What are the common tests used for the identification of cations in the Cambridge IGCSE Coordinated Science syllabus?

In the Cambridge IGCSE Coordinated Science syllabus, common tests for identifying cations include flame tests and reactions with sodium hydroxide or ammonia. For example, a flame test can identify sodium ions by a yellow flame, while copper ions produce a blue-green flame. Reactions with sodium hydroxide can help identify cations like iron(II) and iron(III) by forming green and brown precipitates, respectively.

How can anions be identified in a laboratory setting according to the IGCSE curriculum?

Anions can be identified using specific chemical tests. For instance, adding dilute nitric acid followed by silver nitrate can test for halide ions: chloride forms a white precipitate, bromide a cream precipitate, and iodide a yellow precipitate. Sulfate ions can be identified by adding barium chloride, resulting in a white precipitate of barium sulfate. These tests are part of the practical skills developed in the IGCSE Coordinated Science curriculum.

What methods are used to identify gases in the IGCSE Coordinated Science course?

In the IGCSE Coordinated Science course, gases are identified using simple tests. For example, hydrogen gas can be identified by its 'pop' sound when a lit splint is held near it. Oxygen relights a glowing splint, while carbon dioxide turns limewater milky. Ammonia gas can be detected by its characteristic pungent smell and its ability to turn damp red litmus paper blue.

Why is it important to learn about the identification of ions and gases in the context of acids, bases, and salts?

Understanding the identification of ions and gases is crucial because it helps students grasp the fundamental concepts of chemical reactions and properties of substances. In the context of acids, bases, and salts, knowing how to identify ions and gases allows students to predict reaction outcomes, understand neutralization processes, and apply this knowledge in practical laboratory settings, which are key components of the IGCSE Coordinated Science curriculum.

What safety precautions should be taken during the identification of ions and gases in a lab?

When identifying ions and gases, safety precautions are essential. Always wear safety goggles and gloves to protect against chemical splashes. Work in a well-ventilated area or fume hood to avoid inhaling harmful gases. Handle acids and bases with care, as they can be corrosive. Ensure that all test tubes and equipment are clean to avoid contamination, and dispose of chemicals according to your teacher's instructions. These practices are emphasized in the IGCSE Coordinated Science curriculum to ensure a safe learning environment.

Why is "Testing for sulfate without first acidifying with dilute HCl" a common mistake in Identification of Ions and Gases?

Why it happens: Students add BaCl₂ directly without removing carbonate or sulfite ions. How to avoid it: Add excess dilute HCl first (removes CO₃²⁻ and SO₃²⁻ that would give false white precipitate), THEN add BaCl₂. Source: 0654 Examiner Report 2022

Why is "Confusing the colours of precipitates formed with NaOH" a common mistake in Identification of Ions and Gases?

Why it happens: Students memorise the list imprecisely and mix up Fe²⁺/Fe³⁺/Cu²⁺ colours. How to avoid it: Fe²⁺: green; Fe³⁺: brown/rust; Cu²⁺: blue; Ca²⁺: white (dissolves in excess NaOH for Al³⁺ and Zn²⁺ only). Learn these exactly as given in the data booklet.

Why is "Saying limewater turns milky because of dissolved CO₂ — not writing the equation" a common mistake in Identification of Ions and Gases?

Why it happens: Students know the observation but cannot explain the chemistry. How to avoid it: CO₂ + Ca(OH)₂ → CaCO₃(s) + H₂O. CaCO₃ is the white insoluble precipitate that makes the solution appear milky. Always give the equation for full marks.

Acids, Bases and SaltsCambridge IGCSE Coordinated Sciences 0654

Identification of Ions and Gases

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Identification of Ions and Gases — Cambridge IGCSE Coordinated Science 0654

Chemical tests identify the presence of specific ions and gases. Cambridge tests are systematic: flame tests (cations), precipitate colour with NaOH and aqueous ammonia, and gas tests for H₂, O₂, CO₂, Cl₂, SO₂, NH₃.

What you’ll learn

Mapped to the Cambridge IGCSE 0654 syllabus (2025-2027).

