Why is "Swapping the iron precipitate colours (Fe²⁺ vs Fe³⁺)" a common mistake in Tests for cations and anions?

Why it happens: Both irons give a coloured precipitate, and students forget which is which. How to avoid it: Lock it in: **Fe²⁺ → green**, **Fe³⁺ → brown**. The '+2' green, '+3' brown order is fixed — and 'brown' matches the rust colour of iron(III). Source: Edexcel Chemistry examiner reports — recurring error

Why is "Failing to add EXCESS NaOH to distinguish Al³⁺ from Ca²⁺" a common mistake in Tests for cations and anions?

Why it happens: Both give a white precipitate, so students think the test cannot separate them. How to avoid it: Always add **excess** NaOH: **Al(OH)₃ dissolves** (colourless solution), **Ca(OH)₂ stays**. The word 'excess' earns the mark. Source: Edexcel Chemistry examiner reports

Why is "Saying 'it fizzes' but not testing the gas with limewater" a common mistake in Tests for cations and anions?

Why it happens: Fizzing looks like proof, so students stop there. How to avoid it: Fizzing only shows a gas is given off. State that the gas **turns limewater milky** to confirm it is **carbon dioxide** — that is what identifies a carbonate. Source: Edexcel Chemistry examiner reports

Why is "Muddling the silver halide precipitate colours" a common mistake in Tests for cations and anions?

Why it happens: White, cream and yellow are close in shade and easy to confuse. How to avoid it: Memorise in order down the group: **Cl⁻ white → Br⁻ cream → I⁻ yellow** (the colour deepens as you go down). Source: Edexcel Chemistry examiner reports

Why is "Using the wrong acid (or forgetting the acid-first step)" a common mistake in Tests for cations and anions?

Why it happens: Students mix up the two anion tests or skip the acid entirely. How to avoid it: **Halide → dilute nitric acid + silver nitrate**; **sulfate → dilute hydrochloric acid + barium chloride**. Never use HCl in the halide test (its chloride would give a white precipitate). Source: Edexcel Chemistry examiner reports

Why is "Using DRY red litmus paper in the ammonium test" a common mistake in Tests for cations and anions?

Why it happens: Students forget the gas must dissolve in water to act on the litmus. How to avoid it: Always write **DAMP** red litmus paper, and remember to **WARM** the mixture so ammonia is released. Dry paper shows no colour change. Source: Edexcel Chemistry examiner reports

Acids, bases and salt preparationsEdexcel IGCSE Science(Double Award)-Chemistry (4WSD0-1C)

Tests for cations and anions

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Tests for Cations and Anions — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

A toolkit of standard tests that identify the ions in an unknown salt. CATIONS with sodium hydroxide solution: Cu²⁺ blue ppt, Fe²⁺ green ppt, Fe³⁺ brown ppt, Al³⁺ white (dissolves in EXCESS), Ca²⁺ white (stays), NH₄⁺ gives ammonia on warming (damp red litmus → blue). ANIONS: carbonate fizzes with acid (CO₂ → limewater milky); halides with dilute nitric acid + silver nitrate give white (Cl⁻), cream (Br⁻), yellow (I⁻) precipitates; sulfate with dilute HCl + barium chloride gives a white precipitate. Covers Double Award Chemistry spec 2.52, 2.53, 2.55, 2.56, assessed on Chemistry Paper 1 (4WSD0/1C).

What you’ll learn

Mapped to the Edexcel IGCSE 4WSD0-1C syllabus (2026 onwards).

2.52 — Describe tests for the cations Cu2+, Fe2+, Fe3+, Al3+, Ca2+ and NH4+ using sodium hydroxide solution.
2.53 — Describe the test for carbonate ions using dilute acid.
2.55 — Describe the test for halide ions using silver nitrate and dilute nitric acid.
2.56 — Describe the test for sulfate ions using barium chloride and dilute hydrochloric acid.

Testing cations with sodium hydroxide (spec 2.52)

Add NaOH(aq) drop by drop — a coloured hydroxide precipitate names the metal cation.

A cation is a positive ion (here, a metal ion or the ammonium ion). To identify it, add a few drops of sodium hydroxide solution, NaOH(aq). The hydroxide (OH⁻) reacts with the metal cation to form an insoluble metal hydroxide — a coloured precipitate. The colour tells you which cation is present.

Two white precipitates? Add EXCESS NaOH: Al(OH)₃ dissolves, Ca(OH)₂ stays.

