ChemistryICSE Class 10 · Chapter 3

Study of Acids, Bases and Salts ICSE Class 10: Ions, pH and Preparing Salts

Q: Do I need to describe the apparatus for preparing salts in ICSE Class 10?

No. The 2027 syllabus says details of apparatus or procedures are not required; you need the method and the balanced equation.

Q: Are double salts like potash alum in the 2027 syllabus?

The 2027 syllabus lists normal, acid, basic and complex salts. Learn those four with examples.

Q: Is every base an alkali?

No. An alkali is a base that dissolves in water. Copper(II) oxide and iron(III) hydroxide are bases but not alkalis.

Q: Why is iron(III) chloride made by direct combination?

Prepared from solution, it takes up water and is hydrolysed, so the anhydrous salt is made by heating iron in dry chlorine.

Updated 14 October 2026 · Fact-checked

An acid gives hydronium ions as its only positive ions in water; an alkali gives hydroxide ions as its only negative ions. A salt forms when the hydrogen of an acid is replaced by a metal. ICSE Class 10 covers indicators, pH, normal, acid, basic and complex salts, and six methods of preparing normal salts.

What Study of Acids, Bases and Salts is about

This chapter is about ions. In water an acid releases H⁺ ions, which join water molecules to form hydronium ions, H₃O⁺; an alkali releases hydroxide ions, OH⁻. Indicators and pH paper detect them; neutralisation joins them into water; and the leftover ions form a salt.

The CISCE 2027 syllabus covers: (i) definitions of acids, bases and alkalis in terms of their ions, with ionisation and dissociation equations; (ii) the indicators litmus, methyl orange and phenolphthalein, pH paper and universal indicator, and the pH scale; (iii) types of salts: normal, acid, basic and complex salts; (iv) the action of dilute acids on carbonates, hydrogen carbonates, sulphites, hydrogen sulphites and sulphides; and (v) six methods of preparing normal salts, with equations: direct combination, displacement, precipitation (double decomposition), neutralisation of an insoluble base, neutralisation of an alkali (titration), and the action of dilute acids on carbonates and hydrogen carbonates. Details of apparatus are not required, but you must be able to choose the right method for a given salt, which depends on solubility.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Study of Acids, Bases and Salts is in the 3. Study of Acids, Bases and Salts part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Acids: definition, ionisation and basicity

An acid is a compound which, when dissolved in water, gives hydronium ions (H₃O⁺) as the only positive ions. It turns blue litmus red.

$H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$

$H_{2} S O_{4} + 2 H_{2} O \to 2 H_{3} O^{+} + S O_{4}^{2 -}$

An acid is a covalent compound that ionises in water (new ions form). Basicity is the number of hydronium ions one molecule can give: HCl and HNO₃ are monobasic, H₂SO₄ dibasic, H₃PO₄ tribasic. Acetic acid, CH₃COOH, is monobasic: only the H in –COOH is replaceable.

A strong acid (HCl, HNO₃, H₂SO₄) ionises almost completely, so its solution contains mostly ions. A weak acid (acetic acid, carbonic acid) ionises only partly, so its solution contains both molecules and ions.

Worked example

State the basicity of (a) sulphuric acid, (b) phosphoric acid, (c) acetic acid. Arrange them in increasing order of basicity.

Try it first, then show the solutionSolution

H₂SO₄ gives 2 H₃O⁺ per molecule: dibasic.
H₃PO₄ gives 3: tribasic.
CH₃COOH gives 1 (only the carboxyl H): monobasic.

Answer: 2, 3, 1; increasing order: acetic acid < sulphuric acid < phosphoric acid.

Bases and alkalis: definitions and acidity

A base is a compound that reacts with an acid to give a salt and water only. Metal oxides and hydroxides are bases (CuO, ZnO, NaOH).

An alkali is a base that is soluble in water and gives hydroxide ions (OH⁻) as the only negative ions. It turns red litmus blue. All alkalis are bases, but not all bases are alkalis (CuO is insoluble).

$N a O H \to N a^{+} + O H^{-}$ (dissociation; the ions already exist in the solid)

$N H_{4} O H ⇌ N H_{4}^{+} + O H^{-}$ (a weak alkali, partly ionised)

Acidity of a base is the number of OH⁻ ions per molecule: NaOH monoacidic, Ca(OH)₂ diacidic, Al(OH)₃ triacidic.

Worked example

Why is copper(II) oxide a base but not an alkali?

