🔬 Qualitative Analysis
Secondary School Chemistry • Practical Chemistry
1. Introduction
Qualitative analysis is the identification of substances by observing their physical and chemical properties. In the laboratory, tests are used to identify ions and gases present in an unknown substance.
Learning Objectives
- Explain the meaning of qualitative analysis.
- Identify common positive ions using chemical tests.
- Identify common negative ions.
- Identify common gases.
- Record observations correctly.
- Distinguish an observation from a conclusion.
2. Observation and Conclusion
An observation is what is directly seen, heard or measured during an experiment.
A conclusion is what the observation tells us about the substance.
Observation: A white precipitate forms.
Conclusion: An ion capable of producing that precipitate may be present.
In practical examinations, avoid writing a conclusion as if it were an observation.
3. Flame Tests
Some metal ions produce characteristic flame colours when heated.
| Ion | Flame colour |
|---|---|
| Na⁺ | Yellow |
| K⁺ | Lilac |
| Ca²⁺ | Brick-red / orange-red |
| Cu²⁺ | Blue-green |
A clean wire is used because contamination can produce misleading colours.
4. Testing for Cations
Ammonium ion — NH₄⁺
Add aqueous sodium hydroxide and warm gently. Ammonia gas is produced.
Ammonia turns damp red litmus paper blue.
Copper(II) ion — Cu²⁺
Add sodium hydroxide solution.
Observation: A light-blue precipitate forms.
Iron(II) ion — Fe²⁺
Add sodium hydroxide solution.
Observation: A green precipitate forms.
Iron(III) ion — Fe³⁺
Add sodium hydroxide solution.
Observation: A red-brown precipitate forms.
Aluminium ion — Al³⁺
A white precipitate forms with sodium hydroxide. The precipitate dissolves in excess sodium hydroxide.
Calcium ion — Ca²⁺
Calcium can be identified using a brick-red flame test.
5. Testing for Anions
Chloride — Cl⁻
Add dilute nitric acid followed by silver nitrate solution.
Positive result: A white precipitate of silver chloride forms.
Sulfate — SO₄²⁻
Add dilute hydrochloric acid if appropriate, followed by barium chloride solution.
Positive result: A white precipitate forms.
Carbonate — CO₃²⁻
Add dilute acid.
Observation: Effervescence occurs because carbon dioxide is produced.
Nitrate — NO₃⁻
Nitrate identification uses a specific chemical test. In examination work, follow the exact test procedure supplied by the question or laboratory instructions.
6. Gas Tests
| Gas | Test | Positive observation |
|---|---|---|
| Hydrogen | Lighted splint | Squeaky pop |
| Oxygen | Glowing splint | Relights |
| Carbon dioxide | Limewater | Turns milky |
| Ammonia | Damp red litmus | Turns blue |
| Chlorine | Damp litmus | Bleaches the dye |
7. Carbon Dioxide Test
Pass the gas through limewater.
The formation of calcium carbonate causes the limewater to become milky.
8. Precipitation Reactions
A precipitate is an insoluble solid formed when two solutions react.
The symbol (s) shows that the substance is a solid precipitate.
9. Practical Examination Technique
- Read the procedure before beginning.
- Use clean apparatus.
- Add reagents carefully.
- Record colour changes accurately.
- Record precipitate colours.
- Do not confuse a gas with a vapour.
- Write observations before conclusions.
- Balance ionic equations where required.
10. MANEB-style Questions
Question 1: A solution gives a white precipitate when acidified and treated with silver nitrate. Identify the likely ion.
Question 2: A gas turns limewater milky. Identify the gas.
Question 3: A solution gives a red-brown precipitate when sodium hydroxide is added. Which metal ion is likely present?
- Chloride, Cl⁻.
- Carbon dioxide, CO₂.
- Iron(III), Fe³⁺.
11. Practice Exercises
- Define qualitative analysis.
- State the flame colour of sodium ions.
- What observation confirms hydrogen gas?
- What reagent is commonly used to test for chloride ions?
- What is observed when sulfate ions react with barium ions?
- Write the ionic equation for formation of silver chloride.
- How is carbon dioxide identified?
- State the colour of the precipitate formed when Fe³⁺ reacts with sodium hydroxide.
Answers
- Identification of substances using chemical tests and observations.
- Yellow.
- A squeaky pop with a lighted splint.
- Silver nitrate after suitable acidification.
- A white precipitate forms.
- Ag⁺ + Cl⁻ → AgCl.
- It turns limewater milky.
- Red-brown.
12. Common Mistakes
- Writing “chloride present” as an observation instead of a conclusion.
- Confusing Fe²⁺ with Fe³⁺.
- Forgetting to include state symbols in ionic equations when requested.
- Using dirty apparatus during flame tests.
- Giving the gas name without describing the positive test.
13. Exam Tip
For qualitative-analysis questions, learn the pattern reagent → observation → conclusion. This makes practical questions much easier to answer accurately.