Overview

What you'll learn

Test for cations. Use aqueous NaOH and aqueous NH₃ to identify Cu²⁺, Fe²⁺, Fe³⁺, Ca²⁺, Al³⁺, Zn²⁺ and NH₄⁺ from the colour of the precipitate and whether it dissolves in excess.

Test for anions. Identify carbonate, sulfate, chloride, iodide and nitrate — and know why dilute HNO₃ is added first.

Test for gases. Match CO₂, NH₃, Cl₂, H₂, O₂ and SO₂ to their giveaway tests.

Work through QA flowcharts. Combine cation, anion and gas tests to identify an unknown substance.

Tutor's Insight

"Short topic. Memorise it cold."
QA questions ask you to match a colour, a precipitate, or a gas to an ion. There are only three things to test for — cations, anions and gases — and almost no working. It shows up as structured questions in Paper 2 (often as a flowchart) and again in the Paper 3 practical, where you write observations and identify the ion. Once the table's in your head, every mark falls. Get it down early. Lose nothing on test day.

6.1 Tests for Cations & Anions

6.1 Tests for Cations & Anions

What this subtopic asks of you
  • Describe tests to identify the cations Cu²⁺, Fe²⁺, Fe³⁺, Ca²⁺, Al³⁺, Zn²⁺ and NH₄⁺ using aqueous sodium hydroxide and aqueous ammonia.
  • Describe tests to identify the anions carbonate (CO₃²⁻), sulfate (SO₄²⁻), chloride (Cl⁻), iodide (I⁻) and nitrate (NO₃⁻).
  • Explain why dilute nitric acid is added before the sulfate and halide tests, and record what you see alongside what it means.

6.1 Cations & Anions

Every test follows the same shape

Step 01
Add a reagent
Cation — aqueous NaOH or aqueous NH₃. Anion — dilute HNO₃ first, then a reagent. Gas — a colour-changing paper or solution.
Step 02
Watch carefully
Record the colour of any precipitate or solution, any gas given off, and whether the precipitate dissolves in excess. The dissolution step is what separates Al³⁺ from Ca²⁺.
Step 03
Read off the ion
Cross-check your observations against the QA table. When two ions share an observation, run a second test to break the tie — e.g. NaOH then NH₃.
Exam Habit

Always add dilute nitric acid before testing for an anion. If a sample also contains carbonate or hydroxide, AgNO₃ or Ba(NO₃)₂ will precipitate them too — and you'll wrongly call it a chloride or sulfate. Dilute HNO₃ removes the interferers.

6.1 Cation Tests

Add aqueous NaOH — what do you see?

Ion With aq NaOH In excess NaOH Note
Cu²⁺ blue ppt insoluble —
Fe²⁺ green ppt insoluble turns brown on standing
Fe³⁺ reddish-brown ppt insoluble —
Ca²⁺ white ppt insoluble —
Al³⁺ white ppt soluble — colourless solution —
Zn²⁺ white ppt soluble — colourless solution —
NH₄⁺ no precipitate warm → NH₃ gas moist red litmus → blue

Free Notes · O-Level Pure Chemistry

Read the full chapter

The aqueous NH₃ cation table, the dissolve-in-excess shortcut, every anion test, the full gas-test table, QA flowcharts and 4 worked exam questions.

6.1 Cation Tests

Add aqueous NH₃ — what changes?

Ion With aq NH₃ In excess NH₃ Note
Cu²⁺ blue ppt soluble — deep blue solution the giveaway for Cu²⁺
Fe²⁺ green ppt insoluble —
Fe³⁺ reddish-brown ppt insoluble —
Ca²⁺ no precipitate — no reaction
Al³⁺ white ppt insoluble differs from NaOH
Zn²⁺ white ppt soluble — colourless solution —
NH₄⁺ no precipitate — —
Al³⁺ and Zn²⁺ both give a white precipitate in NaOH. NH₃ splits them: Zn²⁺ dissolves in excess, Al³⁺ does not.

6.1 Cation Tests · Memory Aid

The dissolve-in-excess shortcut

Excess NaOH dissolves
Al³⁺ · Zn²⁺ · Pb²⁺
The hydroxides Al(OH)₃, Zn(OH)₂ and Pb(OH)₂ are amphoteric — they dissolve in extra alkali to form a colourless solution. Every other white precipitate (Ca²⁺) stays insoluble.
Excess NH₃ dissolves
Cu²⁺ · Zn²⁺
Cu²⁺ and Zn²⁺ form soluble complex ions with ammonia. Cu²⁺ → deep blue solution. Zn²⁺ → colourless solution.
Tie-breaker

Zn²⁺ is the only ion that dissolves in BOTH excess NaOH and excess NH₃. So if both reagents give a white precipitate that dissolves in excess, the ion is zinc. Every time.