Try it first, then show the solutionSolution

CuO reacts with dilute sulphuric acid to form a salt and water only: it is a base.
It is insoluble in water, so it cannot release OH⁻ ions into solution.

Answer: It neutralises acids but does not dissolve in water.

Indicators and the pH scale

Indicator	Acid	Neutral	Alkali
Litmus	blue to red	no change	red to blue
Methyl orange	pink (red)	orange	yellow
Phenolphthalein	colourless	colourless	pink

The pH scale runs from 0 to 14. pH 7 is neutral (pure water); below 7 is acidic, and the lower the pH, the more H₃O⁺ ions; above 7 is alkaline, and the higher the pH, the more OH⁻ ions. Universal indicator or pH paper shows a range of colours, from red (strong acid) through green (neutral) to violet (strong alkali), so it gives the pH, not just 'acid or alkali'.

Worked example

Solutions P, Q and R have pH 13, 6 and 1. Which (a) contains mainly molecules with a few ions as an acid, (b) turns phenolphthalein pink, (c) is a strong acid?

Try it first, then show the solutionSolution

pH 6 is weakly acidic: a weak acid, partly ionised (molecules + ions).
pH 13 is strongly alkaline: phenolphthalein pink.
pH 1 is a strong acid.

Answer: (a) Q (b) P (c) R

Types of salts: normal, acid, basic and complex

A salt is formed when the hydrogen ions of an acid are partly or completely replaced by a metal ion (or the ammonium ion).

Normal salt: all replaceable H replaced. NaCl, Na₂SO₄, Na₂CO₃.
Acid salt: H only partly replaced, so the salt still has replaceable H. NaHSO₄, NaHCO₃, NaHSO₃. Only a polybasic acid can form acid salts.
Basic salt: OH of a base only partly replaced by an acid radical. Examples: basic lead chloride Pb(OH)Cl, basic magnesium chloride Mg(OH)Cl, basic copper carbonate CuCO₃·Cu(OH)₂.
Complex salt: gives a complex ion (a metal ion bonded to other ions or molecules) in solution. Examples: sodium argentocyanide Na[Ag(CN)₂], which gives the [Ag(CN)₂]⁻ ion, and tetraammine copper(II) sulphate [Cu(NH₃)₄]SO₄, which gives the deep blue [Cu(NH₃)₄]²⁺ ion.

Worked example

Classify: KHSO₄, K₂SO₄, Pb(OH)Cl, Na[Ag(CN)₂].

Try it first, then show the solutionSolution

KHSO₄ still has one replaceable H: acid salt.
K₂SO₄ has none: normal salt.
Pb(OH)Cl has an OH left: basic salt.
Na[Ag(CN)₂] contains a complex ion: complex salt.

Answer: Acid, normal, basic, complex.

Action of dilute acids on carbonates, sulphites and sulphides

Dilute acids decompose these salts, releasing a gas you can identify:

Carbonate / hydrogen carbonate → CO₂: $N a_{2} C O_{3} + 2 H C l \to 2 N a C l + H_{2} O + C O_{2}$ ; $N a H C O_{3} + H C l \to N a C l + H_{2} O + C O_{2}$ . CO₂ turns lime water milky.
Sulphite / hydrogen sulphite → SO₂ (warm): $N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$ ; $N a H S O_{3} + H C l \to N a C l + H_{2} O + S O_{2}$ . SO₂ smells of burning sulphur and turns acidified potassium dichromate paper from orange to green.
Sulphide → H₂S: $F e S + H_{2} S O_{4} \to F e S O_{4} + H_{2} S$ ; $N a_{2} S + 2 H C l \to 2 N a C l + H_{2} S$ . H₂S smells of rotten eggs and turns lead acetate paper silvery black.

Both CO₂ and SO₂ turn lime water milky; the dichromate test tells them apart.

Worked example

A white salt gives a gas with dilute HCl on warming; the gas turns acidified potassium dichromate green. Name the anion and write an equation using sodium as the metal.

Try it first, then show the solutionSolution

Dichromate turning green indicates SO₂.
SO₂ from an acid comes from a sulphite (or hydrogen sulphite).
$N a_{2} S O_{3} + 2 H C l \to 2 N a C l + H_{2} O + S O_{2}$

Answer: Sulphite ion; the equation above.

Choosing a method to prepare a normal salt

The method depends on whether the salt and the base are soluble.