Standard observations.

Cation	Precipitate with NaOH(aq)	Effect of EXCESS NaOH
Cu²⁺	blue precipitate	stays
Fe²⁺	green precipitate	stays
Fe³⁺	brown (orange-brown) precipitate	stays
Al³⁺	white precipitate	DISSOLVES → colourless solution
Ca²⁺	white precipitate	does NOT dissolve (stays)
NH₄⁺	no precipitate	on warming, gives off ammonia (damp red litmus → blue)

Telling the two white precipitates apart — Al³⁺ vs Ca²⁺. Both give a white precipitate, so add EXCESS NaOH: the aluminium hydroxide dissolves to give a colourless solution, while the calcium hydroxide stays as a white precipitate. "Add excess" is the keyword that earns the mark.

Testing for ammonium (NH₄⁺). Ammonium is the odd one out — it gives no precipitate. Instead: add NaOH(aq), warm gently, and hold damp red litmus paper at the mouth of the tube. If ammonia gas is released, the litmus turns blue — this confirms NH₄⁺. The word damp is essential (the gas must dissolve in the water on the paper).

Cu²⁺ → blue ppt; Fe²⁺ → green ppt; Fe³⁺ → brown ppt.
Al³⁺ → white ppt that DISSOLVES in excess NaOH; Ca²⁺ → white ppt that stays.
Add EXCESS NaOH to distinguish Al³⁺ (dissolves) from Ca²⁺ (stays).
NH₄⁺: warm with NaOH → ammonia turns damp red litmus blue (no precipitate).

See the full worked example for tests for cations and anions →

Testing for carbonate ions (spec 2.53)

Add dilute acid — fizzing gives off CO₂, which turns limewater milky.

A carbonate (CO₃²⁻) reacts with dilute acid to release carbon dioxide gas. This is one of the simplest and most reliable anion tests.

Test: add a few drops of dilute acid (e.g. dilute hydrochloric acid) to the solid or solution.
Observation: fizzing / bubbling (effervescence) as a gas is given off.
Confirm the gas: bubble it through limewater — the limewater turns milky (cloudy white), which confirms the gas is carbon dioxide (CO₂).

Ionic equation: CO₃²⁻ + 2H⁺ → H₂O + CO₂

Examiner tip: to score full marks you must give two observations — the fizzing and the limewater turning milky. "It fizzes" alone does not prove carbon dioxide; many gases fizz. The limewater test is what identifies the gas as CO₂.

Add dilute acid → fizzing (effervescence).
The gas turns limewater milky → it is carbon dioxide (CO₂).
Give BOTH observations — fizzing alone does not identify the gas.
Ionic equation: CO₃²⁻ + 2H⁺ → H₂O + CO₂.

See the full worked example for tests for cations and anions →

Testing for halide ions (spec 2.55)

Add dilute nitric acid first, then silver nitrate — white/cream/yellow precipitates.

Halide ions are chloride (Cl⁻), bromide (Br⁻) and iodide (I⁻). To identify them, add dilute nitric acid (HNO₃) FIRST, then add silver nitrate solution (AgNO₃). A coloured silver-halide precipitate forms — and the colour names the halide.

The dilute nitric acid removes carbonate first, so it cannot give a false-positive white precipitate.

Observations.

Halide	Precipitate	Colour
Chloride (Cl⁻)	silver chloride (AgCl)	white
Bromide (Br⁻)	silver bromide (AgBr)	cream
Iodide (I⁻)	silver iodide (AgI)	yellow

Ionic equations.

Ag⁺ + Cl⁻ → AgCl
Ag⁺ + Br⁻ → AgBr
Ag⁺ + I⁻ → AgI

Why add dilute nitric acid first? If a carbonate is present, it would react with silver nitrate to give a white precipitate (silver carbonate) — a false positive that you could mistake for chloride. The dilute nitric acid removes any carbonate (as CO₂) before the silver nitrate is added, so only the true halide precipitate forms. You must never use hydrochloric acid here — its own chloride ions would give a white precipitate.

Add dilute HNO₃ FIRST, then AgNO₃(aq).
Cl⁻ → white ppt; Br⁻ → cream ppt; I⁻ → yellow ppt.
The acid first removes carbonate, preventing a false-positive white ppt.
Never use HCl — its chloride ions would react with AgNO₃.

