; about 450–500 °C; 1–2 atmospheres.

Q: Write balanced equations for the action of dilute sulphuric acid on (a) zinc, (b) sodium hydrogen carbonate, (c) sodium sulphite, (d) iron(II) sulphide.

(a) Zn + H2SO4 → ZnSO4 + H2; (b) 2NaHCO3 + H2SO4 → Na2SO4 + 2H2O + 2CO2; (c) Na2SO3 + H2SO4 → Na2SO4 + H2O + SO2; (d) FeS + H2SO4 → FeSO4 + H2S.

Q: Write the equation for the reaction of hot concentrated sulphuric acid with sulphur. Which property of the acid does it show?

S + 2H2SO4 → 3SO2 + 2H2O. It shows the acid acting as an oxidising agent.

ICSE Class 10 · Chemistry · Chapter 11

What this chapter covers

Sulphuric acid at a glance
Manufacture by the Contact process
Dilute sulphuric acid as an acid
Sulphuric acid as a dibasic acid
Concentrated sulphuric acid as an oxidising agent
Concentrated sulphuric acid as a dehydrating agent
Non-volatile nature of sulphuric acid
Tests for sulphuric acid

Key definitions

Sulphuric acid is an oxyacid of sulphur with the formula H₂SO₄, in which a sulphur atom is joined to four oxygen atoms, two of which also carry a hydrogen atom each.

The Contact process is the industrial method of manufacturing sulphuric acid in which sulphur dioxide is catalytically oxidised to sulphur trioxide, which is then absorbed in concentrated sulphuric acid and diluted.

Oleum is the thick, fuming liquid formed when sulphur trioxide dissolves in concentrated sulphuric acid, also called fuming sulphuric acid or pyrosulphuric acid, with the formula H₂S₂O₇.

The basicity of an acid is the number of hydrogen ions that one molecule of the acid can give when it ionises in water.

A dibasic acid is an acid that gives two hydrogen ions per molecule when it ionises in water, and so can form two kinds of salt with a base.

A normal salt is a salt formed when all the replaceable hydrogen ions of an acid are replaced by a metal or ammonium ion.

An acid salt is a salt formed when only some of the replaceable hydrogen ions of an acid are replaced by a metal or ammonium ion, so that it still contains replaceable hydrogen.

A dehydrating agent is a substance that removes the elements of water, hydrogen and oxygen in the ratio 2 : 1, from a compound, or removes the water of crystallisation from a hydrated salt.

A drying agent is a substance that absorbs moisture from another substance, such as a gas, without changing that substance chemically.

Sugar charcoal is the black, porous carbon left behind when cane sugar is dehydrated by concentrated sulphuric acid.

A volatile acid is an acid that has a low boiling point and turns into vapour easily on warming.

A non-volatile acid is an acid that has a high boiling point and does not readily turn into vapour on heating.

Key points

Sulphuric acid, H₂SO₄, is a dense, oily, colourless liquid that boils at about 338 °C; it is diluted by adding the acid to water, never water to the acid.
Contact process: S + O₂ → SO₂; 2SO₂ + O₂ rightleftharpoons 2SO₃ over V₂O₅ at 450–500 °C and 1–2 atm; SO₃ + H₂SO₄ → H₂S₂O₇ (oleum); H₂S₂O₇ + H₂O → 2H₂SO₄.
Sulphur trioxide is absorbed in concentrated sulphuric acid, not water, to avoid a dense acid mist.
Dilute sulphuric acid is a strong acid: it gives hydrogen with active metals, a salt and water with oxides and hydroxides, CO₂ with carbonates and hydrogen carbonates, SO₂ with sulphites and hydrogen sulphites, and H₂S with sulphides.
Calcium carbonate stops reacting with dilute sulphuric acid because sparingly soluble calcium sulphate coats it.
Sulphuric acid is dibasic: it forms acid salts (NaHSO₄) and normal salts (Na₂SO₄).
Hot concentrated sulphuric acid is an oxidising agent: C + 2H₂SO₄ → CO₂ + 2SO₂ + 2H₂O and S + 2H₂SO₄ → 3SO₂ + 2H₂O.
Concentrated sulphuric acid is a dehydrating agent: it chars sugar to carbon and turns blue copper(II) sulphate crystals white; it also dries gases other than ammonia.
Being non-volatile, it displaces volatile acids from their salts: HCl from NaCl or KCl, and HNO₃ from NaNO₃ or KNO₃, below 200 °C.
Test: dilute sulphuric acid gives a white precipitate of barium sulphate with barium chloride solution, insoluble in dilute hydrochloric or nitric acid.

Questions answered

Name the catalyst used in the Contact process and state the temperature and pressure used in the converter.

Vanadium pentoxide, V₂O₅; about 450–500 °C; 1–2 atmospheres.

Why is sulphur trioxide not dissolved directly in water in the Contact process? Write the equations for the steps actually used.

The reaction is so exothermic that a dense acid mist forms, which is hard to condense. SO₃ + H₂SO₄ → H₂S₂O₇; H₂S₂O₇ + H₂O → 2H₂SO₄.

Define basicity. What is the basicity of sulphuric acid? Name the two salts it forms with potassium hydroxide.

The basicity of an acid is the number of hydrogen ions one molecule gives on ionisation in water. Sulphuric acid has basicity 2. Potassium hydrogen sulphate, KHSO₄ (acid salt), and potassium sulphate, K₂SO₄ (normal salt).