Solubility rules you need: all sodium, potassium and ammonium salts and all nitrates are soluble. Chlorides are soluble except AgCl and PbCl₂ (PbCl₂ dissolves in hot water). Sulphates are soluble except BaSO₄ and PbSO₄ (CaSO₄ is only slightly soluble). Carbonates are insoluble except those of Na, K and NH₄⁺.

1. Insoluble salt → precipitation (two soluble salts, or acid + soluble salt): $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$ ; $P b (N O_{3})_{2} + H_{2} S O_{4} \to P b S O_{4} + 2 H N O_{3}$ . Filter, wash, dry.
2. Soluble salt of Na, K or NH₄⁺ → titration (alkali + acid): $N a O H + H C l \to N a C l + H_{2} O$ ; $K O H + H N O_{3} \to K N O_{3} + H_{2} O$ . An indicator finds the exact volumes, then the experiment is repeated without it.
3. Soluble salt of a moderately active metal (Zn, Fe, Mg) → displacement: $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ .
4. Soluble salt of a metal that does not react with dilute acids (Cu) → insoluble base or carbonate + acid: $C u O + H_{2} S O_{4} \to C u S O_{4} + H_{2} O$ ; $C u C O_{3} + H_{2} S O_{4} \to C u S O_{4} + H_{2} O + C O_{2}$ . Add excess solid, filter, evaporate, crystallise.
5. Salt that is hydrolysed by water or made of two elements → direct combination: $2 F e + 3 C l_{2} \to 2 F e C l_{3}$ (anhydrous iron(III) chloride); $F e + S \to F e S$ .

Lead and calcium are not used with dilute sulphuric acid because the insoluble sulphate coats the metal and stops the reaction.

Worked example

Suggest a method and give the equation for preparing: (a) lead chloride, (b) potassium sulphate, (c) copper(II) nitrate, (d) zinc sulphate.

Try it first, then show the solutionSolution

(a) PbCl₂ is insoluble in cold water: precipitation, $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$ .
(b) Potassium salt: titration, $2 K O H + H_{2} S O_{4} \to K_{2} S O_{4} + 2 H_{2} O$ .
(c) Copper does not react with dilute acids to give H₂: insoluble base, $C u O + 2 H N O_{3} \to C u (N O_{3})_{2} + H_{2} O$ .
(d) Zinc is moderately active: displacement, $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$ .

Answer: (a) Precipitation (b) Titration (c) Neutralisation of an insoluble base (d) Displacement

Chemistry in real life: Study of Acids, Bases and Salts

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.

Action of acids on carbonates

Suppose a student drops a piece of marble chip into dilute hydrochloric acid while testing whether a temple step might be harmed by acidic cleaning liquid.

What will she observe? Write the equation and name a test for the gas.

Try it first, then show the solutionSolution

Marble is calcium carbonate.
Acids decompose carbonates to give a salt, water and carbon dioxide: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$ .
Effervescence is seen; the gas turns lime water milky.

Answer: Brisk effervescence; CO₂, which turns lime water milky; the marble slowly dissolves.

Memory hook: Acid on marble fizzes because carbonate always gives CO₂.

Choosing a preparation method

Suppose a school laboratory has run out of copper(II) sulphate crystals but has copper turnings, copper(II) oxide powder and dilute sulphuric acid.

Which starting material should be used, and why not the other one? Write the equation.

Try it first, then show the solutionSolution

Copper is below hydrogen in the activity series and does not react with dilute sulphuric acid.
Copper(II) oxide is an insoluble base, so it neutralises the acid: $C u O + H_{2} S O_{4} \to C u S O_{4} + H_{2} O$ .
Add excess oxide to warm acid, filter off the excess, evaporate and crystallise.

Answer: Use copper(II) oxide; copper turnings do not react with the dilute acid.

Memory hook: If the metal won't react, use its oxide.

pH in daily life

Suppose a gardener tests three samples with pH paper: tap water (pH 7), a soap solution (pH 10) and vinegar (pH 3), and wants to know which ones would turn phenolphthalein pink and which would turn methyl orange pink.

Predict the colours of both indicators in each sample.

Try it first, then show the solutionSolution

Phenolphthalein is pink only in alkalis: soap solution.
Methyl orange is pink in acids (vinegar), orange in neutral water and yellow in alkali (soap).

Answer: Phenolphthalein: pink in soap, colourless in the others. Methyl orange: pink in vinegar, orange in water, yellow in soap.