C8.11 — Describe and use flame tests to identify cations.
C8.12 — Describe tests for cations using NaOH and aqueous ammonia.
C8.13 — Describe tests for anions (Cl⁻, SO₄²⁻, CO₃²⁻).
C8.14 — Describe tests for common gases.

Tests for Cations

Cations are identified by flame tests (Group I and II metals) and by characteristic precipitate colours with NaOH or aqueous ammonia.

Flame tests (clean platinum wire loop dipped in compound, held in blue Bunsen flame):

Ion	Flame colour
Li⁺	Crimson (red)
Na⁺	Yellow (persistent)
K⁺	Lilac (pale violet)
Ca²⁺	Brick red (orange-red)
Ba²⁺	Green
Cu²⁺	Blue-green

NaOH (sodium hydroxide) test: Add NaOH solution dropwise → observe precipitate; then add excess NaOH.

Ion	Precipitate with NaOH	In excess NaOH
Cu²⁺	Blue	Stays blue (insoluble)
Fe²⁺	Green (slimy)	Stays green
Fe³⁺	Red-brown	Stays red-brown
Al³⁺	White	Dissolves (amphoteric)
Ca²⁺	White	Stays white
Mg²⁺	White	Stays white
Zn²⁺	White	Dissolves (amphoteric)
NH₄⁺	No precipitate; pungent ammonia gas on warming	—

Aqueous ammonia (NH₃(aq)) test:

Cu²⁺: blue precipitate with drops; precipitate dissolves in EXCESS → deep blue solution [Cu(NH₃)₄]²⁺
Fe²⁺: green precipitate (stays green)
Fe³⁺: red-brown precipitate (stays red-brown)
Al³⁺, Mg²⁺, Ca²⁺: white precipitate, does NOT dissolve in excess NH₃

Distinguishing Al³⁺ from Mg²⁺/Ca²⁺: Only Al(OH)₃ dissolves in excess NaOH (amphoteric). Mg(OH)₂ and Ca(OH)₂ do NOT dissolve.

Flame tests: Na⁺ yellow, K⁺ lilac, Ca²⁺ brick red, Cu²⁺ blue-green.
NaOH: Cu²⁺ → blue ppt. Fe²⁺ → green. Fe³⁺ → red-brown. Al³⁺ → white, dissolves in excess.
Cu²⁺ with excess NH₃ → deep blue solution [Cu(NH₃)₄]²⁺.

Tests for Anions and Gases

Anions identified by precipitate formation. Gases identified by specific chemical tests.

Tests for anions:

Anion	Test	Positive result
Cl⁻	Add AgNO₃(aq) (after acidifying with HNO₃)	White precipitate of AgCl — soluble in dilute NH₃
Br⁻	Add AgNO₃(aq)	Cream precipitate of AgBr — slightly soluble in conc. NH₃
I⁻	Add AgNO₃(aq)	Yellow precipitate of AgI — insoluble in NH₃
SO₄²⁻	Add BaCl₂(aq) (after acidifying with HCl)	White precipitate of BaSO₄ — insoluble in dilute HCl
CO₃²⁻	Add dilute acid	Effervescence; CO₂ turns limewater milky
NO₃⁻	Add NaOH and Al powder/Devarda's alloy; warm	Ammonia gas produced (turns damp red litmus blue)

Tests for gases:

Gas	Test	Observation
H₂	Apply lit splint	Burns with squeaky pop
O₂	Apply glowing splint	Relights the glowing splint
CO₂	Bubble through limewater	Turns milky (CaCO₃ precipitate)
Cl₂	Damp litmus paper	Bleaches (turns then decolourises)
NH₃	Damp red litmus paper	Turns blue (alkaline)
SO₂	Potassium dichromate (K₂Cr₂O₇) solution	Orange → green
HCl	Concentrated ammonia nearby	White fumes of NH₄Cl

Cl⁻: AgNO₃ → white AgCl (dissolves in NH₃). SO₄²⁻: BaCl₂ → white BaSO₄ (doesn't dissolve in HCl).
CO₂: milky limewater. H₂: squeaky pop. O₂: relights glowing splint.
Cl₂: bleaches damp litmus. NH₃: turns damp red litmus blue.