6.1 Anion Tests

Two precipitate tests

Test 01 · Carbonate
CO₃²⁻
Add dilute HCl (or dilute HNO₃). See effervescence — bubbles of a colourless gas. To confirm, pass the gas through limewater. A white precipitate (the limewater turns milky) confirms CO₂ — and so a carbonate.
Test 02 · Sulfate
SO₄²⁻
Add a few drops of dilute HNO₃, then aqueous Ba(NO₃)₂. A white precipitate forms — barium sulfate, BaSO₄. The HNO₃ goes first to remove any carbonate that would also precipitate with Ba²⁺.
Effervescence that turns limewater milky = carbonate. White precipitate with acidified Ba(NO₃)₂ = sulfate.

6.1 Anion Tests

Halides — and the nitrate trick

Tests 03 & 04 · Halides
Cl⁻ · I⁻
Add a few drops of dilute HNO₃, then aqueous AgNO₃. Cl⁻ → white precipitate (AgCl). I⁻ → yellow precipitate (AgI).
Test 05 · Nitrate
NO₃⁻
Add aqueous NaOH, then a piece of aluminium foil, and warm. Ammonia gas is evolved — moist red litmus paper turns blue. The aluminium reduces NO₃⁻ to NH₃ in alkaline solution.
Why it matters

The same white precipitate appears in more than one test — silver chloride, barium sulfate, and the white hydroxides of Ca²⁺, Al³⁺ and Zn²⁺ all look alike. That is why QA is never a single test: it's a sequence of observations that each rule something out until only one ion is left.

6.2 Tests for Gases & QA Flowcharts

6.2 Tests for Gases & QA Flowcharts

What this subtopic asks of you
  • Describe tests to identify the gases hydrogen (H₂), oxygen (O₂), carbon dioxide (CO₂), ammonia (NH₃), chlorine (Cl₂) and sulfur dioxide (SO₂).
  • State the observation, not just the test — e.g. "burns with a pop", "relights a glowing splint", "limewater turns milky".
  • Combine cation, anion and gas tests to identify an unknown substance in a QA flowchart.

6.2 Gas Tests

Each gas has one giveaway

Gas Properties Test Observation
H₂ colourless, odourless lighted splint burns with a 'pop'
O₂ colourless, odourless glowing splint splint relights
CO₂ colourless, odourless bubble through limewater limewater turns milky
NH₃ colourless, pungent moist red litmus paper turns blue (alkaline)
Cl₂ yellow-green, pungent moist blue litmus paper turns red, then bleached white
SO₂ colourless, pungent acidified K₂Cr₂O₇ paper orange → green
NH₃ is the only common gas that is alkaline — it's the one that turns red litmus blue. Cl₂ is the only one that bleaches.

6.2 Putting It Together

Read every test as a narrowing funnel

CATION TEST → ANION TEST → IDENTIFY THE SALT
Scenario
A pale-blue solution
Solid X dissolves in water to a pale-blue solution.
• with NaOH → blue precipitate, insoluble in excess.
• with NH₃ → blue precipitate, soluble in excess giving a deep-blue solution.
• with HNO₃ + Ba(NO₃)₂ → white precipitate.
Reasoning
Cation, then anion
Cation — a blue precipitate that dissolves in excess NH₃ to a deep blue solution is the classic Cu²⁺ result. Anion — a white precipitate with Ba(NO₃)₂ (after acidifying) means SO₄²⁻. So X is CuSO₄.
Exam-paper Habit

Write what you SEE first, then what it MEANS. Two things on every line — what you SEE (colour, precipitate, gas) and what it MEANS (the ion identified). Putting them side-by-side wins both marks.

Practice

Exam-style questions

Question 01  ·  AMKSS 2024 Prelim
The Question
MCQ

Ammonium iron(II) sulfate + NaOH

A salt has the formula (NH₄)₂Fe(SO₄)₂ · 12H₂O. Excess aqueous sodium hydroxide is added slowly, with shaking, to its aqueous solution until there is no further change. The boiling tube is then left to stand. Which observation would not be made?

  • A  a green precipitate was produced
  • B  a pungent gas which turned damp red litmus paper blue was produced
  • C  on standing, a brown precipitate was produced
  • D  the precipitate dissolved in excess aqueous sodium hydroxide
Worked Answer
D
  1. Fe²⁺ + 2OH⁻ → Fe(OH)₂ — a green precipitate. So A is observed.
  2. NH₄⁺ + OH⁻ → NH₃ + H₂O; ammonia turns moist red litmus blue. So B is observed.
  3. Fe(OH)₂ slowly oxidises in air to reddish-brown Fe(OH)₃. So C is observed on standing.
  4. Fe(OH)₂ is NOT amphoteric — it does not dissolve in excess NaOH. So D is the false observation.
Question 02  ·  AMKSS 2024 Prelim
The Question
MCQ

Telling two alkalis apart

Both aqueous sodium carbonate (Na₂CO₃) and aqueous sodium hydroxide (NaOH) turn red litmus blue. Which one of the following can be used to differentiate the two solutions?