Memory hook: Phenolphthalein pinks for alkalis; methyl orange pinks for acids.

Topics in this chapter

Preparation of Normal SaltsComing soon
pH Scale and IndicatorsComing soon
Ionisation of Acids, Bases and SaltsComing soon
Acids, Alkalis, Indicators and the pH Scale
Methods of Preparing Normal Salts
Types of Salts and Action of Dilute Acids on Salts

Study of Acids, Bases and Salts formula sheet

Acid in water: $H C l + H_{2} O \to H_{3} O^{+} + C l^{-}$; Hydronium ions are the only positive ions.
Alkali in water: $N a O H \to N a^{+} + O H^{-}$; Hydroxide ions are the only negative ions.
Neutralisation (ionic): $H_{3} O^{+} + O H^{-} \to 2 H_{2} O$; Acid + base → salt + water.
Carbonate + acid: $C a C O_{3} + 2 H C l \to C a C l_{2} + H_{2} O + C O_{2}$; CO₂ turns lime water milky.
Sulphite + acid: $N a_{2} S O_{3} + H_{2} S O_{4} \to N a_{2} S O_{4} + H_{2} O + S O_{2}$; SO₂ turns acidified K₂Cr₂O₇ green.
Sulphide + acid: $F e S + 2 H C l \to F e C l_{2} + H_{2} S$; H₂S blackens lead acetate paper.
Direct combination: $2 F e + 3 C l_{2} \to 2 F e C l_{3}$; Anhydrous iron(III) chloride.

Study of Acids, Bases and Salts: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

pH scale with indicator colours

Revision sketch or to answer pH and indicator MCQs.

Draw

A horizontal bar numbered 0 to 14.
Mark 7 as neutral; label 0-6 'acidic (stronger towards 0)' and 8-14 'alkaline (stronger towards 14)'.
Under the bar, write the universal indicator colours: red, orange, yellow, green (7), blue, violet.

Label

acidic
neutral
alkaline
increasing H₃O⁺
increasing OH⁻

Watch out: Add rows for methyl orange (pink | orange | yellow) and phenolphthalein (colourless | colourless | pink).

How the board asks Study of Acids, Bases and Salts

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
Match salt with preparation method	Section A or B: a list of salts and a list of methods (precipitation, titration, displacement, direct combination, neutralisation of insoluble base).	First decide: is the salt soluble? Is it a Na/K/NH₄ salt? Does the metal react with the acid? Then pick the method.
pH and indicator questions	Solutions with given pH values: which is a weak acid, which turns phenolphthalein pink, which contains only ions.	Below 7 acid (lower = stronger), above 7 alkali; weak = molecules and ions.
Equations for preparing salts	Section A, Question 2 or Section B: write balanced equations for preparing a named salt by a named method.	Write the method's general form and balance; mark the precipitate or gas.
Identify the gas from a salt	A salt + dilute acid gives a gas with a given test; name the gas and the anion.	Lime water only: CO₂ (or SO₂); dichromate green: SO₂; lead acetate black: H₂S.
Types of salts and definitions	Classify NaHCO₃, Pb(OH)Cl, K₂SO₄; define acid salt, basic salt, alkali.	Look for leftover replaceable H (acid salt), leftover OH (basic salt), a bracketed complex ion (complex salt).

Tricks to remember Study of Acids, Bases and Salts

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

Methyl orange: 'Pink acid, Yellow alkali'
Think of a pink lemon sherbet (acid) and a yellow soap (alkali); in the middle it's orange.
Use it when: Indicator colours.
Phenolphthalein only blushes in alkali
Phenolphthalein is colourless in acids and neutral solutions and turns pink only in alkalis.
Use it when: Titration end points and MCQs.
Insoluble? Precipitate it!
If the salt won't dissolve, make it fall out of solution: precipitation. If it's a Na, K or NH₄ salt, titrate.
Use it when: Choosing the method.
Gases from salts: C-S-S
Carbonate → CO₂ (lime water); Sulphite → SO₂ (dichromate green); Sulphide → H₂S (lead acetate black).
Use it when: Identification questions.

Study of Acids, Bases and Salts: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Amphoteric hydroxide · 1 mark · MCQBoard level

In which pair do BOTH hydroxides dissolve in dilute hydrochloric acid and also in excess sodium hydroxide solution?
(a) Mg(OH)₂ and Pb(OH)₂
(b) Zn(OH)₂ and Al(OH)₃
(c) Ca(OH)₂ and Zn(OH)₂
(d) Fe(OH)₃ and Al(OH)₃

Try it first, then show the solutionSolution

A hydroxide that reacts with both an acid and an alkali is amphoteric: those of zinc, aluminium and lead.
Mg(OH)₂, Ca(OH)₂ and Fe(OH)₃ are only basic, so every pair containing one of them is wrong.