How it’s examined

Paper 4 and Paper 6: ion identification questions are very common. 'Describe a test to identify Cl⁻ ions' (2 marks — add AgNO₃, white precipitate AgCl, soluble in dilute NH₃). 'State the colour change observed when NH₃ is tested with damp red litmus paper' (1 mark — turns blue). Paper 6 often asks for full identification schemes — systematic testing in logical order. Memorise all flame colours and precipitate colours.

Sources: Cambridge IGCSE Coordinated Sciences 0654 syllabus 2025-2027 (C8); 0654 Paper 6 past papers 2022-2024; 0654 Examiner Reports 2022-2024. Last reviewed 2026-05-14.

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Step-by-step worked examples — Identification of Ions and Gases

Step-by-step solutions to past-paper-style questions on identification of ions and gases, written exactly the way a tutor would explain them at the board.

1Identifying carbonate anion
Core
Question
A white solid dissolves in water to give a colourless solution. When dilute hydrochloric acid is added, effervescence occurs and the gas turns limewater milky. Identify the anion present.
Step-by-step solution
1. Step 1
  Effervescence with dilute acid → gas produced. Gas turns limewater milky → gas is CO₂.
2. Step 2
  CO₃²⁻ ions react with H⁺ to produce CO₂ gas.
  $\text{CO}_3^{2-} + 2\text{H}^+ \rightarrow \text{H}_2\text{O} + \text{CO}_2$
3. Step 3
  The anion is carbonate (CO₃²⁻).
Answer
Carbonate ion ( $\text{CO}_3^{2-}$ ) — effervescence with acid + CO₂ turns limewater milky.
2Test for sulfate ions
Extended
Question
Describe the test for the presence of sulfate ions in solution.
Step-by-step solution
1. Step 1
  Add excess dilute hydrochloric acid to the test solution. This removes carbonate or sulfite ions that could give a false positive (they also precipitate with Ba²⁺).
2. Step 2
  Add barium chloride solution (BaCl₂). A white precipitate of barium sulfate indicates the presence of sulfate ions.
  $\text{Ba}^{2+}(\text{aq}) + \text{SO}_4^{2-}(\text{aq}) \rightarrow \text{BaSO}_4(\text{s})$
3. Step 3
  The white precipitate is insoluble in excess dilute HCl (this distinguishes it from BaSO₃, which dissolves in acid).
Answer
Add excess dilute HCl, then BaCl₂ solution; white precipitate of BaSO₄ (insoluble in HCl) confirms SO₄²⁻.
Examiner tip
Adding HCl first is essential — without it, carbonate or sulfite ions would also give a white precipitate with Ba²⁺, giving a false positive.
3Identifying Cu²⁺ ions
Extended
Question
A solution gives a blue precipitate when NaOH solution is added. Identify the cation and write the ionic equation.
Step-by-step solution
1. Step 1
  Blue precipitate with NaOH → copper(II) hydroxide, Cu(OH)₂.
2. Step 2
  The cation is Cu²⁺ (copper(II) ion).
  $\text{Cu}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Cu(OH)}_2(\text{s})$
Answer
Cation: Cu²⁺; Ionic equation: $\text{Cu}^{2+}(\text{aq}) + 2\text{OH}^-(\text{aq}) \rightarrow \text{Cu(OH)}_2(\text{s})$
4Flame tests
Core
Question
Describe the flame test procedure and state the flame colours for potassium, sodium, calcium, and copper.
Step-by-step solution
1. Step 1
  Clean a platinum/nichrome wire loop by dipping in concentrated HCl and holding in a roaring Bunsen flame until no colour is imparted.
2. Step 2
  Dip the clean wire into the test solution (or solid), then hold in the blue/non-luminous Bunsen flame and observe colour.
3. Step 3
  Flame colours: potassium (K) → lilac/violet; sodium (Na) → persistent yellow; calcium (Ca) → brick red/orange-red; copper (Cu) → blue-green.
Answer
Dip clean wire in solution; hold in flame. K: lilac; Na: yellow; Ca: brick red; Cu: blue-green.
Examiner tip
Na's persistent yellow can mask other flame colours. If K is suspected alongside Na, use a blue cobalt glass filter to see the lilac colour through the yellow.
5Identifying ammonia gas
Core
Question
A gas with a pungent smell turns damp red litmus paper blue. Identify the gas and describe a further confirmatory test.
Step-by-step solution
1. Step 1
  Pungent smell + turns damp red litmus blue → alkaline gas → ammonia (NH₃).
2. Step 2
  Confirmatory test: hold a glass rod dipped in concentrated HCl near the gas; dense white smoke (fumes) of ammonium chloride forms.
  $\text{NH}_3(\text{g}) + \text{HCl}(\text{g}) \rightarrow \text{NH}_4\text{Cl}(\text{s})$
Answer
Ammonia (NH₃). Confirmation: white fumes/smoke when held near HCl(g); $\text{NH}_3 + \text{HCl} \rightarrow \text{NH}_4\text{Cl}$ .