  • A  aqueous calcium nitrate
  • B  aqueous potassium chloride
  • C  carbon dioxide
  • D  dilute hydrochloric acid
Worked Answer
D — dilute hydrochloric acid
  1. A test 'differentiates' if it gives a different observation for each solution.
  2. HCl + Na₂CO₃ → 2NaCl + H₂O + CO₂↑ — effervescence; the gas turns limewater milky.
  3. HCl + NaOH → NaCl + H₂O — neutralisation; no gas, no visible change.
  4. Effervescence vs no change is the clean distinction — answer D.
Question 03  ·  AMKSS 2024 Prelim
The Question
MCQ

Spot the pair that gives no reaction

Which of the following pairs of solutions will show no visible change when mixed?

  • A  barium nitrate and hydrochloric acid
  • B  lead(II) nitrate and sodium sulfate
  • C  sodium carbonate and magnesium nitrate
  • D  zinc and copper(II) sulfate
Worked Answer
A
  1. A: Ba²⁺ + Cl⁻ → BaCl₂ (soluble); H⁺ + NO₃⁻ → HNO₃ (soluble). No precipitate, no gas — no visible change.
  2. B: Pb²⁺ + SO₄²⁻ → PbSO₄, a white precipitate.
  3. C: Mg²⁺ + CO₃²⁻ → MgCO₃, a white precipitate.
  4. D: Zn + Cu²⁺ → Zn²⁺ + Cu (red-brown deposit; blue solution fades). Answer: A.
Question 04
The Question
Structured

Identifying an unknown white solid

A white solid Y dissolves in water to give a colourless solution. Two tests are carried out:

  • 1  — Adding aqueous NaOH gives a white precipitate that dissolves in excess to a colourless solution. Adding aqueous NH₃ gives a white precipitate that also dissolves in excess.
  • 2  — Adding dilute HNO₃ then aqueous AgNO₃ gives a yellow precipitate.

Identify the cation and the anion, and hence name the compound Y.

Worked Answer
Zinc iodide, ZnI₂
  1. The cation gives a white precipitate with both NaOH and NH₃ that dissolves in excess of each. Only Zn²⁺ dissolves in both excess NaOH and excess NH₃, so the cation is Zn²⁺.
  2. The anion gives a yellow precipitate (AgI) with acidified AgNO₃. A yellow silver precipitate means the anion is iodide, I⁻.
  3. Combining a Zn²⁺ cation with an I⁻ anion gives zinc iodide, ZnI₂.

Frequently Asked Questions

Qualitative Analysis — FAQ

What is Qualitative Analysis in the O-Level Chemistry syllabus (6092)?
Qualitative Analysis (Topic 6 of Syllabus 6092) is about identifying the ions and gases present in an unknown substance. It covers three sets of tests: tests for cations (Cu²⁺, Fe²⁺, Fe³⁺, Ca²⁺, Al³⁺, Zn²⁺ and NH₄⁺) using aqueous NaOH and aqueous NH₃; tests for anions (carbonate, sulfate, chloride, iodide and nitrate); and tests for gases (H₂, O₂, CO₂, NH₃, Cl₂ and SO₂). It is largely memory work, tested in MCQ, structured and practical papers.
How do you tell Al³⁺ and Zn²⁺ apart from Ca²⁺ in the cation tests?
All three give a white precipitate with aqueous NaOH. The difference is in excess: Al³⁺ and Zn²⁺ hydroxides are amphoteric and dissolve in excess NaOH to give a colourless solution, while Ca(OH)₂ stays insoluble. To separate Al³⁺ from Zn²⁺, add aqueous NH₃ instead — Zn²⁺ dissolves in excess NH₃ but Al³⁺ does not. Zn²⁺ is the only ion that dissolves in both excess NaOH and excess NH₃.
Why must you add dilute nitric acid before testing for sulfate or chloride?
If the sample also contains carbonate or hydroxide ions, they would precipitate with Ba(NO₃)₂ or AgNO₃ too, and you would wrongly conclude a sulfate or chloride is present. A few drops of dilute nitric acid (HNO₃) first removes these interfering ions, so any precipitate you then see must be barium sulfate or silver chloride.
How do you test for the nitrate ion (NO₃⁻)?
Add aqueous sodium hydroxide, then a piece of aluminium foil, and warm gently. The aluminium reduces the nitrate ion to ammonia gas in the alkaline solution. Ammonia gas is given off, which turns moist red litmus paper blue — confirming nitrate.
What is the giveaway test for each common gas?
H₂ burns with a 'pop' with a lighted splint; O₂ relights a glowing splint; CO₂ turns limewater milky; NH₃ turns moist red litmus paper blue; Cl₂ turns moist blue litmus paper red then bleaches it white; and SO₂ turns acidified potassium dichromate(VI) paper from orange to green.
How do you identify an unknown salt using a QA flowchart?
Work it as a narrowing funnel. First identify the cation using aqueous NaOH and, if needed, aqueous NH₃ — matching the colour of the precipitate and whether it dissolves in excess. Then identify the anion using the acid, barium and silver tests. For example, a blue precipitate that dissolves in excess NH₃ to a deep blue solution is Cu²⁺; a white precipitate with Ba(NO₃)₂ after acidifying is SO₄²⁻, so the salt is CuSO₄.

O-Level Pure Chemistry  ·  Syllabus 6092  ·  Topic 6 of 12  ·  © 2026 Overmugged. For personal study use only.