Answer: (b) Zn(OH)₂ and Al(OH)₃

Example 2: Insoluble salt · 1 mark · MCQHarder board level

The best pair of reactants for preparing barium sulphate is:
(a) barium + dilute sulphuric acid
(b) barium chloride solution + sodium sulphate solution
(c) barium oxide + sodium sulphate
(d) barium carbonate + sulphuric acid

Try it first, then show the solutionSolution

BaSO₄ is insoluble, so it should be made by precipitation from two soluble compounds.
(a) and (d) would coat the solid with insoluble sulphate and stop.

Answer: (b): $B a C l_{2} + N a_{2} S O_{4} \to B a S O_{4} + 2 N a C l$

Example 3: Weak and strong · 1 mark · Assertion-reasonBoard level

Assertion (A): A solution of acetic acid contains both ions and molecules.
Reason (R): Acetic acid is a weak acid that ionises only partially in water.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

Partial ionisation leaves un-ionised molecules alongside the ions.

Answer: (a)

Example 4: Preparation equations · 3 marks · Short answerBoard level

Write balanced equations for preparing: (a) sodium sulphate by titration, (b) iron(II) sulphide by direct combination, (c) calcium carbonate by precipitation.

Try it first, then show the solutionSolution

(a) $2 N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O$
(b) $F e + S \to F e S$ (heat)
(c) $C a C l_{2} + N a_{2} C O_{3} \to C a C O_{3} + 2 N a C l$

Answer: As above.

Example 5: Identify by method · 3 marks · Short answerHarder board level

Name the method used to prepare each salt and give a reason: (a) ammonium sulphate, (b) silver chloride, (c) anhydrous iron(III) chloride.

Try it first, then show the solutionSolution

(a) Ammonium salt, soluble: titration, $2 N H_{4} O H + H_{2} S O_{4} \to (N H_{4})_{2} S O_{4} + 2 H_{2} O$ .
(b) AgCl is insoluble: precipitation, $A g N O_{3} + N a C l \to A g C l + N a N O_{3}$ .
(c) Iron(III) chloride takes up water and is hydrolysed, so it is made dry by direct combination: $2 F e + 3 C l_{2} \to 2 F e C l_{3}$ .

Answer: (a) Titration (b) Precipitation (c) Direct combination

Example 6: Unknown salt · 3 marks · Short answerHOTS / competency

A white salt X gives a colourless gas with dilute H₂SO₄ without heating. The gas turns lime water milky but has no effect on acidified potassium dichromate. A solution of X is only weakly alkaline (pH about 8).(a) Name the gas. (b) Name the anion in X. (c) Suggest X if its cation is sodium and it is an acid salt.

Try it first, then show the solutionSolution

Milky lime water, no dichromate change: CO₂, not SO₂.
CO₂ from an acid: carbonate or hydrogen carbonate anion.
An acid salt of sodium giving CO₂: sodium hydrogen carbonate (its solution is weakly alkaline).

Answer: (a) CO₂ (b) hydrogen carbonate (c) NaHCO₃

Example 7: Why not lead + sulphuric acid? · 2 marks · Short answerHarder board level

Explain why lead sulphate is not prepared by adding lead to dilute sulphuric acid. Suggest a method with an equation.

Try it first, then show the solutionSolution

PbSO₄ is insoluble; it forms a coating on the lead which stops further reaction.
Precipitation: $P b (N O_{3})_{2} + H_{2} S O_{4} \to P b S O_{4} + 2 H N O_{3}$ (or with Na₂SO₄).

Answer: Insoluble coating; use precipitation from lead nitrate.

Example 8: Acid salt from a dibasic acid · 3 marks · Short answerHarder board level

Sodium hydroxide is added to sulphuric acid in two different mole ratios. Write the equations for the formation of (a) an acid salt and (b) a normal salt. Why can hydrochloric acid not form an acid salt?

Try it first, then show the solutionSolution

(a) 1 : 1: $N a O H + H_{2} S O_{4} \to N a H S O_{4} + H_{2} O$
(b) 2 : 1: $2 N a O H + H_{2} S O_{4} \to N a_{2} S O_{4} + 2 H_{2} O$
HCl is monobasic: it has only one replaceable H, so it is either fully replaced or not at all.