See the full worked example for tests for cations and anions →

Testing for sulfate ions (spec 2.56)

Add dilute hydrochloric acid first, then barium chloride — white precipitate.

A sulfate (SO₄²⁻) is identified using barium chloride. Add dilute hydrochloric acid (HCl) FIRST, then add barium chloride solution (BaCl₂). A white precipitate of barium sulfate (BaSO₄) forms.

Test: add a few drops of dilute HCl, then BaCl₂(aq).
Observation: a white precipitate (barium sulfate).
Ionic equation: Ba²⁺ + SO₄²⁻ → BaSO₄

Why add dilute hydrochloric acid first? If a carbonate is present, it would react with barium chloride to give a white precipitate (barium carbonate) — another false positive. The dilute HCl removes the carbonate (as CO₂) before the barium chloride is added, so only a true sulfate gives the white precipitate.

Spotting the difference. Both the halide test and the sulfate test add an acid first, then a metal-salt reagent. Keep the pairs straight:

Anion	Acid first	Reagent	Result
Carbonate (CO₃²⁻)	—	dilute acid	fizzes → CO₂ (limewater milky)
Chloride (Cl⁻)	dilute HNO₃	AgNO₃	white precipitate
Bromide (Br⁻)	dilute HNO₃	AgNO₃	cream precipitate
Iodide (I⁻)	dilute HNO₃	AgNO₃	yellow precipitate
Sulfate (SO₄²⁻)	dilute HCl	BaCl₂	white precipitate

Add dilute HCl FIRST, then BaCl₂(aq) → white precipitate (BaSO₄).
Ionic equation: Ba²⁺ + SO₄²⁻ → BaSO₄.
Acid first removes carbonate, which would also give a white ppt (false positive).
Halide uses HNO₃ + AgNO₃; sulfate uses HCl + BaCl₂ — don't mix them up.

See the full worked example for tests for cations and anions →

How it’s examined

Double Award Chemistry is assessed on one untiered paper — Chemistry Paper 1 (4WSD0/1C), 2 hours, 110 marks, 33.3% of the Double Award (calculator allowed). Ion tests are high-value identification marks: you may be asked to tabulate observations for cation tests with NaOH, describe how to identify an unknown anion, distinguish two similar ions (e.g. Fe²⁺ vs Fe³⁺, or Al³⁺ vs Ca²⁺ using excess NaOH; Cl⁻ vs I⁻ by precipitate colour), or explain why dilute acid is added first in the halide and sulfate tests. The observations must be EXACT — a wrong precipitate colour loses the mark outright.

Sources: Pearson Edexcel International GCSE Science (Double Award) 4SD0 specification — Issue 4 (valid for 2026 onwards); Pearson Edexcel International GCSE Chemistry 4CH1 specification — Issue 4 (shared Chemistry Paper 1 content); Pearson Edexcel 4SD0 / 4CH1 examiner reports 2022–2024. Last reviewed 2026-06-07.

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Step-by-step worked examples — Tests for cations and anions

Step-by-step solutions to past-paper-style questions on tests for cations and anions, written exactly the way a tutor would explain them at the board.

