Study of Acids, Bases and Salts

Q: Write the equation for the ionisation of sulphuric acid in water and for the dissociation of calcium hydroxide.

H2SO4 + 2H2O → 2H3O+ + SO42-; Ca(OH)2 → Ca2+ + 2OH-.

Q: Explain how a hydronium ion is formed.

The H+ ion released by an acid accepts a lone pair of electrons from the oxygen atom of a water molecule, forming a coordinate bond and giving H3O+.

ICSE Class 10 · Chemistry · Chapter 3

What this chapter covers

Acids, bases and alkalis: definitions and properties
Ions in acids, alkalis and salts
Indicators and the pH scale
Salts and types of salts
Action of dilute acids on salts
Preparation of normal salts

Key definitions

An acid is a compound which, when dissolved in water, produces hydronium ions (H₃O⁺) as the only positive ions.

Basicity of an acid is the number of hydronium ions that one molecule of the acid can produce in aqueous solution, which is the same as the number of replaceable hydrogen atoms in one molecule.

A base is a compound which reacts with an acid to form a salt and water only.

An alkali is a base which is soluble in water and which, on dissolving, produces hydroxyl ions (OH^-) as the only negative ions.

Acidity of a base is the number of hydroxyl ions that one molecule of the base can produce, or the number of molecules of a monobasic acid needed to neutralise one molecule of the base.

A strong acid is an acid which ionises almost completely in water, so that its solution contains mainly ions and very few un-ionised molecules.

A concentrated acid is a solution that contains a large amount of the acid and very little water, while a dilute acid contains a small amount of the acid in a large amount of water.

Neutralisation is the reaction between an acid and a base to form a salt and water only.

Ionisation is the process in which a polar covalent compound, which contains no ions, forms ions when it is dissolved in water.

Dissociation is the process in which the ions already present in an electrovalent compound separate from one another when the compound is melted or dissolved in water.

The hydronium ion is the ion H₃O⁺, formed when a hydrogen ion from an acid combines with a water molecule through a coordinate bond.

The hydroxyl ion is the negatively charged ion OH^-, made of one oxygen atom and one hydrogen atom, which every alkali produces in water.

Key points

An acid gives hydronium ions as the only positive ions in water; an alkali is a soluble base that gives hydroxyl ions as the only negative ions.
A base reacts with an acid to form a salt and water only; every alkali is a base, but most bases are not alkalis.
Basicity is the number of replaceable hydrogen ions in an acid molecule; acidity of a base is the number of hydroxyl ions it gives.
Strength depends on how completely an acid or alkali ionises; concentration depends on how much of it is present in the water.
Acids ionise in water; electrovalent alkalis and salts dissociate into ions they already contain.
A hydronium ion forms when H⁺ accepts a lone pair from the oxygen of water through a coordinate bond.
Litmus is red in acid and blue in alkali; methyl orange is pink in acid and yellow in alkali; phenolphthalein is colourless in acid and pink in alkali.
On the pH scale, 7 is neutral, below 7 is acidic and above 7 is alkaline; each unit is a tenfold change. Universal indicator and pH paper show the pH by colour.
A salt is formed by partial or complete replacement of the replaceable hydrogen of an acid by a metal or ammonium ion.
Salts are normal, acid, basic or complex; only polybasic acids form acid salts.
Dilute acids decompose carbonates and hydrogen carbonates to give CO₂, sulphites and hydrogen sulphites to give SO₂, and sulphides to give H₂S.
Normal salts are prepared by direct combination, displacement, precipitation, neutralisation of an insoluble base, titration of an alkali, or the action of dilute acids on carbonates and hydrogen carbonates.
Insoluble salts are made by precipitation; soluble salts of sodium, potassium and ammonium by titration; other soluble salts with an excess of insoluble base or carbonate that is then filtered off.

Questions answered

Define an acid in terms of the ions it produces in water, and explain why dry hydrogen chloride gas does not turn dry blue litmus red.

An acid is a compound which, when dissolved in water, gives hydronium ions as the only positive ions. Dry HCl gas is covalent and contains no ions; without water, no hydronium ions form, so it shows no acidic property.

Distinguish between a base and an alkali with one example of each.

A base reacts with an acid to give a salt and water only, for example copper(II) oxide, which is insoluble. An alkali is a base that dissolves in water and gives hydroxyl ions as the only negative ions, for example sodium hydroxide.

Acetic acid, CH₃COOH, has four hydrogen atoms in each molecule. Why is it monobasic?

Only the hydrogen attached to the oxygen of the –COOH group can ionise to form a hydronium ion; the three hydrogens bonded to carbon cannot be replaced.

Write the equation for the ionisation of sulphuric acid in water and for the dissociation of calcium hydroxide.

H₂SO₄ + 2H₂O → 2H₃O⁺ + SO₄^2-; Ca(OH)₂ → Ca²⁺ + 2OH^-.

