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Preparation of salts — KCSE Chemistry

KCSE Chemistry · 62 practice questions · 4 syllabus objectives · 4 revision lessons

24 easy26 medium12 hard

Last updated · Aligned to the KNEC KCSE syllabus

What You'll Learn

Key learning outcomes for this topic, aligned to the KNEC KCSE syllabus.

Describe the preparation of soluble salts by neutralisation, reaction of acid with metal and reaction of acid with carbonate

Describe the preparation of insoluble salts by precipitation (double decomposition)

Write ionic equations for salt preparation reactions and select the appropriate method for a given salt

Preparation of salts

Revision Notes

Concise lesson notes for Preparation of salts, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Preparation of Soluble Salts

Soluble salts can be prepared through three main methods: neutralisation, reaction of acid with metal, and reaction of acid with carbonate.

  1. Neutralisation: This involves the reaction between an acid and a base. For example, when hydrochloric acid (HCl) reacts with sodium hydroxide (NaOH), sodium chloride (NaCl) and water (H2O) are formed. The reaction is represented as:

    HCl + NaOH → NaCl + H2O

  2. Reaction of Acid with Metal: In this method, an acid reacts with a reactive metal to produce a soluble salt and hydrogen gas. For instance, when zinc (Zn) reacts with sulfuric acid (H2SO4), zinc sulfate (ZnSO4) and hydrogen gas are produced:

    Zn + H2SO4 → ZnSO4 + H2↑

  3. Reaction of Acid with Carbonate: Here, an acid reacts with a carbonate to produce a soluble salt, carbon dioxide, and water. For example, when calcium carbonate (CaCO3) reacts with hydrochloric acid (HCl), calcium chloride (CaCl2), carbon dioxide, and water are formed:

    CaCO3 + 2HCl → CaCl2 + CO2↑ + H2O

Understanding these methods is essential for preparing soluble salts in the laboratory.

Key points to remember

  • Neutralisation involves acid reacting with a base to form salt and water.
  • Acid and metal reaction produces salt and hydrogen gas.
  • Acid and carbonate reaction yields salt, carbon dioxide, and water.
  • Common examples include HCl with NaOH, Zn, and CaCO3.

Worked example

Describe how to prepare sodium chloride using neutralisation.

  • Mix hydrochloric acid with sodium hydroxide.
  • The reaction produces sodium chloride and water.

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More lessons in this topic

Lesson 2: Preparation of Insoluble Salts by Precipitation

Objective: Describe the preparation of insoluble salts by precipitation (double decomposition)

Insoluble salts can be prepared through a process called precipitation, which is a type of double decomposition reaction. This occurs when two soluble salts in solution react to form an insoluble salt that precipitates out of the solution. Key steps in the process include:

  1. Mixing Solutions: Combine two solutions containing the ions that will form the insoluble salt. For example, mixing sodium sulfate (Na2SO4) and barium nitrate (Ba(NO3)2).

  2. Formation of Precipitate: The insoluble salt, barium sulfate (BaSO4), forms and appears as a solid precipitate. This can be represented by the equation:

    Na2SO4(aq) + Ba(NO3)2(aq) → BaSO4(s) + 2NaNO3(aq)

  3. Separation: The precipitate can be separated from the solution by filtration.

  4. Washing and Drying: The collected precipitate is then washed to remove impurities and dried for use.

This method is useful in the laboratory for preparing various insoluble salts necessary for experiments and applications in industry.

  • Insoluble salts form through double decomposition reactions.
  • Two soluble salts react to produce an insoluble salt.
  • The insoluble salt precipitates out of the solution.
  • Separation is done via filtration, followed by washing.

Describe how to prepare barium sulfate using sodium sulfate and barium nitrate.

  • Mix equal volumes of sodium sulfate and barium nitrate solutions.
  • Observe the formation of a white precipitate (barium sulfate).
  • Filter to collect the barium sulfate, wash, and dry.
Lesson 3: Ionic Equations for Salt Preparation

Objective: Write ionic equations for salt preparation reactions and select the appropriate method for a given salt

In chemistry, preparing salts involves reactions between acids and bases or acids and metals. Writing ionic equations helps us understand the species involved in these reactions. Common methods for salt preparation include:

  • Neutralization: Acid + Base → Salt + Water
  • Precipitation: Mixing two solutions to form an insoluble salt

When writing ionic equations, focus on the ions that participate in the reaction. For example, consider the preparation of sodium chloride (NaCl) from hydrochloric acid (HCl) and sodium hydroxide (NaOH):

  1. Balanced molecular equation: HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)

  2. Ionic equation: H⁺(aq) + Cl⁻(aq) + Na⁺(aq) + OH⁻(aq) → Na⁺(aq) + Cl⁻(aq) + H₂O(l)

  3. Net ionic equation (removing spectator ions): H⁺(aq) + OH⁻(aq) → H₂O(l)

To select the appropriate method for a given salt, consider the solubility of the reactants and the desired salt's properties. For instance, to prepare silver sulfate (Ag₂SO₄), the precipitation method is suitable due to the low solubility of Ag₂SO₄ in water.

