What are the common methods for the preparation of salts in the Cambridge IGCSE Coordinated Science curriculum?

In the Cambridge IGCSE Coordinated Science curriculum, salts can be prepared using several methods, including neutralization reactions between acids and bases, reaction of acids with metals, reaction of acids with metal carbonates, and precipitation reactions. Each method is chosen based on the solubility of the salt and the reactants available.

How do you prepare a soluble salt using an acid and a base?

To prepare a soluble salt using an acid and a base, you perform a neutralization reaction. For example, to make sodium chloride, you can react hydrochloric acid with sodium hydroxide. The reaction produces water and sodium chloride. The resulting solution is then evaporated to crystallize the salt.

What is the role of titration in the preparation of salts?

Titration is used in the preparation of salts to accurately determine the amount of acid and base needed for complete neutralization. This method is particularly useful for preparing soluble salts, ensuring that the resulting solution is neutral and free from excess acid or base.

How can insoluble salts be prepared through precipitation reactions?

Insoluble salts can be prepared through precipitation reactions by mixing two aqueous solutions containing ions that form an insoluble compound. For example, mixing solutions of barium chloride and sulfuric acid will produce barium sulfate, an insoluble salt, which precipitates out of the solution.

Why is it important to understand the solubility rules when preparing salts?

Understanding solubility rules is crucial when preparing salts because it helps predict whether a salt will dissolve in water or form a precipitate. This knowledge guides the selection of appropriate methods for salt preparation, ensuring the desired product is obtained efficiently.

Why is "Not using excess of the insoluble base when making a salt by acid + base" a common mistake in Preparation of Salts?

Why it happens: Students add a calculated amount of base, leaving some acid unreacted in the filtrate. How to avoid it: Always add excess of the insoluble base (e.g. CuO) to ensure all the acid reacts; then filter off the unreacted base. Source: 0654 Examiner Report 2023

Why is "Proposing Cu metal + HCl → CuCl₂ + H₂ to make copper chloride" a common mistake in Preparation of Salts?

Why it happens: Students apply the 'metal + acid → salt + hydrogen' rule without checking reactivity. How to avoid it: This reaction does not occur because Cu is below H in the reactivity series. Use CuO + 2HCl → CuCl₂ + H₂O instead.

Why is "Forgetting to wash the precipitate with distilled water" a common mistake in Preparation of Salts?

Why it happens: Students filter and dry without removing soluble impurities trapped on the precipitate. How to avoid it: Always wash the precipitate on the filter paper with distilled water to remove any ions from the mother liquor before drying.

Acids, Bases and SaltsCambridge IGCSE Coordinated Sciences 0654

Preparation of Salts

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Preparation of Salts — Cambridge IGCSE Coordinated Science 0654

The method used to prepare a salt depends on whether it is soluble or insoluble. Cambridge tests four preparation methods: titration (soluble from strong acid + alkali), excess solid method (soluble from acid + metal/carbonate/oxide), and precipitation (insoluble).

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C8.8 — Describe methods for preparing soluble salts (titration, excess solid method).
C8.9 — Describe how to prepare insoluble salts by precipitation.
C8.10 — Use solubility rules to predict whether a precipitate forms.

Preparing Soluble Salts

Two methods for soluble salts: titration (acid + alkali) or excess solid (acid + metal/oxide/carbonate). Both end with filtration and crystallisation.

Method 1 — Titration (acid + alkali): Used when BOTH reactants are soluble (and the salt is also soluble).

Fill burette with alkali (e.g. NaOH)
Add known volume of acid (e.g. HCl) to conical flask; add indicator
Titrate: add alkali slowly until indicator shows endpoint (neutralisation)
Record volumes (titre); repeat WITHOUT indicator (indicator would contaminate product)
Use exact volumes found → mix acid and alkali → neutralise completely
Evaporate solution to concentrate; allow to crystallise; filter; dry

Why no indicator in the repeat? The indicator itself is a coloured organic compound that would contaminate the pure salt crystals.

Method 2 — Excess solid method (acid + insoluble metal/oxide/carbonate): Used when reactant is an insoluble solid (metal, metal oxide, metal carbonate).

Warm acid (e.g. H₂SO₄) in a beaker
Add excess solid (e.g. CuO) until no more dissolves and some solid remains — ensures all acid is used up (no excess acid in product)
Filter off excess solid (removes unreacted solid and any impurities)
Evaporate filtrate to concentrate; cool → crystals form; filter; dry at low temperature

Example: Copper(II) sulfate: CuO + H₂SO₄ → CuSO₄ + H₂O → blue crystals (CuSO₄·5H₂O) after crystallisation

Titration: acid + alkali; use indicator to find exact volumes; repeat without indicator; crystallise.
Excess solid: add excess metal/oxide/carbonate to acid; filter; crystallise.
Excess solid ensures all acid used up — prevents acidic product.

