Overview
What you’ll learn
- Describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the cations of aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc by their precipitates and solubility.
- Describe tests to identify carbonate, chloride, nitrate and sulfate anions.
- Describe tests to identify ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide gases.
Key points
- 1Qualitative analysis identifies which ions are present — never how much.
- 2With NaOH(aq): Cu²⁺ gives a light blue precipitate, Fe²⁺ green, Fe³⁺ red-brown; Al³⁺, Ca²⁺ and Zn²⁺ all give white precipitates.
- 3Only the white precipitates of Al³⁺ and Zn²⁺ dissolve in excess NaOH — amphoteric hydroxides; Ca(OH)₂ does not.
- 4Aqueous ammonia separates the whites too: Zn²⁺ dissolves in excess, Al³⁺ does not; Cu²⁺ dissolves to a deep blue solution.
- 5NH₄⁺: no precipitate with NaOH, but warming releases ammonia, which turns damp red litmus blue.
- 6Anions: carbonate fizzes with acid (CO₂), chloride gives a white ppt with silver nitrate, sulfate a white ppt with barium chloride, nitrate gives ammonia with aluminium foil + NaOH on warming.
- 7Gas tests: H₂ pops, O₂ relights a glowing splint, CO₂ turns limewater milky, NH₃ turns red litmus blue, Cl₂ bleaches damp litmus, SO₂ decolourises acidified potassium manganate(VII).
- 8Always add dilute nitric acid before silver nitrate (chloride test) and before barium chloride (sulfate test) to remove interfering carbonate ions.
Notes · 1 of 5
What qualitative analysis is
Identify ions from the evidence: precipitates, colour changes and gas tests.
Qualitative analysis answers the question "what is in this sample?" — which cations and anions make up an unknown salt. It is qualitative because the answer is an identity, not an amount; measuring how much is a different job. The evidence comes from three kinds of test: adding aqueous sodium hydroxide or aqueous ammonia to name a cation from its precipitate, running anion tests with specific reagents, and applying gas tests to anything that fizzes off.
The method never changes: add a few drops of reagent, record the colour of any precipitate, then add the reagent in excess and record whether the precipitate dissolves. Those two observations — colour and solubility in excess — do nearly all the cation work.
Notes · 2 of 5
Cation tests with NaOH and aqueous ammonia
Colour names the transition-metal ion; solubility in excess splits the whites.
| Cation | Effect of a few drops of NaOH(aq) | Effect of excess NaOH(aq) |
|---|---|---|
| Aluminium, Al³⁺ | white precipitate | precipitate dissolves, colourless solution |
| Ammonium, NH₄⁺ | no precipitate | on warming, ammonia gas given off (damp red litmus turns blue) |
| Calcium, Ca²⁺ | white precipitate | precipitate insoluble |
| Copper(II), Cu²⁺ | light blue precipitate | precipitate insoluble |
| Iron(II), Fe²⁺ | green precipitate | precipitate insoluble (turns red-brown near the surface on standing) |
| Iron(III), Fe³⁺ | red-brown precipitate | precipitate insoluble |
| Zinc, Zn²⁺ | white precipitate | precipitate dissolves, colourless solution |
The precipitates are the insoluble metal hydroxides: Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s) light blue, Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s) red-brown. Al(OH)₃ and Zn(OH)₂ are amphoteric — the same property you met in Chapter 5 — so they react with the excess hydroxide and dissolve: Zn(OH)₂(s) + 2OH⁻(aq) → Zn(OH)₄²⁻(aq). Calcium hydroxide is white but only slightly soluble, so it stays as a precipitate.
| Cation | Effect of a few drops of NH₃(aq) | Effect of excess NH₃(aq) |
|---|---|---|
| Aluminium, Al³⁺ | white precipitate | insoluble in excess |
| Calcium, Ca²⁺ | no precipitate | no precipitate |
| Copper(II), Cu²⁺ | light blue precipitate | dissolves to a deep blue solution |
| Iron(II), Fe²⁺ | green precipitate | insoluble in excess |
| Iron(III), Fe³⁺ | red-brown precipitate | insoluble in excess |
| Zinc, Zn²⁺ | white precipitate | dissolves, colourless solution |
Compare the two reagents and the pairs fall apart: Al³⁺ vs Zn²⁺ — Zn²⁺ dissolves in excess ammonia, Al³⁺ does not. Ca²⁺ vs Al³⁺ (both white with NaOH, insoluble in excess) — Ca²⁺ gives nothing with ammonia, Al³⁺ gives a white precipitate. Cu²⁺ vs the rest — the deep blue solution in excess ammonia is unique.