Key Definitions and Keywords — Identification of Ions and Gases

Definitions to memorise and the exact keywords mark schemes credit for identification of ions and gases answers — sharpened from recent examiner reports for the 2026 Cambridge IGCSE sitting.

Precipitate test
Examiner keyword
A chemical test in which a reagent is added to a solution to form a distinctive coloured insoluble precipitate, identifying a particular ion.
Flame test
Examiner keyword
A qualitative test in which a sample is introduced to a hot flame; metal ions emit characteristic colours due to electron transitions.
Limewater test
Examiner keyword
A test for CO₂ in which the gas is bubbled through limewater (Ca(OH)₂ solution); CO₂ reacts to form a white precipitate of CaCO₃, turning the solution milky.
Related: CO₂, carbonates
Squeaky pop test
Examiner keyword
The test for hydrogen gas: a burning splint held at the mouth of a test tube produces a squeaky pop sound when H₂ ignites.
Glowing splint test
Examiner keyword
A test for oxygen: a glowing (not burning) splint relights in the presence of O₂.
Bleaching of litmus (Cl₂ test)
Examiner keyword
Chlorine gas bleaches damp litmus paper white (after briefly turning red). This is the confirmatory test for Cl₂.

Common Mistakes and Misconceptions — Identification of Ions and Gases

The traps other students keep falling into on identification of ions and gases questions — taken from recent Cambridge IGCSE examiner reports and mark schemes — and how to avoid them.

✕Testing for sulfate without first acidifying with dilute HCl
0654 Examiner Report 2022
Why it happens
Students add BaCl₂ directly without removing carbonate or sulfite ions.
How to avoid it
Add excess dilute HCl first (removes CO₃²⁻ and SO₃²⁻ that would give false white precipitate), THEN add BaCl₂.
✕Confusing the colours of precipitates formed with NaOH
Why it happens
Students memorise the list imprecisely and mix up Fe²⁺/Fe³⁺/Cu²⁺ colours.
How to avoid it
Fe²⁺: green; Fe³⁺: brown/rust; Cu²⁺: blue; Ca²⁺: white (dissolves in excess NaOH for Al³⁺ and Zn²⁺ only). Learn these exactly as given in the data booklet.
✕Saying limewater turns milky because of dissolved CO₂ — not writing the equation
Why it happens
Students know the observation but cannot explain the chemistry.
How to avoid it
CO₂ + Ca(OH)₂ → CaCO₃(s) + H₂O. CaCO₃ is the white insoluble precipitate that makes the solution appear milky. Always give the equation for full marks.

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Identification of Ions and Gases

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Short Study Notes — Identification of Ions and Gases

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What you’ll learn

How it’s examined

1Identifying carbonate anion

2Test for sulfate ions

3Identifying Cu²⁺ ions

4Flame tests

5Identifying ammonia gas

Precipitate test

Flame test

Limewater test

Squeaky pop test

Glowing splint test

Bleaching of litmus (Cl₂ test)

✕Testing for sulfate without first acidifying with dilute HCl

✕Confusing the colours of precipitates formed with NaOH

✕Saying limewater turns milky because of dissolved CO₂ — not writing the equation

Practice questions

Past paper style quiz

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Identification of Ions and Gases — frequently asked questions