Answer: NaHSO₄ (acid salt) and Na₂SO₄ (normal salt); monobasic acids give only normal salts.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 1 + 2 marks.

Five reagents, four salts

Suppose a technician has these reagents: zinc granules, copper(II) carbonate, lead nitrate solution, potassium hydroxide solution, and dilute hydrochloric, nitric and sulphuric acids.

(i) · 1 mark
Write the equation for preparing zinc chloride and name the method.
Answer
Answer: Displacement: $Z n + 2 H C l \to Z n C l_{2} + H_{2}$
(ii) · 1 mark
Write the equation for preparing copper(II) sulphate from these reagents and name the method.
Answer
Answer: Action of acid on a carbonate: $C u C O_{3} + H_{2} S O_{4} \to C u S O_{4} + H_{2} O + C O_{2}$
(iii) · 1 mark
Which reagent pair gives lead chloride? Name the method.
Answer
Answer: Lead nitrate solution + dilute hydrochloric acid: precipitation. $P b (N O_{3})_{2} + 2 H C l \to P b C l_{2} + 2 H N O_{3}$
(iv) · 2 marks
Potassium nitrate is made by titration. Why is an indicator used, and why is the experiment repeated without it?
Answer
Answer: The indicator shows the exact volumes needed for neutralisation; the experiment is then repeated with those volumes and no indicator so the salt is not contaminated by it.

pH of five solutions

Suppose a student records: A pH 1, B pH 4, C pH 7, D pH 9, E pH 14.

(i) · 1 mark
Which is likely to be a weak acid solution? State what particles it contains.
Answer
Answer: B; both molecules and ions.
(ii) · 1 mark
Which solutions turn phenolphthalein pink?
Answer
Answer: D and E.
(iii) · 2 marks
Equal volumes of A and E of matching strength are mixed. What is the likely pH, and what does the mixture contain?
Answer
Answer: About 7; a salt solution and water (neutralisation).

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Write balanced equations for the action of dilute hydrochloric acid on (a) sodium hydrogen sulphite, (b) sodium sulphide. (2 marks) (2 marks)

Step	Marks
$N a H S O_{3} + H C l \to N a C l + H_{2} O + S O_{2}$	1
$N a_{2} S + 2 H C l \to 2 N a C l + H_{2} S$	1

Define acid salt and basic salt and give one example of each. State the colour of methyl orange in an alkali. (3 marks) (3 marks)

Step	Marks
Acid salt: hydrogen of the acid partly replaced by a metal; e.g. NaHSO₄	1
Basic salt: hydroxide of a base partly replaced by an acid radical; e.g. Pb(OH)Cl	1
Yellow	1

For each salt, name the method and write the equation: (a) zinc sulphate, (b) barium sulphate, (c) copper(II) chloride, (d) sodium chloride. (4 marks) (4 marks)

Step	Marks
Displacement: $Z n + H_{2} S O_{4} \to Z n S O_{4} + H_{2}$	1
Precipitation: $B a C l_{2} + H_{2} S O_{4} \to B a S O_{4} + 2 H C l$	1
Insoluble base: $C u O + 2 H C l \to C u C l_{2} + H_{2} O$	1
Titration: $N a O H + H C l \to N a C l + H_{2} O$	1