Notes · 3 of 5
Tests for anions
Carbonate, chloride, nitrate and sulfate each have one signature reaction.
| Anion | Test | Positive result |
|---|---|---|
| Carbonate, CO₃²⁻ | add dilute acid | fizzing; gas turns limewater milky (CO₂) |
| Chloride, Cl⁻ | acidify with dilute nitric acid, then add silver nitrate solution | white precipitate (AgCl) |
| Nitrate, NO₃⁻ | add aqueous NaOH and aluminium foil, warm | ammonia given off — damp red litmus turns blue |
| Sulfate, SO₄²⁻ | acidify with dilute nitric acid, then add barium chloride solution | white precipitate (BaSO₄) |
The ionic equations behind the precipitates: Ag⁺(aq) + Cl⁻(aq) → AgCl(s) and Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s). The carbonate reaction is just Chapter 5's acid + carbonate pattern read as a test: CO₃²⁻(aq) + 2H⁺(aq) → H₂O(l) + CO₂(g).
Why the acidifying step? Carbonate ions also form a white precipitate with silver and barium ions (silver carbonate, barium carbonate). Adding dilute nitric acid first destroys any carbonate — it fizzes away as CO₂ — so the only precipitate left afterwards means chloride or sulfate was really there. Nitric acid is chosen because nitrate never interferes.
The nitrate test works by reduction: aluminium (with NaOH) converts nitrate ions all the way down to ammonia, which you detect with damp red litmus. Warm the mixture or the reaction drags.
Notes · 4 of 5
Tests for gases
Six gases, six unmistakable results.
| Gas | Test | Positive result |
|---|---|---|
| Ammonia, NH₃ | damp red litmus paper | turns blue (the only alkaline gas) |
| Carbon dioxide, CO₂ | bubble through limewater | turns milky (white CaCO₃ forms) |
| Chlorine, Cl₂ | damp litmus paper | bleaches it white (first turns red) |
| Hydrogen, H₂ | lighted splint | burns with a squeaky pop |
| Oxygen, O₂ | glowing splint | relights |
| Sulfur dioxide, SO₂ | acidified potassium manganate(VII) solution | purple colour decolourises |
Pair up the look-alikes. H₂ and O₂ both involve splints — but hydrogen needs a lighted splint (pop) while oxygen relights a glowing one. NH₃ and Cl₂ both attack litmus — ammonia turns damp red litmus blue, chlorine bleaches it white. SO₂ also bleaches some dyes, which is why the manganate test is the safe identifier: the purple colour fades to colourless.
Notes · 5 of 5
Putting the tests together
A short sequence names an unknown salt completely.
A complete identification runs in a fixed order. Step 1 — anion first: does it fizz with dilute acid (carbonate)? If not, dissolve a sample and run the chloride and sulfate tests (acidify, then silver nitrate or barium chloride). Step 2 — cation second: take a fresh solution and add NaOH dropwise then in excess; add ammonia to a second portion if the whites need separating; warm with NaOH if ammonium is suspected. Step 3 — name the salt by pairing the two answers: copper(II) sulfate, zinc chloride, ammonium carbonate…
Colour often shortcuts step 2: a blue solution already shouts Cu²⁺, a pale green one Fe²⁺, a yellow-brown one Fe³⁺. Colourless solutions hide the common Group metal ions and the ammonium ion — exactly the cases the precipitate tables were built for.
Revise
Key terms
The words examiners expect you to use precisely.
- qualitative analysis
- Experimental tests that identify which ions or substances are present in a sample, without measuring amounts.
- precipitate
- An insoluble solid formed when two solutions are mixed and react.
- amphoteric hydroxide
- A hydroxide (Al(OH)₃ or Zn(OH)₂) that reacts with both acids and bases, so its white precipitate dissolves in excess NaOH.
- aqueous ammonia
- Ammonia dissolved in water, NH₃(aq), a weak alkali used to identify cations by precipitate colour and solubility.
- limewater
- A clear solution of calcium hydroxide that turns milky when carbon dioxide bubbles through it.
- litmus test for ammonia
- Damp red litmus turns blue in ammonia — the only common alkaline gas.
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- acidified potassium manganate(VII)
- A purple oxidising agent used to test for sulfur dioxide, which decolourises it.
- in excess
- Adding more reagent after the precipitate forms, to test whether the precipitate dissolves — the key cation separator.
Revise
Common mistakes
Where students most often lose marks in this chapter — and what to do instead.
Mistake: Quoting only the precipitate colour for Al³⁺, Ca²⁺ or Zn²⁺.
All three give white precipitates — the colour alone cannot tell them apart.
Instead: The solubility in excess decides: Al³⁺ and Zn²⁺ dissolve in excess NaOH; Ca²⁺ does not; with excess ammonia Zn²⁺ dissolves but Al³⁺ does not.
