Mini Chemistry
Part of Mini EducationSwitch subject

Qualitative Analysis

6. Qualitative Analysis

  • SEC G3 Combined Science Chemistry component 2027
On this page

Learning objectives

  • describe the use of aqueous sodium hydroxide and/or aqueous ammonia to identify the following aqueous cations through the formation of precipitates (if any) and their subsequent solubility: aluminium, ammonium (together with evolution of ammonia gas upon warming), calcium, copper(II), iron(II), iron(III) and zinc (formulae of complex ions are not required)
  • describe tests to identify the following anions: carbonate (by the addition of dilute acid and subsequent use of limewater); chloride (by reaction of an aqueous solution with nitric acid and aqueous silver nitrate); nitrate (by reduction with aluminium in aqueous sodium hydroxide to ammonia and subsequent use of damp red litmus paper) and sulfate (by reaction of an aqueous solution with nitric acid and aqueous barium nitrate)
  • describe tests to identify the following gases: ammonia (using damp red litmus paper); carbon dioxide (using limewater); chlorine (using damp litmus paper); hydrogen (using a burning splint); oxygen (using a glowing splint) and sulfur dioxide (using acidified potassium manganate(VII)).

This lesson joins the prescribed observation-to-inference method, cation and anion tests, and gas tests for G3 Science (Chemistry).

1. Definition

Qualitative analysis identifies substances from characteristic chemical tests rather than measuring their amount.

  • Reagent: substance added to perform a test.
  • Observation: directly seen, heard or measured result.
  • Inference: identity supported by the observation.
  • Precipitate: insoluble solid formed from aqueous reactants.

2. Key Ideas

Use fresh portions of an unknown for different tests so one reagent does not contaminate the next result.

Test familyRequired procedureEvidence to record
cationsadd aqueous sodium hydroxide and/or aqueous ammonia dropwise, then in excessprecipitate colour and solubility in excess
chlorideacidify with dilute nitric acid, then add aqueous silver nitratewhite precipitate
sulfateacidify with dilute nitric acid, then add aqueous barium nitratewhite precipitate
carbonateadd dilute acid and test the evolved gaseffervescence; carbon dioxide turns limewater milky
ammonium / nitrateuse the prescribed alkaline warming sequenceammonia turns damp red litmus blue
gasesuse the named splint, limewater, damp litmus or redox reagentexact positive observation

3. Detailed Explanations

Cations

Add aqueous sodium hydroxide dropwise, then in excess. Repeat with aqueous ammonia where prescribed. The initial precipitate and its behaviour in excess are separate observations.

CationAqueous sodium hydroxideAqueous ammonia
aluminium, Al³⁺white precipitate; soluble in excess to a colourless solutionwhite precipitate; insoluble in excess
ammonium, NH₄ +ammonia produced on warmingno test
calcium, Ca²⁺white precipitate; insoluble in excessno precipitate
copper(II), Cu²⁺light blue precipitate; insoluble in excesslight blue precipitate; soluble in excess to a dark blue solution
iron(II), Fe²⁺green precipitate; insoluble in excessgreen precipitate; insoluble in excess
iron(III), Fe³⁺red-brown precipitate; insoluble in excessred-brown precipitate; insoluble in excess
zinc, Zn²⁺white precipitate; soluble in excess to a colourless solutionwhite precipitate; soluble in excess to a colourless solution

Do not try to identify every white precipitate from one result. Aluminium and zinc are separated by their behaviour in excess aqueous ammonia; calcium gives no precipitate with aqueous ammonia.

For copper(II) ions:

Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)

The light blue precipitate is insoluble in excess sodium hydroxide but dissolves in excess aqueous ammonia to form a dark blue solution.

Anions and reagent order

Use dilute nitric acid before silver nitrate or barium nitrate. Hydrochloric acid would introduce chloride ions; sulfuric acid would introduce sulfate ions and could create false positive evidence.