Write balanced equations for the action of dilute sulphuric acid on (a) zinc, (b) sodium hydrogen carbonate, (c) sodium sulphite, (d) iron(II) sulphide.

(a) Zn + H₂SO₄ → ZnSO₄ + H₂; (b) 2NaHCO₃ + H₂SO₄ → Na₂SO₄ + 2H₂O + 2CO₂; (c) Na₂SO₃ + H₂SO₄ → Na₂SO₄ + H₂O + SO₂; (d) FeS + H₂SO₄ → FeSO₄ + H₂S.

Why does the reaction between marble chips and dilute sulphuric acid stop after a short time?

The calcium sulphate formed is only slightly soluble; it coats the marble chips and prevents the acid from reaching the calcium carbonate underneath.

Copper does not react with dilute sulphuric acid. Give the reason.

Copper is below hydrogen in the activity series, so it cannot displace hydrogen from the acid.

Write the equation for the reaction of hot concentrated sulphuric acid with sulphur. Which property of the acid does it show?

S + 2H₂SO₄ → 3SO₂ + 2H₂O. It shows the acid acting as an oxidising agent.

Describe what you see when concentrated sulphuric acid is added to cane sugar, and write the equation.

The sugar turns yellow, then brown, then black; the mixture becomes hot, steam is given off, and a black, spongy mass of carbon rises up. C₁₂H₂₂O₁₁ → 12C + 11H₂O.

Past board exam questions

ICSE 2027 sample paper · 1 mark

The flow chart given below represents the industrial process for the manufacture of concentrated sulphuric acid by the Contact process. sulphur → sulphur dioxide → sulphur trioxide —(step R)→ sulphuric acid What is used in step R to obtain sulphuric acid from sulphur trioxide?

vanadium pentoxide
water only
water followed by concentrated sulphuric acid
concentrated sulphuric acid followed by water

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concentrated sulphuric acid followed by water

ICSE 2027 sample paper · 1 mark

The following statements are related to the properties of sulphuric acid: 1. Barium chloride reacts with sulphuric acid to give a white precipitate. 2. Sulphur dioxide gas is evolved when sulphuric acid reacts with carbon. 3. Sulphuric acid has a pH value of less than 7.0. 4. Copper (II) oxide reacts with sulphuric acid to form a blue solution. Which two statements confirm the acidic nature of sulphuric acid?

1 and 2
1 and 3
3 and 4
2 and 4

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3 and 4

ICSE 2027 sample paper · 3 marks

A student was given a crystalline salt ‘B’ for analysis which was blue in colour. He added a few drops of dense oily hygroscopic liquid to it and observed a colourless neutral gas/vapour being released and the salt turning white. (a) Identify the blue crystalline salt B. (b) Which property of the hygroscopic liquid is demonstrated in this reaction? (c) Write a balanced chemical equation for the above reaction.

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(a) Copper sulphate pentahydrate, CuSO₄·5H₂O (b) Dehydrating property of concentrated sulphuric acid (c) CuSO₄·5H₂O —(conc. H₂SO₄)→ CuSO₄ + 5H₂O

ICSE 2026 board paper · 1 mark

A non-metal which reacts with concentrated sulphuric acid to form two gases which turn lime water milky is __________.

Sulphur
Carbon
Oxygen
Nitrogen

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Carbon

ICSE 2026 board paper · 1 mark

Assertion (A): Dilute Sulphuric acid is a stronger electrolyte than concentrated Sulphuric acid. Reason (R): Dilute Sulphuric acid has a higher concentration of mobile ions.

(A) is true but (R) is false.
(A) is false but (R) is true.
Both (A) and (R) are true and (R) is the correct explanation of (A).
Both (A) and (R) are true, but (R) is not the correct explanation of (A).

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Both (A) and (R) are true and (R) is the correct explanation of (A).

ICSE 2026 sample paper · 3 marks

State the property exhibited by sulphuric acid in each of the following reactions: (a) Sulphur with concentrated sulphuric acid. (b) Conversion of ferrous sulphide to hydrogen sulphide gas using sulphuric acid. (c) Ethanol with concentrated sulphuric acid.

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(a) Oxidising property (S + 2H₂SO₄ → 3SO₂ + 2H₂O) (b) Acidic property (FeS + H₂SO₄ → FeSO₄ + H₂S) (c) Dehydrating property (C₂H₅OH → C₂H₄ + H₂O)

ICSE 2026 board paper · 3 marks

During the manufacture of sulphuric acid, a chemist collected the gas formed after catalytic oxidation of sulphur dioxide. (a) Name the gas collected. (b) Which acid is formed when the gas collected by the chemist is dissolved directly in water? (c) Why should we not dissolve this gas directly in water?

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(a) Sulphur trioxide (SO₃): 2SO₂ + O₂ ⇌ 2SO₃ (b) Sulphuric acid: SO₃ + H₂O → H₂SO₄ (c) The reaction is highly exothermic; the heat produced forms a dense fog (mist) of sulphuric acid droplets that does not condense easily, so absorption is incomplete. Hence SO₃ is dissolved in concentrated sulphuric acid to form oleum, which is then diluted.

ICSE 2025 board paper · 1 mark

In the reaction given below sulphuric acid acts as a/an: S + 2H₂SO₄ → 3SO₂ + 2H₂O

Non-volatile acid
Dibasic acid
Oxidising agent
Reducing agent

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Oxidising agent