Mistake: Saying ammonia gas turns limewater milky.
Mixing up the CO₂ test with the NH₃ test.
Instead: CO₂ turns limewater milky; NH₃ turns damp red litmus blue.
Show 10 more mistakesShow fewer mistakes
Mistake: Using a lighted splint to test for oxygen.
A lighted splint tests hydrogen (pop); oxygen needs a glowing one.
Instead: H₂: lighted splint, squeaky pop. O₂: glowing splint relights.
Mistake: Forgetting to acidify before adding silver nitrate or barium chloride.
Carbonate ions also give white precipitates with Ag⁺ and Ba²⁺, faking a positive result.
Instead: Add dilute nitric acid first to remove carbonate, then the test reagent.
Mistake: Testing for a nitrate by adding silver nitrate.
Silver nitrate tests chloride; nitrate ions form no precipitate with silver.
Instead: Nitrate is reduced by aluminium foil + NaOH on warming to ammonia, seen on damp red litmus.
Mistake: Saying Fe²⁺ gives a red-brown precipitate.
Swapping the two iron ions.
Instead: Fe²⁺ → green ppt; Fe³⁺ → red-brown ppt. (Green Fe(OH)₂ browns on standing as air oxidises it.)
Mistake: Claiming calcium gives a precipitate with aqueous ammonia.
Guessing by symmetry with NaOH.
Instead: Ca²⁺ gives no precipitate with NH₃(aq) — the reason the ammonia table separates Ca²⁺ from Al³⁺.
Mistake: Calling the deep blue solution with excess ammonia "copper metal formed".
The colour is from a dissolved copper–ammonia complex, not a new element.
Instead: Cu(OH)₂ dissolves in excess NH₃ giving a deep blue solution — still copper(II) in solution.
Mistake: Using hydrochloric acid to acidify before the chloride test.
HCl adds chloride ions of its own — a white precipitate would mean nothing.
Instead: Acidify with dilute nitric acid, which contributes no chloride or sulfate.
Mistake: Saying chlorine turns litmus blue.
That is ammonia; chlorine is an acidic, bleaching gas.
Instead: Cl₂ bleaches damp litmus white (it may redden it first).
Mistake: Writing "the test is positive for a metal".
Vague answers earn no marks; the tables name ions, not families.
Instead: Name the ion and the evidence: "Zn²⁺, because the white precipitate dissolves in excess NaOH and in excess NH₃".
Mistake: Testing for the ammonium ion without warming.
At room temperature the ammonia comes off too slowly to detect.
Instead: Add NaOH and warm gently; ammonia released turns damp red litmus blue.
Check yourself
Quick quiz
20 questions with instant explanations. About 5 minutes.
Question 1 of 3
- Question 1: current question
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- Question 3: not answered
Describe the use of aqueous sodium hydroxide and aqueous ammonia to identify the cations of aluminium, ammonium, calcium, copper(II), iron(II), iron(III) and zinc by their precipitates and solubility.
A colourless solution gives a white precipitate with a few drops of NaOH(aq), and the precipitate dissolves in excess to a colourless solution. Which cations could the solution contain?
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5 multiple-choice questions and structured questions with answer lines. Chapter 6 (cation tests with aqueous sodium hydroxide and aqueous ammonia, anion tests for carbonate, chloride, nitrate and sulfate, gas tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide).
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Questions
Frequently asked questions
How do I learn the cation table without mixing it up?
Learn three colours first (Cu²⁺ blue, Fe²⁺ green, Fe³⁺ red-brown), then the three whites (Al³⁺, Ca²⁺, Zn²⁺). For the whites, only two questions matter: does it dissolve in excess NaOH (Al³⁺ and Zn²⁺ yes, Ca²⁺ no) and does it dissolve in excess ammonia (Zn²⁺ yes, Al³⁺ no, Ca²⁺ gives nothing at all).
Why do I acidify before the chloride and sulfate tests?
To destroy carbonate ions, which would otherwise form white precipitates with silver and barium ions and fake a positive result. Use dilute nitric acid — hydrochloric acid would add chloride ions and sulfuric acid would add sulfate ions of its own.
Which gases are alkaline, and how do I test for them?
Ammonia is the only common alkaline gas: damp red litmus turns blue. Chlorine also attacks litmus but bleaches it white, so litmus alone tells them apart if you watch the colour carefully.
What is the quickest way to identify an unknown salt?
Test the anion first (acid fizz for carbonate; acidify + silver nitrate for chloride; acidify + barium chloride for sulfate; aluminium foil + warm NaOH for nitrate), then run the cation table on a fresh solution with NaOH and ammonia. Pair the two names to name the salt.