Carbonate releases carbon dioxide with dilute acid. Nitrate is tested by adding aqueous sodium hydroxide and aluminium foil, warming, then testing the evolved ammonia with damp red litmus paper.

AnionComplete positive test
carbonate, CO₃²⁻add dilute acid; effervescence occurs and the gas turns limewater milky
chloride, Cl⁻acidify with dilute nitric acid, then add aqueous silver nitrate; a white precipitate forms
nitrate, NO₃⁻add aqueous sodium hydroxide and aluminium foil, warm carefully; ammonia is produced and turns damp red litmus blue
sulfate, SO₄²⁻acidify with dilute nitric acid, then add aqueous barium nitrate; a white precipitate forms

Gases

  • ammonia: damp red litmus turns blue;
  • carbon dioxide: limewater turns milky; the precipitate dissolves in excess carbon dioxide;
  • chlorine: damp litmus is bleached;
  • hydrogen: a lighted splint gives a squeaky pop;
  • oxygen: a glowing splint relights;
  • sulfur dioxide: aqueous acidified potassium manganate(VII) changes from purple to colourless.

Practical task 1: Use fresh portions and preserve evidenceOptional

Label separate small portions, add only the prescribed reagent, record the initial observation, then add excess reagent or perform the confirmatory gas test where required. Wear eye protection and do not inhale gases directly.

4. Common Mistakes

  • Giving an ion name without a reagent and observation.
  • Forgetting “dropwise, then in excess”.
  • Reporting only “a precipitate forms” without its colour.
  • Acidifying a chloride test with hydrochloric acid.
  • Using dry litmus paper for ammonia or chlorine.
  • Treating nitrate-produced ammonia as proof when ammonium ions were not excluded using a fresh portion.

5. Exam Tips

Preserve the evidence order

Write reagent and condition → observation → inference. If two ions share an initial result, include the excess-reagent behaviour or a second prescribed test.

Exam question 1: Resolve a white precipitateCore

An unknown gives a white precipitate with aqueous sodium hydroxide. Explain why this is not yet a unique identification.

Show Answer

Several cations give white hydroxide precipitates. Add sodium hydroxide in excess and use aqueous ammonia as prescribed, recording whether the precipitate dissolves.

6. Worked Examples

Modelled example 1

Identify Copper(II) Ions

Core

Problem

An unknown gives a light blue precipitate that is insoluble in excess aqueous sodium hydroxide but dissolves in excess aqueous ammonia to give a dark blue solution. Identify the ion and show the evidence chain.
Study the worked solution

  1. Record the observations

    Method

    Keep the colours and excess-reagent results together.

    Reason

    The complete pattern is more discriminating than “a precipitate forms”.

    Working

    Light blue precipitate; insoluble in excess NaOH(aq); dissolves in excess NH₃(aq) to a dark blue solution.

  2. Infer the ion

    Method

    Identify Cu²⁺.

    Reason

    This prescribed combination of observations is characteristic of copper(II) ions.

    Working

    Inference: copper(II) ions, Cu²⁺, are present.

Guided practice 2

Test for Chloride Ions

About 5 min

Problem

State the complete prescribed test for chloride ions and its positive observation.

Build the sequence

Acid used first
Reagent added next
Positive observation

Hints

Hint 1: avoid-contamination
The acid must not add the ion being tested.
Hint 2: silver-halide
After acidifying, add a soluble source of silver ions.
View solution step by step

  1. Prepare a fresh portion

    Method

    Acidify it with dilute nitric acid.

    Reason

    Hydrochloric acid would introduce chloride and corrupt the evidence.

    Working

    Fresh portion + dilute HNO₃(aq).

  2. Add the test reagent

    Method

    Add aqueous silver nitrate.

    Reason

    Silver ions form the prescribed halide precipitate.

    Working

    Add AgNO₃(aq).

  3. State observation and inference

    Working

    A white precipitate forms; chloride ions are present.

