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Qualitative analysis — KCSE Chemistry

KCSE Chemistry · 58 practice questions · 4 syllabus objectives · 4 revision lessons

11 easy29 medium18 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 tests for common anions: carbonate (CO₃²⁻), sulphate (SO₄²⁻), chloride (Cl⁻) and nitrate (NO₃⁻)

Describe tests for common cations using aqueous sodium hydroxide and aqueous ammonia

Identify an unknown salt by systematic qualitative analysis and write equations for the reactions observed

Qualitative analysis

Revision Notes

Concise lesson notes for Qualitative analysis, written to the KCSE Chemistry marking standard. Read the first lesson free below.

Tests for Common Anions in Chemistry

In qualitative analysis, identifying common anions is essential. Here are the tests for four important anions:

  • Carbonate (CO₃²⁻): Add dilute hydrochloric acid (HCl) to the sample. A brisk effervescence indicates the presence of carbon dioxide (CO₂), confirming carbonate ions.

  • Sulphate (SO₄²⁻): Add barium chloride (BaCl₂) to the sample solution. A white precipitate of barium sulfate (BaSO₄) indicates the presence of sulfate ions.

  • Chloride (Cl⁻): Add silver nitrate (AgNO₃) to the sample. A white precipitate of silver chloride (AgCl) confirms the presence of chloride ions.

  • Nitrate (NO₃⁻): Add concentrated sulfuric acid (H₂SO₄) and a few granules of copper(II) sulfate (CuSO₄) to the sample. A brown ring forms at the interface, indicating the presence of nitrate ions.

These tests are simple yet effective for identifying anions in a laboratory setting.

Key points to remember

  • Effervescence with HCl indicates carbonate ions.
  • White precipitate with BaCl₂ confirms sulfate ions.
  • White precipitate with AgNO₃ indicates chloride ions.
  • Brown ring test with H₂SO₄ confirms nitrate ions.

Worked example

Question: Describe the test for chloride ions in a sample. Answer: Add silver nitrate solution to the sample; a white precipitate of silver chloride confirms chloride ions.

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Lesson 2: Tests for Common Cations

Objective: Describe tests for common cations using aqueous sodium hydroxide and aqueous ammonia

In qualitative analysis, we can identify common cations through specific tests using aqueous sodium hydroxide (NaOH) and aqueous ammonia (NH₃). Here are the key tests:

  1. Sodium Hydroxide (NaOH) Test:

    • Copper(II) ions (Cu²⁺): Add NaOH; a blue precipitate of copper(II) hydroxide forms.
    • Iron(II) ions (Fe²⁺): Add NaOH; a green precipitate of iron(II) hydroxide forms.
    • Iron(III) ions (Fe³⁺): Add NaOH; a brown precipitate of iron(III) hydroxide forms.
    • Aluminium ions (Al³⁺): Add NaOH; a white precipitate forms, which dissolves in excess NaOH to give a colourless solution.

  2. Aqueous Ammonia (NH₃) Test:

    • Copper(II) ions (Cu²⁺): Add NH₃; a deep blue solution forms due to the formation of tetraamminecopper(II) complex.
    • Iron(II) ions (Fe²⁺): Add NH₃; a green precipitate of iron(II) hydroxide forms.
    • Aluminium ions (Al³⁺): Add NH₃; a white precipitate forms, which does not dissolve in excess NH₃.

These tests help distinguish between different cations based on the precipitate colors formed.

  • Copper(II) ions yield a blue precipitate with NaOH.
  • Iron(II) ions produce a green precipitate with NaOH.
  • Iron(III) ions give a brown precipitate with NaOH.
  • Aluminium ions form a white precipitate with NaOH.
  • Copper(II) ions form a deep blue solution with NH₃.

Question: Describe the test for copper(II) ions using aqueous sodium hydroxide.
Answer: Add aqueous sodium hydroxide to the solution; a blue precipitate of copper(II) hydroxide forms.

Lesson 3: Identifying Unknown Salts in Qualitative Analysis

Objective: Identify an unknown salt by systematic qualitative analysis and write equations for the reactions observed

In qualitative analysis, identifying an unknown salt involves systematic tests to determine its components. The process includes:

  1. Preliminary Tests: Observe physical properties such as color, solubility, and state.
  2. Anion Tests: Conduct tests for anions, such as:
    • Chloride (Cl⁻): Add dilute nitric acid and silver nitrate; a white precipitate indicates chloride.
    • Sulfate (SO₄²⁻): Add barium chloride; a white precipitate indicates sulfate.
  3. Cation Tests: Conduct tests for cations, such as:
    • Sodium (Na⁺): Flame test shows a bright yellow flame.
    • Copper (II) (Cu²⁺): Flame test shows a blue-green flame.