Explain how a hydronium ion is formed.

The H⁺ ion released by an acid accepts a lone pair of electrons from the oxygen atom of a water molecule, forming a coordinate bond and giving H₃O⁺.

State the colour of methyl orange and of phenolphthalein in (a) dilute hydrochloric acid and (b) sodium hydroxide solution.

(a) Methyl orange pink, phenolphthalein colourless. (b) Methyl orange yellow, phenolphthalein pink.

Solution A has a pH of 2 and solution B has a pH of 5. Which is more acidic, and by what factor?

A is more acidic; a difference of 3 pH units means its hydronium ion concentration is 10 × 10 × 10 = 1000 times that of B.

Why is universal indicator more useful than litmus for testing a solution?

Litmus shows only whether a solution is acidic or alkaline, while universal indicator shows a different colour at each pH value and so tells you how strongly acidic or alkaline the solution is.

Past board exam questions

ICSE 2027 sample paper · 1 mark

A student tested the nature of four different solutions P, Q, R, and S using pH paper. He noted the following observations: | | P | Q | R | S | |---|---|---|---|---| | Colour of the pH paper after testing | Blue | Green | Red | Violet | Which of the above solutions is neutral in nature?

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ICSE 2027 sample paper · 1 mark

A teacher demonstrated four different reactions as shown below: 1. Zinc placed in dilute hydrochloric acid in a conical flask. 2. Calcium carbonate heated in a dish on a tripod stand over a burner. 3. Ethanol burning in a spirit lamp. 4. Magnesium carbonate placed in dilute sulphuric acid in a conical flask. Which reactions will produce water?

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

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3 and 4

ICSE 2027 sample paper · 5 marks

Complete the following sentences by choosing the correct option from the brackets: (a) The general formula of an alkyl group is __________ (CₙH₂ₙ₊₁/CₙH₂ₙ₋₁). (b) The aqueous solution of ___________ (zinc chloride/ sodium sulphate) gives white precipitate with barium nitrate. (c) The gas liberated when alkalis are heated with ammonium salts is __________ (N₂/NH₃). (d) The alloy that contains a non-metal as one of its constituents is __________ (bronze/steel). (e) A salt prepared by neutralization in which titration is involved is __________ (CaCl₂/ NH₄Cl).

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(a) CₙH₂ₙ₊₁ (b) sodium sulphate (Na₂SO₄ + Ba(NO₃)₂ → BaSO₄↓ + 2NaNO₃, white precipitate; the marking scheme prints 'zinc chloride', but zinc chloride gives no precipitate with barium nitrate) (c) NH₃ (d) steel (e) NH₄Cl

ICSE 2027 sample paper · 3 marks

N is a metal which forms a basic oxide. N has atomic no. 11. (a) What is the molecular formula of the oxide of N? (b) Write the balanced equation for the reaction that takes place when the oxide of N is treated with water. (c) State the formula of the product obtained other than water when sulphuric acid is added to the product obtained in (b) above.

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N (atomic number 11, configuration 2, 8, 1) is sodium, valency 1. (a) N₂O (i.e. Na₂O) (b) N₂O + H₂O → 2NOH (i.e. Na₂O + H₂O → 2NaOH) (c) N₂SO₄ (i.e. Na₂SO₄); 2NOH + H₂SO₄ → N₂SO₄ + 2H₂O

ICSE 2027 sample paper · 3 marks

Rohit has three solutions X, Y and Z that have pH 2, 7 and 13 respectively. Which solution out of X, Y or Z will: (a) liberate sulphur dioxide gas when heated with sodium sulphite? (b) liberate ammonia gas when reacted with ammonium chloride? (c) not have any effect on litmus paper?

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(a) X (pH 2, acidic) (b) Z (pH 13, alkaline) (c) Y (pH 7, neutral)

ICSE 2027 sample paper · 3 marks

Write balanced chemical equations for the following conversions (A to C): Zn —A→ ZnSO₄ Zn —B→ ZnS ZnSO₄ —C→ Zn(OH)₂

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A: Zn + H₂SO₄ (dil.) → ZnSO₄ + H₂ B: Zn + S → ZnS (on heating) C: ZnSO₄ + 2NaOH → Zn(OH)₂ + Na₂SO₄ (or ZnSO₄ + 2NH₄OH → Zn(OH)₂ + (NH₄)₂SO₄)

ICSE 2026 sample paper · 1 mark

Which of the following will dissociate in aqueous solution, to give a positive ion other than hydronium ion, and a negative ion other than hydroxyl ion?

KOH
dil. HCl
NaCl
CH₃COOH

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NaCl

ICSE 2026 board paper · 1 mark

The electronic configuration of an element is 2,8,2. The hydroxide of this element can produce __________ hydroxyl ions per molecule.

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