Where students lose marks in Study of Acids, Bases and Salts

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

Mistake: Choosing displacement for an insoluble salt
CISCE examiners' comments, 2025: for lead sulphate, and for a method-matching table, candidates chose simple displacement instead of precipitation.
Wrong working
Lead sulphate: Pb + H₂SO₄ → PbSO₄ + H₂
Right working
Precipitation: $P b (N O_{3})_{2} + H_{2} S O_{4} \to P b S O_{4} + 2 H N O_{3}$
Do this: Ask 'is the salt insoluble?' first; if yes, precipitate it.
Usually costs: 1 mark per salt.
Mistake: Wrong method for K, Na and NH₄ salts
CISCE examiners' comments, 2024: for potassium nitrate from KOH, candidates chose direct combination or displacement.
Wrong working
KNO₃ by displacement.
Right working
KNO₃ by titration: $K O H + H N O_{3} \to K N O_{3} + H_{2} O$
Do this: A soluble alkali + acid always means titration.
Usually costs: 1 mark.
Mistake: Not linking pH to weak acid particles
CISCE examiners' comments, 2024: given solutions of pH 13, 5 and 2, candidates could not link the weakly acidic solution (pH 5) with a mixture of molecules and ions.
Wrong working
pH 2 solution contains molecules and ions.
Right working
pH 5 (weak acid): molecules and ions; pH 2 (strong acid): mainly ions.
Do this: Weak = partly ionised = molecules + ions.
Usually costs: 1 mark.
Mistake: Confusing ionisation with dissociation
CISCE examiners' comments, 2024: candidates mixed up the equation for electrolytic dissociation with that for ionisation.
Wrong working
NaOH ionises in water to form ions.
Right working
NaOH (ionic) dissociates; HCl (covalent) ionises.
Do this: Ions already there: dissociation. Ions newly made: ionisation.
Usually costs: 1 mark.
Mistake: Not recognising an amphoteric hydroxide
CISCE examiners' comments, 2024: candidates chose calcium or magnesium hydroxide for the hydroxide that reacts with both acids and alkalis.
Wrong working
Mg(OH)₂ reacts with acids and alkalis.
Right working
Zn(OH)₂ (also Al(OH)₃, Pb(OH)₂) is amphoteric.
Do this: Learn the amphoteric trio: Al, Zn, Pb.
Usually costs: 1 mark.

Practice questions on Study of Acids, Bases and Salts

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
Which indicator turns yellow in an alkali?
(a) Litmus
(b) Phenolphthalein
(c) Methyl orange
(d) Universal indicator at pH 7
Hint
Pink-orange-yellow.
Answer
(c) Methyl orange
Q2 · 1 mark · MCQBoard level
Which of the following is an acid salt?
(a) Na₂CO₃
(b) NaHCO₃
(c) Pb(OH)Cl
(d) Na[Ag(CN)₂]
Hint
Replaceable H left.
Answer
(b) NaHCO₃
Q3 · 1 mark · MCQBoard level
Dilute sulphuric acid is added to a salt and the gas formed turns lead acetate paper black. The salt is:
(a) sodium sulphite
(b) iron(II) sulphide
(c) sodium carbonate
(d) sodium hydrogen sulphite
Hint
Black lead sulphide.
Answer
(b) iron(II) sulphide
Q4 · 1 mark · Assertion-reasonHarder board level
Assertion (A): Copper(II) sulphate cannot be prepared by adding copper to dilute sulphuric acid.
Reason (R): Copper is less reactive than hydrogen.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.
Hint
Activity series.
Answer
(a)
Q5 · 3 marks · Short answerBoard level
Write balanced equations for preparing: (a) magnesium sulphate from magnesium carbonate; (b) potassium sulphate by titration; (c) lead chloride by precipitation.
Hint
Balance the K and Cl.
Answer
(a) $M g C O_{3} + H_{2} S O_{4} \to M g S O_{4} + H_{2} O + C O_{2}$ (b) $2 K O H + H_{2} S O_{4} \to K_{2} S O_{4} + 2 H_{2} O$ (c) $P b (N O_{3})_{2} + 2 N a C l \to P b C l_{2} + 2 N a N O_{3}$
Q6 · 4 marks · Short answerHarder board level
Name the method of preparation for each, giving a reason: (a) zinc nitrate from zinc oxide; (b) iron(II) sulphate from iron; (c) silver chloride; (d) ammonium chloride.
Hint
Solubility and reactivity.
Answer
(a) Neutralisation of an insoluble base (ZnO is insoluble). (b) Displacement (iron reacts with dilute acid). (c) Precipitation (AgCl insoluble). (d) Titration (ammonium salt from NH₄OH and HCl).
Q7 · 3 marks · Short answerHarder board level
Classify each as strong acid, weak acid, strong alkali or weak alkali: HNO₃, NH₄OH, CH₃COOH, KOH. Write the equation for the weak alkali in water.
Hint
Partly ionised = weak.
Answer
HNO₃ strong acid; NH₄OH weak alkali; CH₃COOH weak acid; KOH strong alkali. $N H_{4} O H ⇌ N H_{4}^{+} + O H^{-}$
Q8 · 3 marks · Short answerHOTS / competency
A solution of salt Y gives a white precipitate with barium chloride solution that does not dissolve in dilute HCl. On treating solid Y with dilute HCl, there is no gas. Y is a potassium salt. (a) Name the anion. (b) Name Y. (c) Suggest how Y could be made.
Hint
BaSO₄ is insoluble in acid.
Answer
(a) Sulphate (b) Potassium sulphate (c) Titration of potassium hydroxide with dilute sulphuric acid: $2 K O H + H_{2} S O_{4} \to K_{2} S O_{4} + 2 H_{2} O$ .
Q9 · 2 marks · Short answerHarder board level
Explain why (a) an aqueous solution of an acid conducts electricity but dry HCl gas does not, (b) all alkalis are bases but all bases are not alkalis.
Hint
Ions only in water.
Answer
(a) HCl ionises only in water, giving mobile H₃O⁺ and Cl⁻; dry HCl is molecular. (b) An alkali must dissolve in water; many bases (CuO, Fe(OH)₃) are insoluble.
Q10 · 3 marks · Short answerHOTS / competency
Write the equations to show the stepwise ionisation of sulphuric acid in water and use them to explain why it forms two series of salts.
Hint
One H at a time.
Answer
$H_{2} S O_{4} + H_{2} O \to H_{3} O^{+} + H S O_{4}^{-}$ ; $H S O_{4}^{-} + H_{2} O \to H_{3} O^{+} + S O_{4}^{2 -}$ . Replacing only the first H gives acid salts (NaHSO₄); replacing both gives normal salts (Na₂SO₄).
Q11 · 3 marks · Short answerHOTS / competency
A student wants copper(II) chloride but has only copper and dilute HCl. Describe two steps (with equations) that use extra reagents available in any laboratory.
Hint
Make an oxide first.
Answer
Heat copper in air to form copper(II) oxide: $2 C u + O_{2} \to 2 C u O$ . Add the oxide to dilute HCl: $C u O + 2 H C l \to C u C l_{2} + H_{2} O$ ; filter, evaporate, crystallise.
Q12 · 2 marks · Very short answerBoard level
Give one example each of: (a) a basic salt, (b) a complex salt, (c) a dibasic acid, (d) a triacidic base.
Hint
One formula each.
Answer
(a) Pb(OH)Cl (b) [Cu(NH₃)₄]SO₄ (c) H₂SO₄ (d) Al(OH)₃