Once the anions and cations are identified, you can write balanced equations for the reactions observed. For example:

  • If the salt is sodium chloride (NaCl):
    • Reaction with silver nitrate:
    • NaCl (aq) + AgNO₃ (aq) → AgCl (s) + NaNO₃ (aq)

By systematically testing and recording observations, you can accurately identify the unknown salt and write the corresponding equations.

  • Conduct preliminary tests for physical properties of the salt.
  • Identify anions and cations through specific tests.
  • Write balanced equations for observed reactions.
  • Ensure to record all observations systematically.
  • Conclude with the identification of the unknown salt.

Identify the unknown salt if a white precipitate forms when adding silver nitrate to a sample.

  • The unknown salt is likely sodium chloride (NaCl).
  • Reaction: NaCl (aq) + AgNO₃ (aq) → AgCl (s) + NaNO₃ (aq).
Lesson 4: Understanding Qualitative Analysis in Chemistry

Objective: Qualitative analysis

Qualitative analysis is a method used to identify the components of a substance. It focuses on determining the presence of specific ions or functional groups in a sample rather than quantifying them. Key techniques include:

  • Precipitation reactions: Adding reagents to form insoluble compounds, indicating specific ions.
  • Flame tests: Observing the color of a flame when a sample is heated, revealing certain metal ions.
  • Litmus tests: Using litmus paper to assess acidity or alkalinity, indicating the presence of acids or bases.

In qualitative analysis, it is essential to follow a systematic approach:

  1. Sample preparation: Ensure the sample is clean and suitable for testing.
  2. Testing for cations: Use specific reagents to identify metal ions.
  3. Testing for anions: Apply different tests to confirm the presence of non-metal ions.

Remember to document your observations clearly, as they are crucial for drawing conclusions about the composition of the sample.

  • Qualitative analysis identifies components in a substance.
  • Key methods include precipitation and flame tests.
  • Systematic approach ensures accurate results.
  • Observations must be documented for conclusions.
  • Focus on presence, not quantity, of substances.

Question: Describe how you would test for the presence of chloride ions in a solution. Answer: - Add a few drops of dilute nitric acid to the solution.

  • Then, add silver nitrate solution.
  • A white precipitate of silver chloride indicates the presence of chloride ions.

Sample Questions

Read 3 questions and answers free. Sign up to access all 58 questions with full KNEC-style marking schemes and a personalised study plan.

1
easySHORT ANSWER4 marks

List the steps and observations to confirm if a solution contains lead(II) ions using aqueous sodium hydroxide and aqueous ammonia. (4 marks)

Answer & marking scheme

Part (a) — 4 marks
Add a few drops of aqueous sodium hydroxide to the solution — a white precipitate of lead(II) hydroxide, Pb(OH)₂, forms (1 mk)
Add excess sodium hydroxide — the precipitate does not dissolve (1 mk)
Add a few drops of aqueous ammonia to the same solution — a white precipitate of lead(II) hydroxide, Pb(OH)₂, is also formed (1 mk)
Add excess ammonia — the precipitate does not dissolve (1 mk)
2
easySHORT ANSWER2 marks

State the expected observation when aqueous ammonia is added to a solution containing iron(III) ions. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
A reddish-brown precipitate of iron(III) hydroxide, Fe(OH)₃, is formed (1 mk)
The precipitate does not dissolve in excess ammonia (1 mk)
3
easySHORT ANSWER2 marks

List the observations made when a few drops of aqueous sodium hydroxide are added to a solution containing zinc ions. (2 marks)

Answer & marking scheme

Part (a) — 2 marks
A white precipitate of zinc hydroxide, Zn(OH)₂, is formed (1 mk)
The precipitate is soluble in excess sodium hydroxide, forming a clear solution (1 mk)
4

Define the procedure to test for the presence of iron(II) ions (Fe²⁺) in a solution. (4 marks)

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Frequently asked questions

What does the KCSE Chemistry topic "Qualitative analysis" cover?

Qualitative analysis covers Describe tests for common anions: carbonate (CO₃²⁻), sulphate (SO₄²⁻), chloride (Cl⁻) and nitrate (NO₃⁻); Describe tests for common cations using aqueous sodium hydroxide and aqueous ammonia; Identify an unknown salt by systematic qualitative analysis and write equations for the reactions observed, and more, all aligned to the official KNEC KCSE Chemistry syllabus.

How many practice questions are available for Qualitative analysis?

HighMarks has 58 Qualitative analysis 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 Qualitative analysis for the KCSE exam?

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Why Practise Qualitative analysis?

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