1Identify a cation from its NaOH precipitate (Getting started)
Getting started• 4WSD0/1C style — short answer• cations, spec-2.52
Question
A solution gives a brown precipitate when a few drops of sodium hydroxide solution are added. Name the cation present. (1 mark)
Step-by-step solution
1. Step 1
  Recall the colour code (1). With NaOH(aq): Cu²⁺ → blue, Fe²⁺ → green, Fe³⁺ → brown, Al³⁺/Ca²⁺ → white.
2. Step 2
  Match the observation. A brown precipitate matches iron(III), Fe³⁺.
Answer
Fe³⁺ (iron(III) ion).
Examiner tip
Learn the four coloured outcomes exactly: blue Cu²⁺, green Fe²⁺, brown Fe³⁺. A vague answer like 'iron' is not enough — you must specify Fe³⁺ because Fe²⁺ gives a different (green) precipitate.
2Test for a carbonate (Getting started)
Getting started• 4WSD0/1C style — short answer• carbonate, spec-2.53
Question
Describe a test to show that a white solid is a carbonate, and give the result. (2 marks)
Step-by-step solution
1. Step 1
  Test (1). Add dilute acid (e.g. dilute hydrochloric acid) to the solid.
2. Step 2
  Result (1). It fizzes and the gas given off turns limewater milky, showing carbon dioxide is produced.
Answer
Add dilute acid: the solid fizzes and the gas turns limewater milky, confirming a carbonate.
Examiner tip
Both marks need the test AND the result. 'It fizzes' alone is incomplete — the limewater test identifies the gas as CO₂, which is what proves a carbonate.
3Distinguish chloride from iodide (Building confidence)
Building confidence• 4WSD0/1C style — structured• halides, spec-2.55, application
Question
Two unlabelled solutions contain sodium chloride and sodium iodide. Describe a test that would tell them apart, and state the result for each. (3 marks)
Step-by-step solution
1. Step 1
  Reagents (1). To each solution add dilute nitric acid, then silver nitrate solution (AgNO₃).
2. Step 2
  Chloride result (1). Sodium chloride gives a white precipitate (silver chloride).
3. Step 3
  Iodide result (1). Sodium iodide gives a yellow precipitate (silver iodide).
Answer
Add dilute nitric acid then silver nitrate to each. Chloride gives a white precipitate; iodide gives a yellow precipitate.
Examiner tip
The discriminator is the precipitate colour: white (Cl⁻) vs yellow (I⁻). Both marks for the results need the correct colours; saying 'they go cloudy' is too vague to distinguish them.
4Distinguish Fe²⁺ from Fe³⁺ (Building confidence)
Building confidence• 4WSD0/1C style — structured• cations, spec-2.52, application
Question
Two solutions contain iron(II) sulfate and iron(III) sulfate. Describe how sodium hydroxide solution could be used to tell them apart. (3 marks)
Step-by-step solution
1. Step 1
  Method (1). Add sodium hydroxide solution to each — a coloured hydroxide precipitate forms in both.
2. Step 2
  Iron(II) result (1). Iron(II), Fe²⁺, gives a green precipitate.
3. Step 3
  Iron(III) result (1). Iron(III), Fe³⁺, gives a brown precipitate.
Answer
Add NaOH(aq) to each: iron(II) gives a green precipitate and iron(III) gives a brown precipitate.
Examiner tip
Marks are won on the exact colours — green for Fe²⁺, brown for Fe³⁺. Mixing up which iron ion gives which colour is one of the most common errors in this topic.
5Tell apart two white precipitates (Stretch)
Stretch• 4WSD0/1C style — extended• cations, spec-2.52, reasoning
Question
Two solutions each give a white precipitate when sodium hydroxide solution is added. One contains Al³⁺ and the other contains Ca²⁺. Describe how you could find out which is which, and state what you would see. (3 marks)
Step-by-step solution
1. Step 1
  The problem. Both Al³⁺ and Ca²⁺ give a white precipitate, so the initial test cannot tell them apart.
2. Step 2
  Add EXCESS NaOH (1). Continue adding sodium hydroxide solution until it is in excess.
3. Step 3
  Al³⁺ result (1). The Al³⁺ precipitate dissolves to give a colourless solution.
4. Step 4
  Ca²⁺ result (1). The Ca²⁺ precipitate does not dissolve — it stays as a white precipitate.
Answer
Add excess sodium hydroxide solution: the aluminium precipitate dissolves (colourless solution), while the calcium precipitate stays as a white solid.
Examiner tip
The mark-scheme keyword is 'EXCESS'. Students who only add a little more NaOH, or who forget that only Al(OH)₃ dissolves, lose the marks. State clearly which one dissolves.
6Explain the 'acid first' step (Stretch)
Stretch• 4WSD0/1C style — extended• anions, spec-2.55, spec-2.56, reasoning
Question
In the test for sulfate ions, dilute hydrochloric acid is added before the barium chloride solution. Explain why the dilute acid is added first. (2 marks)
Step-by-step solution
1. Step 1
  The interferent (1). A carbonate, if present, would also react with barium chloride to give a white precipitate (barium carbonate).
2. Step 2
  The fix (1). The dilute acid removes the carbonate (it fizzes off as CO₂) before the barium chloride is added, so only a genuine sulfate gives the white precipitate — preventing a false positive.
Answer
Carbonate ions would also give a white precipitate with barium chloride. The dilute acid removes any carbonate first, so the white precipitate only appears if a sulfate is genuinely present.
Examiner tip
The high-value idea is 'carbonate would give a false-positive white precipitate, so acid removes it first'. The same reasoning applies to dilute nitric acid in the halide test. Just saying 'to clean the solution' scores nothing.