Preparing Insoluble Salts — Precipitation

Insoluble salts are made by mixing two solutions whose ions form an insoluble product. The precipitate is filtered, washed, and dried.

Precipitation method — for insoluble salts:

Choose two soluble salts whose ions will form the insoluble product when mixed
Mix solutions → precipitate forms immediately
Filter the precipitate
Wash with distilled water (removes soluble impurities)
Dry (gently, in oven)

Example — Barium sulfate (BaSO₄):

BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq) Ionic equation: Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)

BaSO₄ is the precipitate (white solid); NaCl remains in solution

Example — Silver chloride (AgCl):

AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq) Ionic equation: Ag⁺(aq) + Cl⁻(aq) → AgCl(s)

Solubility rules (for predicting precipitates):

Ion	Solubility rule
NO₃⁻ (nitrate)	All soluble
Cl⁻ (chloride)	All soluble EXCEPT AgCl, PbCl₂
SO₄²⁻ (sulfate)	All soluble EXCEPT BaSO₄, PbSO₄, CaSO₄ (sparingly)
CO₃²⁻ (carbonate)	Insoluble EXCEPT Na₂CO₃, K₂CO₃, (NH₄)₂CO₃
OH⁻ (hydroxide)	Insoluble EXCEPT NaOH, KOH, Ca(OH)₂ (sparingly)

Insoluble salts: mix two solutions → precipitate → filter → wash → dry.
Ionic equation: only the precipitating ions, with (aq) and (s) state symbols.
AgCl (insoluble), BaSO₄ (insoluble) — formed by precipitation. All nitrates soluble.

How it’s examined

Paper 4: 'Describe how to prepare pure crystals of copper(II) sulfate from copper(II) oxide and dilute sulfuric acid' (5 marks — excess solid, warming, filter, evaporate, crystallise, filter, dry). 'Write the ionic equation for the formation of barium sulfate precipitate' (2 marks — Ba²⁺ + SO₄²⁻ → BaSO₄ with state symbols). 'Identify which of these pairs of solutions would form a precipitate when mixed, and name the precipitate' — tests solubility rules.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C8); 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Preparation of Salts

Step-by-step solutions to past-paper-style questions on preparation of salts, written exactly the way a tutor would explain them at the board.

1Preparing copper sulfate crystals from copper oxide
Extended
Question
Describe how to prepare a pure, dry sample of copper(II) sulfate crystals from copper(II) oxide and dilute sulfuric acid.
Step-by-step solution
1. Step 1
  Add excess copper(II) oxide (black powder) to warm dilute sulfuric acid in a beaker. Stir/warm gently.
  $\text{CuO} + \text{H}_2\text{SO}_4 \rightarrow \text{CuSO}_4 + \text{H}_2\text{O}$
2. Step 2
  Continue adding CuO until no more dissolves (excess ensures all acid is used up). The solution turns blue.
3. Step 3
  Filter the mixture to remove excess unreacted CuO (residue on filter paper); the blue filtrate is copper sulfate solution.
4. Step 4
  Evaporate the filtrate gently until saturated (test: a glass rod dipped in the solution forms crystals on cooling). Allow to cool to form blue crystals.
5. Step 5
  Filter off the crystals and dry them between filter paper (or in a warm oven at low temperature).
Answer
Add excess CuO to warm H₂SO₄; filter off excess CuO; evaporate filtrate to saturation; cool to crystallise; filter; dry.
Examiner tip
Must use excess CuO to ensure all acid is used (otherwise the salt would be contaminated with acid). Do not boil to dryness.
2Preparing an insoluble salt — lead(II) sulfate
Extended
Question
Describe how to prepare a pure sample of lead(II) sulfate (an insoluble salt).
Step-by-step solution
1. Step 1
  Prepare a solution of lead(II) nitrate (Pb(NO₃)₂) and a solution containing sulfate ions (e.g. dilute sulfuric acid or sodium sulfate solution).
2. Step 2
  Mix the two solutions together in a beaker. A white precipitate of lead(II) sulfate immediately forms.
  $\text{Pb}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{PbSO}_4(\text{s})$
3. Step 3
  Filter the mixture; the white precipitate (PbSO₄) is collected on the filter paper.
4. Step 4
  Wash the precipitate with distilled water to remove any soluble impurities (e.g. remaining Pb²⁺ or SO₄²⁻ ions).
5. Step 5
  Dry the precipitate in a warm oven or leave in air.
Answer
Mix Pb(NO₃)₂ solution with H₂SO₄ or Na₂SO₄ solution; white PbSO₄ precipitate forms; filter; wash with distilled water; dry.
3Why copper cannot react with HCl to make copper chloride
Extended
Question
State why copper metal cannot be reacted directly with dilute hydrochloric acid to prepare copper(II) chloride.
Step-by-step solution
1. Step 1
  Copper is below hydrogen in the metal reactivity series.
2. Step 2
  Metals below hydrogen in the reactivity series do not react with dilute acids; they cannot displace hydrogen from acid.
3. Step 3
  Instead, copper(II) oxide or copper(II) carbonate can be used: CuO + 2HCl → CuCl₂ + H₂O.
Answer
Copper is below hydrogen in the reactivity series and does not react with dilute HCl; use copper oxide or copper carbonate + HCl instead.
Examiner tip
Never use copper metal + acid. This is a classic exam trap for 'preparing copper salts'.
4Ionic equation for neutralisation
Extended
Question
Write the ionic equation for the neutralisation reaction between hydrochloric acid and sodium hydroxide solution.
Step-by-step solution
1. Step 1
  Full equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
2. Step 2
  All aqueous species are split into ions: H⁺ + Cl⁻ + Na⁺ + OH⁻ → Na⁺ + Cl⁻ + H₂O
3. Step 3
  Cancel spectator ions (Na⁺ and Cl⁻ appear on both sides).
  $\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(l)$
Answer
$\text{H}^+(\text{aq}) + \text{OH}^-(\text{aq}) \rightarrow \text{H}_2\text{O}(l)$