Common misconception 3

Correct a Precipitate-Only Answer

Find and correct the mistake

Learner response

After adding aqueous sodium hydroxide dropwise and then in excess, a learner writes only, “A precipitate forms, so zinc ions are present.” Explain why the conclusion is unsupported and improve the evidence plan.

Diagnose the missing evidence

First missing observation
Further required evidence

View solution step by step

  1. Reject the shortcut

    Method

    State that precipitate formation alone is not unique.

    Reason

    Several prescribed cations form hydroxide precipitates.

    Working

    “A precipitate forms” cannot identify Zn²⁺.

  2. Collect discriminating observations

    Method

    Record colour and solubility in excess sodium hydroxide, then use aqueous ammonia as prescribed on a fresh portion.

    Reason

    The combined reagent history and excess behaviour support the ion inference.

    Working

    Procedure → precise observations → zinc-ion inference.

Examiner practice 4

Write a Markable Ammonium Test

3 marks

Examination question

Describe how to test an aqueous sample for ammonium ions and state the positive result. [3 marks]

Write the three-mark answer

View solution step by step

  1. State reagent and condition

    1 mark

    Method

    Add aqueous sodium hydroxide and warm gently.

    Reason

    Alkali and warming release ammonia from ammonium ions.

    Working

    Add aqueous sodium hydroxide and warm gently.

  2. State the positive observation

    1 mark

    Method

    Test the evolved gas with damp red litmus paper.

    Reason

    Ammonia dissolves in the moisture and gives an alkaline response.

    Working

    Damp red litmus paper turns blue.

  3. State the inference

    1 mark

    Method

    Infer ammonium ions from the confirmed ammonia gas.

    Reason

    The prescribed reagent, condition and positive gas test support that conclusion.

    Working

    Ammonia is produced, so ammonium ions are present.

Challenge 5

Separate Nitrate from Ammonium Evidence

Minimal support

Interference problem

An unknown may contain nitrate ions, ammonium ions or both. Explain why a fresh portion should be warmed with aqueous sodium hydroxide before aluminium foil is introduced, then state how the nitrate test continues.

Track the source of ammonia

First test excludes
Then add for nitrate

Hints

Hint 1: same-gas
Both routes can produce ammonia, so establish whether alkali alone does so.
Hint 2: fresh-portion
Use a fresh portion so the nitrate evidence has a known reagent history.
View solution step by step

  1. Check for ammonium first

    Method

    Warm a fresh portion with aqueous sodium hydroxide and test any gas with damp red litmus.

    Reason

    Ammonium ions release ammonia with alkali alone and would otherwise imitate the nitrate-test product.

    Working

    A positive result here establishes ammonium interference.

  2. Continue the nitrate test

    Method

    Only after accounting for ammonium, add aluminium foil to the alkaline test mixture and warm as prescribed.

    Reason

    Ammonia produced after the aluminium step can support nitrate only when pre-existing ammonium has been excluded.

    Working

    Damp red litmus turns blue after the nitrate-test sequence.

  3. Limit the conclusion

    Method

    Attribute the result only to the controlled sequence used.

    Reason

    The identity of a gas is not enough; its reagent history determines which ion it evidences.

    Working

    Alkali-first control → aluminium nitrate test → supported inference.

7. Mind Stretchers

Mind stretcher 1: Design an efficient sequenceExtension

Three solutions contain copper(II), chloride and carbonate ions in different combinations. Explain how fresh portions preserve trustworthy evidence.

Show Answer

Use separate portions for the cation test, the acidified silver-nitrate test and the carbonate acid/gas test. Otherwise hydroxide, chloride or acid added in an earlier test could change the sample or introduce an ion used as later evidence.

Mind stretcher 2: Separate three representation levelsExtension

Classify “light blue solid forms”, “copper(II) ions are present” and “copper(II) hydroxide particles formed”.

Show Answer

The first is an observation, the second is an inference and the third is a particle-level explanation.

8. Quiz

Use Check your understanding, then practise the test sequences. Before moving on, cover the tables and reconstruct each answer as reagent and condition → observation → inference.