Model Answers — Tests for cations and anions

High-scoring sample answers for tests for cations and anions on the Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) paper, with examiner-style notes mapping each response to the mark scheme and assessment objectives.

Question 1
4WSD0/1C style2 marks
State the colour of the precipitate formed when sodium hydroxide solution is added to a solution containing: (a) Cu²⁺; (b) Fe²⁺. (2 marks)
Model answer
(a) Cu²⁺ → blue precipitate.

(b) Fe²⁺ → green precipitate.
Why this scores
1 mark each. These are recall marks — the colours must be exact. Blue is Cu²⁺; green is Fe²⁺ (not Fe³⁺, which is brown).
Question 2
4WSD0/1C style3 marks
Describe the test for a carbonate, including the reagent used and the observations you would make. (3 marks)
Model answer
Add dilute acid (e.g. dilute hydrochloric acid) to the sample. (1)

The sample fizzes / bubbles as a gas is given off. (1)

Bubble the gas through limewater — it turns milky, confirming the gas is carbon dioxide. (1)
Why this scores
All three parts are needed: reagent, fizzing, and limewater turning milky. The limewater step is what proves the gas is CO₂ — fizzing alone is not enough.
Question 3
4WSD0/1C style4 marks
A student has three solutions: potassium chloride, potassium bromide and potassium iodide. Describe a single test that would identify each one, and give the result for all three. (4 marks)
Model answer
To each solution add dilute nitric acid, then silver nitrate solution (AgNO₃). (1)

Chloride (Cl⁻) gives a white precipitate (silver chloride). (1)

Bromide (Br⁻) gives a cream precipitate (silver bromide). (1)

Iodide (I⁻) gives a yellow precipitate (silver iodide). (1)
Why this scores
1 mark for the correct reagents (nitric acid + silver nitrate) and 1 for each precipitate colour. The three colours — white, cream, yellow — must be exact and in the right order, as they are easily confused.
Question 4
4WSD0/1C style4 marks
Describe how you would test a solution for sulfate ions, explaining why dilute hydrochloric acid is added before the barium chloride. (4 marks)
Model answer
Add a few drops of dilute hydrochloric acid, then add barium chloride solution (BaCl₂). (1)

A white precipitate (barium sulfate) confirms a sulfate is present. (1)

The dilute acid is added first because a carbonate, if present, would also give a white precipitate with barium chloride. (1)

The acid removes any carbonate (as carbon dioxide) before the test, preventing a false positive. (1)
Why this scores
Marks for: reagents, white precipitate, identifying carbonate as the interferent, and explaining the acid removes it. 'To clean the test tube' is a non-answer; the point is the carbonate false positive.
Question 5
4WSD0/1C style — extended6 marks
An unknown white solid X dissolves in water. When sodium hydroxide solution is added, a white precipitate forms that dissolves when excess NaOH is added. A separate sample of X, treated with dilute hydrochloric acid then barium chloride solution, gives a white precipitate. Identify both ions in X and name the compound. Explain your reasoning. (6 marks)
Model answer
Cation. With NaOH a white precipitate forms that dissolves in excess NaOH. White rules out Cu²⁺ (blue), Fe²⁺ (green) and Fe³⁺ (brown). Of the two white-precipitate cations, only Al³⁺ dissolves in excess NaOH (Ca²⁺ would stay). So the cation is Al³⁺. (2)

Anion. Adding dilute hydrochloric acid then barium chloride gives a white precipitate. After the acid removes any carbonate, a white precipitate with barium chloride identifies a sulfate, SO₄²⁻. (2)

Compound. The salt contains Al³⁺ and SO₄²⁻, so X is aluminium sulfate, Al₂(SO₄)₃. (2)
Why this scores
Top-band answers state the observation, eliminate the alternatives, and then name the ion. The 'dissolves in excess' detail is what pins the cation to Al³⁺ rather than Ca²⁺. Combining the ions with the correct charges (2 × Al³⁺ + 3 × SO₄²⁻) gives the formula.
Question 6
4WSD0/1C style — extended6 marks
A solution may contain ammonium ions (NH₄⁺), iodide ions (I⁻), or both. Describe a scheme of tests to find out which ions are present, giving the reagents and the expected observations. (6 marks)
Model answer
Test for ammonium (NH₄⁺). Add sodium hydroxide solution to a sample and warm gently. Hold damp red litmus paper at the mouth of the tube. If ammonia gas is released, the litmus turns blue, confirming NH₄⁺ is present. (3)

Test for iodide (I⁻). To a fresh sample add dilute nitric acid, then silver nitrate solution. A yellow precipitate (silver iodide) confirms iodide is present. (3)

If both observations are seen, both ions are present; if only one is seen, only that ion is present.
Why this scores
Each ion test scores up to 3 marks (reagents + procedure + observation). Use a SEPARATE sample for each test. For ammonium the 'warm' and 'damp red litmus → blue' details are essential; for iodide the yellow precipitate must be stated exactly.