Key Definitions and Keywords — Preparation of Salts

Definitions to memorise and the exact keywords mark schemes credit for preparation of salts answers — sharpened from recent examiner reports for the 2026 0654 sitting.

Salt
Examiner keyword
An ionic compound formed when the hydrogen in an acid is replaced by a metal (or ammonium) ion; produced by the reaction of an acid with a base, carbonate, or reactive metal.
Precipitation
Examiner keyword
The formation of an insoluble product (precipitate) when two solutions are mixed; used to prepare insoluble salts.
Excess reactant
A reactant added in greater than the stoichiometric amount to ensure complete reaction of the other reactant. The excess is removed by filtration after the reaction.
Soluble salt
A salt that dissolves significantly in water (>1 g per 100 g water); prepared by acid + reactive metal, acid + insoluble base, or acid + carbonate, then crystallisation.
Insoluble salt
A salt that does not dissolve significantly in water; prepared by mixing two solutions of soluble ionic compounds (precipitation).

Common Mistakes and Misconceptions — Preparation of Salts

The traps other students keep falling into on preparation of salts questions — taken from recent Cambridge IGCSE 0654 examiner reports and mark schemes — and how to avoid them.

✕Not using excess of the insoluble base when making a salt by acid + base
0654 Examiner Report 2023
Why it happens
Students add a calculated amount of base, leaving some acid unreacted in the filtrate.
How to avoid it
Always add excess of the insoluble base (e.g. CuO) to ensure all the acid reacts; then filter off the unreacted base.
✕Proposing Cu metal + HCl → CuCl₂ + H₂ to make copper chloride
Why it happens
Students apply the 'metal + acid → salt + hydrogen' rule without checking reactivity.
How to avoid it
This reaction does not occur because Cu is below H in the reactivity series. Use CuO + 2HCl → CuCl₂ + H₂O instead.
✕Forgetting to wash the precipitate with distilled water
Why it happens
Students filter and dry without removing soluble impurities trapped on the precipitate.
How to avoid it
Always wash the precipitate on the filter paper with distilled water to remove any ions from the mother liquor before drying.

Practice questions

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Preparation of Salts — frequently asked questions

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Preparation of Salts

Short Study Notes in the form of Slides

Short Study Notes — Preparation of Salts

Detailed Notes

What you’ll learn

How it’s examined

1Preparing copper sulfate crystals from copper oxide

2Preparing an insoluble salt — lead(II) sulfate

3Why copper cannot react with HCl to make copper chloride

4Ionic equation for neutralisation

Salt

Precipitation

Excess reactant

Soluble salt

Insoluble salt

✕Not using excess of the insoluble base when making a salt by acid + base

✕Proposing Cu metal + HCl → CuCl₂ + H₂ to make copper chloride

✕Forgetting to wash the precipitate with distilled water

Practice questions

Past paper style quiz

Assess your understanding

Preparation of Salts — frequently asked questions