OCR A Level Chemistry A: Qualitative analysis, 5.3.2 (H432) – Revision Notes

For full explanations and worked examples, read the 5.3.2 study guide first. These notes are for the final weeks.

They cover section 5.3.2, Qualitative analysis, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. The section has one learning outcome, 5.3.2(a), with anions in part (i) and cations in part (ii), plus links to PAG4 and HSW4. It sits in Module 5, which the assessment overview places in component 01 and component 03. H432 is the full A level.

Course hub: OCR A Level chemistry. Progress tracker: printable checklist. Gap finder: free diagnostics. Exam-style practice: 5.3.2 practice questions.

The outcome in one line

Given an unknown compound, use test-tube tests to identify CO₃²⁻, Cl⁻, Br⁻, I⁻ and SO₄²⁻ (part (i)) and NH₄⁺, Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺ and Cr³⁺ (part (ii)). The anion tests and the ammonium test come from 3.1.4(a). The transition metal tests use the precipitation and complex-formation reactions of 5.3.1(j). Revise those sections in 3.1.4 Qualitative analysis and 5.3.1 Transition elements.

Anions: the order matters

Order	Test for	Add	Positive result
1	CO₃²⁻	dilute HNO₃(aq)	fizzing; gas turns limewater milky
2	SO₄²⁻	Ba(NO₃)₂(aq) or another Ba²⁺ source	white precipitate, BaSO₄
3	Cl⁻ / Br⁻ / I⁻	AgNO₃(aq), then NH₃(aq)	white / cream / yellow precipitate

Silver halides with ammonia: AgCl dissolves in dilute NH₃; AgBr dissolves only in concentrated NH₃; AgI does not dissolve in either.

Why this order: BaCO₃ and Ag₂SO₄ are both insoluble (the specification states this). Remove carbonate with acid before adding Ba²⁺; deal with sulfate before adding Ag⁺. Use nitric acid, because HCl adds Cl⁻ and H₂SO₄ adds SO₄²⁻.

Transition metal carbonates are insoluble in water. If a coloured solid fizzes in dilute nitric acid and dissolves, you now have its metal ion in solution for the cation tests.

Cations

Ammonium

Method in steps

1. Add NaOH(aq) to the unknown.
2. Warm gently.
3. Hold a strip of damp red litmus just inside the tube’s mouth, clear of the glass.
4. Litmus turns blue: NH₃ is given off, so NH₄⁺ is present.

NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Transition metal ions: the master table

Ion (solution colour)	a little NaOH(aq) or NH₃(aq)	excess NaOH(aq)	excess NH₃(aq)
Cu²⁺ (pale blue)	pale blue precipitate	stays	deep blue solution, [Cu(NH₃)₄(H₂O)₂]²⁺
Fe²⁺ (pale green)	green precipitate; orange-brown at the surface on standing	stays	stays
Fe³⁺ (pale yellow to orange-brown)	orange-brown precipitate	stays	stays
Mn²⁺ (very pale pink)	light brown precipitate; darkens on standing in air	stays	stays
Cr³⁺ (violet, often green)	grey-green precipitate	dark green solution, [Cr(OH)₆]³⁻	purple solution, [Cr(NH₃)₆]³⁺

Memory hook from the specification’s guidance: excess NaOH dissolves one hydroxide (chromium). Excess NH₃ dissolves two (chromium and copper).

Equations to be able to write

Process	Equation
Cu²⁺ precipitation	Cu²⁺(aq) + 2OH⁻(aq) → Cu(OH)₂(s)
Fe²⁺ precipitation	Fe²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(s)
Fe³⁺ precipitation	Fe³⁺(aq) + 3OH⁻(aq) → Fe(OH)₃(s)
Mn²⁺ precipitation	Mn²⁺(aq) + 2OH⁻(aq) → Mn(OH)₂(s)
Cr³⁺ precipitation	Cr³⁺(aq) + 3OH⁻(aq) → Cr(OH)₃(s)
Cr(OH)₃ in excess NaOH	Cr(OH)₃(s) + 3OH⁻(aq) → [Cr(OH)₆]³⁻(aq)
Cr(OH)₃ in excess NH₃	Cr(OH)₃(s) + 6NH₃(aq) → [Cr(NH₃)₆]³⁺(aq) + 3OH⁻(aq)
Cu(OH)₂ in excess NH₃	Cu(OH)₂(s) + 4NH₃(aq) + 2H₂O(l) → [Cu(NH₃)₄(H₂O)₂]²⁺(aq) + 2OH⁻(aq)

Either formula style is acceptable for precipitation, according to the specification: Fe³⁺(aq) or [Fe(H₂O)₆]³⁺, Fe(OH)₃(s) or Fe(OH)₃(H₂O)₃. Complexed version for Fe²⁺:

[Fe(H₂O)₆]²⁺(aq) + 2OH⁻(aq) → Fe(OH)₂(H₂O)₄(s) + 2H₂O(l)

Method in steps: a full unknown

1. Look at the solid and try to dissolve it in distilled water. Record the colour of any solution.
2. If it will not dissolve, try dilute nitric acid and watch for fizzing (carbonate).
3. Acidify a portion with dilute HNO₃(aq). Add Ba(NO₃)₂(aq) for sulfate.
4. To a fresh acidified portion (or the filtrate from step 3), add AgNO₃(aq), then NH₃(aq), for halides.
5. To a fresh portion, add NaOH(aq) dropwise, then in excess. Warm and test for ammonia with damp red litmus.
6. To another fresh portion, add NH₃(aq) dropwise, then in excess.
7. Write each observation, then the ion it shows, then an ionic equation.