Key Definitions and Keywords — Tests for cations and anions

Definitions to memorise and the exact keywords mark schemes credit for tests for cations and anions answers — sharpened from recent examiner reports for the 2026 Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) sitting.

Cation
Examiner keyword
A positively charged ion — here a metal ion (Cu²⁺, Fe²⁺, Fe³⁺, Al³⁺, Ca²⁺) or the ammonium ion (NH₄⁺). Identified using sodium hydroxide solution.
Anion
Examiner keyword
A negatively charged ion — here carbonate (CO₃²⁻), the halides (Cl⁻, Br⁻, I⁻) or sulfate (SO₄²⁻).
Precipitate
Examiner keyword
An insoluble solid that forms in a solution during a reaction, making it cloudy. The colour of a hydroxide or silver-halide precipitate identifies the ion.
Halide ion
Examiner keyword
The negative ion of a Group 7 element: chloride (Cl⁻), bromide (Br⁻) or iodide (I⁻). Tested with dilute nitric acid followed by silver nitrate.
Excess (of a reagent)
Examiner keyword
Adding more than enough of a reagent. Adding EXCESS NaOH distinguishes Al³⁺ (white precipitate dissolves) from Ca²⁺ (white precipitate stays).
False positive
A result that wrongly suggests an ion is present. Carbonate can cause a false-positive white precipitate, which is why dilute acid is added before silver nitrate or barium chloride.
Limewater test
Examiner keyword
Bubbling a gas through limewater (calcium hydroxide solution); it turns milky (cloudy white) if the gas is carbon dioxide.
Damp red litmus test
Examiner keyword
Holding damp red litmus paper in a gas; it turns blue if ammonia is present. The paper must be damp so the gas can dissolve.
Ammonium ion (NH₄⁺)
A positive ion that gives no precipitate with NaOH but, on warming, releases ammonia gas that turns damp red litmus blue.
Aluminium hydroxide dissolving in excess
Aluminium hydroxide is the only white hydroxide precipitate here that dissolves in excess sodium hydroxide — used to tell Al³⁺ from Ca²⁺.

Common Mistakes and Misconceptions — Tests for cations and anions

The traps other students keep falling into on tests for cations and anions questions — taken from recent Pearson Edexcel IGCSE Chemistry Paper 1 (4WSD0/1C) examiner reports and mark schemes — and how to avoid them.

✕Swapping the iron precipitate colours (Fe²⁺ vs Fe³⁺)
Edexcel Chemistry examiner reports — recurring error
Why it happens
Both irons give a coloured precipitate, and students forget which is which.
How to avoid it
Lock it in: Fe²⁺ → green, Fe³⁺ → brown. The '+2' green, '+3' brown order is fixed — and 'brown' matches the rust colour of iron(III).
✕Failing to add EXCESS NaOH to distinguish Al³⁺ from Ca²⁺
Edexcel Chemistry examiner reports
Why it happens
Both give a white precipitate, so students think the test cannot separate them.
How to avoid it
Always add excess NaOH: Al(OH)₃ dissolves (colourless solution), Ca(OH)₂ stays. The word 'excess' earns the mark.
✕Saying 'it fizzes' but not testing the gas with limewater
Edexcel Chemistry examiner reports
Why it happens
Fizzing looks like proof, so students stop there.
How to avoid it
Fizzing only shows a gas is given off. State that the gas turns limewater milky to confirm it is carbon dioxide — that is what identifies a carbonate.
✕Muddling the silver halide precipitate colours
Edexcel Chemistry examiner reports
Why it happens
White, cream and yellow are close in shade and easy to confuse.
How to avoid it
Memorise in order down the group: Cl⁻ white → Br⁻ cream → I⁻ yellow (the colour deepens as you go down).
✕Using the wrong acid (or forgetting the acid-first step)
Edexcel Chemistry examiner reports
Why it happens
Students mix up the two anion tests or skip the acid entirely.
How to avoid it
Halide → dilute nitric acid + silver nitrate; sulfate → dilute hydrochloric acid + barium chloride. Never use HCl in the halide test (its chloride would give a white precipitate).
✕Using DRY red litmus paper in the ammonium test
Edexcel Chemistry examiner reports
Why it happens
Students forget the gas must dissolve in water to act on the litmus.
How to avoid it
Always write DAMP red litmus paper, and remember to WARM the mixture so ammonia is released. Dry paper shows no colour change.