  • Identify the reactants and products in salt preparation reactions.
  • Write balanced molecular and ionic equations accurately.
  • Select the appropriate method based on salt properties.

Question: Write the ionic equation for the preparation of potassium sulfate (K₂SO₄) from sulfuric acid and potassium hydroxide. Answer: H₂SO₄(aq) + 2KOH(aq) → K₂SO₄(aq) + 2H₂O(l) Ionic equation: H⁺(aq) + SO₄²⁻(aq) + 2K⁺(aq) + 2OH⁻(aq) → 2K⁺(aq) + SO₄²⁻(aq) + 2H₂O(l) Net ionic equation: H⁺(aq) + OH⁻(aq) → H₂O(l)

Lesson 4: Methods for Preparing Salts

Objective: Preparation of salts

In chemistry, salts can be prepared through various methods. The most common methods include:

  • Neutralization: This involves the reaction between an acid and a base. For example, hydrochloric acid reacts with sodium hydroxide to form sodium chloride (table salt) and water.
  • Precipitation: This occurs when two soluble salts react to form an insoluble salt (precipitate). For example, mixing silver nitrate and sodium chloride forms silver chloride, which precipitates out of solution.
  • Direct Combination: Some salts can be prepared by directly combining their constituent elements. For instance, sodium and chlorine react to form sodium chloride.
  • Evaporation: This method involves evaporating a solution to obtain a salt. For example, evaporating seawater yields sodium chloride.

Understanding these methods is crucial for practical applications and experiments in the lab.

  • Salts can be prepared by neutralization of acids and bases.
  • Precipitation occurs when two soluble salts form an insoluble salt.
  • Direct combination of elements can yield certain salts.
  • Evaporation of solutions can concentrate salts for collection.

Question: Describe the method of preparing sodium chloride using hydrochloric acid and sodium hydroxide. Answer:

  • Mix hydrochloric acid (HCl) with sodium hydroxide (NaOH).
  • This reaction produces sodium chloride (NaCl) and water (H2O).
  • The reaction is a neutralization process.

Sample Questions

Read 3 questions and answers free. Sign up to access all 62 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

Define the term 'precipitation reaction' and give one example of how to prepare an insoluble salt. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
A precipitation reaction is a chemical reaction in which two soluble salts react to form an insoluble salt (precipitate). (1 mk)
The insoluble salt usually appears as a solid that settles out of the solution. (1 mk)
Part (b) — 2 marks
An example is the preparation of barium sulfate by mixing barium chloride with sodium sulfate. (1 mk)
The reactants used are barium chloride (BaCl2) and sodium sulfate (Na2SO4). (1 mk)
2
easySHORT ANSWER4 marks

Explain how you would prepare silver chloride using the precipitation method. (4 marks)

Answer & marking scheme

Part (a) — 2 marks
Mix a solution of silver nitrate with a solution of sodium chloride. (1 mk)
Stir the mixture to ensure complete reaction leading to the formation of the white precipitate of silver chloride. (1 mk)
Part (b) — 2 marks
To separate the insoluble silver chloride precipitate from the soluble sodium nitrate solution. (1 mk)
Filtering ensures that only the desired product (silver chloride) is collected for further use. (1 mk)
3
easySHORT ANSWER4 marks

Identify two properties of the salt formed when hydrochloric acid reacts with sodium hydroxide. (4 marks)

Answer & marking scheme

Part (a) — 4 marks
The salt is soluble in water (1 mk)
The salt has a neutral pH when dissolved in water (1 mk)
The salt forms a colourless solution (1 mk)
The salt can be crystallised from its solution (1 mk)
4

Identify the method used to prepare copper(II) sulfate from copper(II) oxide and sulfuric acid. (2 marks)

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Frequently asked questions

What does the KCSE Chemistry topic "Preparation of salts" cover?

Preparation of salts covers Describe the preparation of soluble salts by neutralisation, reaction of acid with metal and reaction of acid with carbonate; Describe the preparation of insoluble salts by precipitation (double decomposition); Write ionic equations for salt preparation reactions and select the appropriate method for a given salt, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Preparation of salts?

HighMarks has 62 Preparation of salts practice questions for KCSE Chemistry, each with a full marking scheme. The first 3 are free; sign up to access the rest, plus all KCSE mock exams and past papers.

Are these aligned with the KNEC KCSE syllabus?

Yes. Every objective on this page is taken directly from the official KNEC KCSE Chemistry syllabus. Practice questions match the KCSE exam format and are graded against the standard KNEC marking scheme.

How should I revise Preparation of salts for the KCSE exam?

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