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.Define an acid in terms of ions.Show answer
A compound that gives hydronium ions as the only positive ions in water.
2.Define an alkali.Show answer
A base soluble in water that gives hydroxide ions as the only negative ions.
3.Basicity of acetic acid?Show answer
1 (monobasic).
4.Colour of phenolphthalein in an acid?Show answer
Colourless.
5.Example of a complex salt?Show answer
Sodium argentocyanide, Na[Ag(CN)₂].
6.Method to prepare an insoluble salt?Show answer
Precipitation (double decomposition).
7.Gas from a sulphide with dilute acid?Show answer
Hydrogen sulphide (rotten egg smell).
8.Which salts are all soluble?Show answer
All sodium, potassium and ammonium salts, and all nitrates.

Best order to study it

Learn the ion definitions of acid, base and alkali, and basicity/acidity.
Learn the three indicators and the pH scale.
Learn the four types of salt with two examples each.
Practise the five acid + salt reactions with their gas tests.
Learn the solubility rules, then the six preparation methods, and practise choosing the method.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you write ionisation equations for HCl and H₂SO₄ and the dissociation of NaOH?
Do you know litmus, methyl orange and phenolphthalein colours in acid, neutral and alkali?
Can you classify any salt as normal, acid, basic or complex?
Can you write the equations for dilute acid on a carbonate, hydrogen carbonate, sulphite, hydrogen sulphite and sulphide?
Can you choose the right method for any named salt and write its equation?

Study of Acids, Bases and Salts: frequently asked questions

Do I need to describe the apparatus for preparing salts in ICSE Class 10?

No. The 2027 syllabus says details of apparatus or procedures are not required; you need the method and the balanced equation.

Are double salts like potash alum in the 2027 syllabus?

The 2027 syllabus lists normal, acid, basic and complex salts. Learn those four with examples.

Is every base an alkali?

No. An alkali is a base that dissolves in water. Copper(II) oxide and iron(III) hydroxide are bases but not alkalis.

Why is iron(III) chloride made by direct combination?

Prepared from solution, it takes up water and is hydrolysed, so the anhydrous salt is made by heating iron in dry chlorine.

Based on: CISCE ICSE 2027 Chemistry (Science Paper 2) syllabus; CISCE ICSE 2027 Chemistry (Science Paper 2) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2025; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2024