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Tests for Cations and Anions — Edexcel International GCSE Science (Double Award) Chemistry (4WSD0-1C) Study Notes

Cation

Precipitate with NaOH(aq)

Effect of EXCESS NaOH

Cu²⁺

blue precipitate

stays

Fe²⁺

green precipitate

stays

Fe³⁺

brown (orange-brown) precipitate

stays

Al³⁺

white precipitate

DISSOLVES → colourless solution

Ca²⁺

white precipitate

does NOT dissolve (stays)

NH₄⁺

no precipitate

on warming, gives off ammonia (damp red litmus → blue)

A carbonate (CO₃²⁻) reacts with dilute acid to release carbon dioxide gas. This is one of the simplest and most reliable anion tests.

Test: add a few drops of dilute acid (e.g. dilute hydrochloric acid) to the solid or solution.
Observation: fizzing / bubbling (effervescence) as a gas is given off.
Confirm the gas: bubble it through limewater — the limewater turns milky (cloudy white), which confirms the gas is carbon dioxide (CO₂).

Ionic equation: CO₃²⁻ + 2H⁺ → H₂O + CO₂

Halide

Precipitate

Colour

Chloride (Cl⁻)

silver chloride (AgCl)

white

Bromide (Br⁻)

silver bromide (AgBr)

cream

Iodide (I⁻)

silver iodide (AgI)

yellow

Anion

Acid first

Reagent

Result

Carbonate (CO₃²⁻)

—

dilute acid

fizzes → CO₂ (limewater milky)

Chloride (Cl⁻)

dilute HNO₃

AgNO₃

white precipitate

Bromide (Br⁻)

dilute HNO₃

AgNO₃

cream precipitate

Iodide (I⁻)

dilute HNO₃

AgNO₃

yellow precipitate

Sulfate (SO₄²⁻)

dilute HCl

BaCl₂

white precipitate

Tests for cations and anions

Detailed Notes

What you’ll learn

How it’s examined

1Identify a cation from its NaOH precipitate (Getting started)

2Test for a carbonate (Getting started)

3Distinguish chloride from iodide (Building confidence)

4Distinguish Fe²⁺ from Fe³⁺ (Building confidence)

5Tell apart two white precipitates (Stretch)

6Explain the 'acid first' step (Stretch)

Cation

Anion

Precipitate

Halide ion

Excess (of a reagent)

False positive

Limewater test

Damp red litmus test

Ammonium ion (NH₄⁺)

Aluminium hydroxide dissolving in excess

✕Swapping the iron precipitate colours (Fe²⁺ vs Fe³⁺)

✕Failing to add EXCESS NaOH to distinguish Al³⁺ from Ca²⁺

✕Saying 'it fizzes' but not testing the gas with limewater

✕Muddling the silver halide precipitate colours

✕Using the wrong acid (or forgetting the acid-first step)

✕Using DRY red litmus paper in the ammonium test

Past paper style quiz

Tests for cations and anions

Detailed Notes

What you’ll learn

How it’s examined

1Identify a cation from its NaOH precipitate (Getting started)

2Test for a carbonate (Getting started)

3Distinguish chloride from iodide (Building confidence)

4Distinguish Fe²⁺ from Fe³⁺ (Building confidence)

5Tell apart two white precipitates (Stretch)

6Explain the 'acid first' step (Stretch)

Cation

Anion

Precipitate

Halide ion

Excess (of a reagent)

False positive

Limewater test

Damp red litmus test

Ammonium ion (NH₄⁺)

Aluminium hydroxide dissolving in excess

✕Swapping the iron precipitate colours (Fe²⁺ vs Fe³⁺)

✕Failing to add EXCESS NaOH to distinguish Al³⁺ from Ca²⁺

✕Saying 'it fizzes' but not testing the gas with limewater

✕Muddling the silver halide precipitate colours

✕Using the wrong acid (or forgetting the acid-first step)

✕Using DRY red litmus paper in the ammonium test

Past paper style quiz