Worked reminder

A violet solution shows no fizzing with acid and no precipitate with Ba²⁺ or Ag⁺. NaOH(aq) gives a grey-green precipitate that dissolves in excess to a dark green solution; no gas turns litmus blue on warming.

• Cation: Cr³⁺, because only chromium(III) hydroxide dissolves in excess NaOH.
• Ammonium: absent.
• Anion: none of the five listed anions is present. The compound could be a nitrate, for example, but the 5.3.2 tests cannot confirm that. Say only what the evidence supports.

Must-know distinctions

• A little vs excess. “A little” is a few drops; you see the hydroxide precipitate. “Excess” means several cm³ more, shaken; this is where Cu and Cr show their extra reactions.
• NaOH vs NH₃. Both precipitate hydroxides, because ammonia gives OH⁻ through NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. In excess, NH₃ can act as a ligand; NaOH cannot.
• Fe²⁺ vs Fe³⁺. Green precipitate vs orange-brown precipitate. A green one that browns on standing is still Fe²⁺: air is oxidising it to iron(III) hydroxide.
• Fe²⁺ vs Cr³⁺. Both solutions can look green. Excess NaOH settles it: Cr(OH)₃ dissolves, Fe(OH)₂ does not.
• Cu²⁺ in excess ammonia. The precipitate dissolves. The deep blue is a solution, not a precipitate.
• Precipitate vs solution. Always say which. “Turns blue” is incomplete until you say what is blue.

Practical points (PAG4, HSW4)

• Note the colour of the unknown solution before adding anything.
• Use a fresh portion of the unknown for each reagent.
• After a carbonate test with excess acid, the first drops of NaOH only neutralise the acid. Keep adding.
• Leave Fe²⁺ and Mn²⁺ precipitates to stand, then look again.
• Eye protection for NaOH(aq); keep concentrated ammonia bottles closed and use dilute ammonia where it works.

Quick self-test

1. Which two hydroxides dissolve in excess aqueous ammonia?
2. State the colour of the precipitate formed when NaOH(aq) is added to Fe³⁺(aq).
3. Write the ionic equation for the precipitation of manganese(II) hydroxide.
4. Give the formula of the complex formed when Cr(OH)₃ dissolves in excess NaOH(aq), and the colour of the solution.
5. A precipitate formed from a pale green solution turns orange-brown at the top after ten minutes. Name the cation in the original solution.
6. Why does aqueous ammonia, a weak base, still precipitate metal hydroxides?
7. Why must the litmus paper be damp in the ammonium test?
8. A blue solution gives a pale blue precipitate with NaOH(aq) that does not dissolve in excess. With excess NH₃(aq) a deep blue solution forms. Identify the cation and the deep blue species.
9. Calculate the minimum volume of 0.200 mol dm⁻³ NaOH(aq) needed to precipitate all the Fe³⁺ from 15.0 cm³ of 0.0400 mol dm⁻³ FeCl₃(aq).
10. Why is barium chloride a poor choice of sulfate reagent if you will test the same solution for halides next?
11. Write the equation for Cr(OH)₃ dissolving in excess aqueous ammonia.

Answers

1. Cu(OH)₂ and Cr(OH)₃.
2. Orange-brown.
3. Mn²⁺(aq) + 2OH⁻(aq) → Mn(OH)₂(s)
4. [Cr(OH)₆]³⁻; dark green.
5. Fe²⁺ (iron(II)). The darkening is air oxidation to iron(III) hydroxide.
6. NH₃ + H₂O ⇌ NH₄⁺ + OH⁻ gives a low concentration of OH⁻, which is enough to precipitate the hydroxides.
7. The indicator only responds once NH₃ has dissolved in the water on the paper, giving OH⁻(aq).
8. Cu²⁺; [Cu(NH₃)₄(H₂O)₂]²⁺.
9. n(Fe³⁺) = 0.0400 × 0.0150 = 6.00 × 10⁻⁴ mol; n(OH⁻) = 3 × 6.00 × 10⁻⁴ = 1.80 × 10⁻³ mol; volume = 1.80 × 10⁻³ ÷ 0.200 = 9.00 × 10⁻³ dm³ = 9.00 cm³.
10. It adds Cl⁻ ions, so silver nitrate would then give a white precipitate whether or not the unknown contained chloride.
11. Cr(OH)₃(s) + 6NH₃(aq) → [Cr(NH₃)₆]³⁺(aq) + 3OH⁻(aq); the solution is purple.

Where marks are usually lost

• Saying a hydroxide “dissolves in excess” for Fe²⁺, Fe³⁺ or Mn²⁺. Only Cr(OH)₃ dissolves in excess NaOH; only Cr(OH)₃ and Cu(OH)₂ dissolve in excess NH₃.
• Writing “blue precipitate” for the Cu²⁺ + excess NH₃ result. It is a deep blue solution.
• Giving colour without state, such as “goes green”, instead of “green precipitate”.
• Missing the second observation for Fe²⁺ or Mn²⁺: the change on standing in air.
• Leaving off charges on complex ions, or writing [Cr(OH)₆]³⁺ with the wrong sign. Six OH⁻ around Cr³⁺ gives 3 − 6 = −3.
• Writing (aq) for a precipitated hydroxide.
• In the ammonium test: not warming, or using dry or blue litmus paper.
• Acidifying with HCl or H₂SO₄ before anion tests, then reporting the ion you added.
• Testing for sulfate or halide before removing carbonate.

Official syllabus

OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017, section 5.3.2 Qualitative analysis, learning outcome (a)(i) and (a)(ii), with additional guidance (PAG4, HSW4) and the linked statements 3.1.4(a) and 5.3.1(j). Published by OCR.