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

These revision notes condense section 3.1.4, Qualitative analysis, of the OCR A Level Chemistry A (H432) specification, Version 3.1 (May 2026), for first assessment in 2017. They cover the single learning outcome 3.1.4(a), both the anion part (i) and the ammonium part (ii), with the additional guidance on test order and the PAG4 link. H432 is the full A level. For full explanations and longer worked examples, read the Qualitative analysis study guide.

Test yourself afterwards with the Qualitative analysis practice questions. Other H432 sections are reached from the OCR chemistry hub. Use the printable checklist to record what you have revised, and the free diagnostics for a quick course-wide check.

The six ions in one table

Ion	Reagent and conditions	Positive observation	Ionic equation
CO₃²⁻	dilute acid (HNO₃ if more tests follow); pass gas into limewater	effervescence; limewater turns milky	CO₃²⁻(aq) + 2H⁺(aq) → CO₂(g) + H₂O(l)
SO₄²⁻	acidify with dilute HNO₃, then Ba²⁺(aq)	white precipitate	Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
Cl⁻	acidify with dilute HNO₃, then AgNO₃(aq)	white precipitate	Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
Br⁻	as for Cl⁻	cream precipitate	Ag⁺(aq) + Br⁻(aq) → AgBr(s)
I⁻	as for Cl⁻	yellow precipitate	Ag⁺(aq) + I⁻(aq) → AgI(s)
NH₄⁺	NaOH(aq), warm; damp red litmus at mouth of tube	litmus turns blue	NH₄⁺(aq) + OH⁻(aq) → NH₃(g) + H₂O(l)

Limewater equation: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l).

Silver halides and ammonia

Precipitate	Dilute NH₃(aq)	Concentrated NH₃(aq)
AgCl (white)	soluble	soluble
AgBr (cream)	insoluble	soluble
AgI (yellow)	insoluble	insoluble

Memory hook: solubility in ammonia falls from Cl to Br to I. The specification asks only for these observations, not the formulae of the ammonia complexes (3.1.3(g) guidance).

The order of tests (3.1.4(a)(i) guidance)

The specification sets the sequence as carbonate, then sulfate, then halide, and notes that BaCO₃ and Ag₂SO₄ are both insoluble.

Method in steps: one solution, several possible anions

1. Dissolve the unknown in distilled water.
2. Add dilute nitric acid a little at a time. Fizzing that turns limewater milky shows CO₃²⁻. Keep adding acid until fizzing stops, so all carbonate is gone.
3. Add barium nitrate solution in excess. A white precipitate shows SO₄²⁻. Filter it off and keep the filtrate.
4. Add silver nitrate solution to the filtrate. Note the colour of any precipitate.
5. Add dilute ammonia, then concentrated ammonia if needed, to tell Cl⁻, Br⁻ and I⁻ apart.
6. On a separate fresh sample, test for NH₄⁺ with warm NaOH(aq) and damp red litmus.

Why each choice

• Acid before barium: CO₃²⁻ + Ba²⁺ would also give a white precipitate (BaCO₃).
• Acid before silver: CO₃²⁻ would also precipitate with Ag⁺.
• Sulfate before halide: Ag₂SO₄ is insoluble, so sulfate must be removed before Ag⁺ goes in.
• Nitric, not hydrochloric or sulfuric acid: HCl adds Cl⁻; H₂SO₄ adds SO₄²⁻. Nitrate ions precipitate with none of the reagents.
• Barium nitrate, not barium chloride, in a sequence: BaCl₂ adds Cl⁻ before the halide test.
• Warm in the NH₄⁺ test: ammonia is very soluble in water; heat drives it out as a gas.
• Damp litmus: NH₃ must dissolve in water to give OH⁻ ions before litmus responds.

Test-tube technique (PAG4)

• Small volumes: a few cm³ of solution, reagents dropwise from a pipette.
• Distilled water only; tap water may hold Cl⁻ or SO₄²⁻.
• Fresh sample for each independent test.
• Litmus held at the tube mouth, not touching the glass, which may carry NaOH.
• A blank with distilled water helps when a result is faint.
• Record: observation first (“cream precipitate”), then inference (“Br⁻ present”).

The PAG4 example activity in the specification is identifying the anions and cations in a mixture of Group 2 salts.

Calculations that use these tests

Precipitation tests become quantitative when the solid is filtered, dried and weighed. Gas tests become quantitative when the gas volume is measured.

Situation	Mole link	Then
mass of BaSO₄	n(SO₄²⁻) = n(BaSO₄)	mass or % of the sulfate salt
mass of AgX	n(X⁻) = n(AgX)	mass of halide, or identity of X
volume of CO₂ at RTP	n(CO₃²⁻) = n(CO₂) = V ÷ 24.0 dm³ mol⁻¹	mass or purity of the carbonate

Small worked reminder. A salt solution gives 0.0150 mol of chloride ions. With excess silver nitrate, mass of AgCl = 0.0150 × 143.4 = 2.15 g (Mr(AgCl) = 107.9 + 35.5 = 143.4).

Reading a results grid

Work row by row, writing one inference per observation. Negative results count as evidence.

Observation on solution Z	Inference
dilute HNO₃: no effervescence	no CO₃²⁻
Ba(NO₃)₂(aq): white precipitate	SO₄²⁻ present
filtrate + AgNO₃(aq): white precipitate, dissolves in dilute NH₃(aq)	Cl⁻ present
warm with NaOH(aq): damp red litmus stays red	no NH₄⁺

Conclusion: Z contains sulfate and chloride ions, with no carbonate or ammonium ions. The barium salt had to be the nitrate here. Barium chloride would have put chloride into the filtrate, so the silver nitrate result would prove nothing.

Where this section fits

The assessment overview places Module 3 in component 01, Periodic table, elements and physical chemistry, and in component 03, Unified chemistry. The halide test is also listed in 3.1.3(g), and the carbonate reaction links to 2.1.4(c). Section 5.3.2 returns to these tests and adds transition metal cations: see the 5.3.2 Qualitative analysis study guide.

Must-know distinctions

• Effervescence vs a gas test. Fizzing shows a gas; only limewater turning milky shows that gas is CO₂.
• White BaSO₄ vs white BaCO₃. Same colour, so remove carbonate with acid first.
• White AgCl vs white BaSO₄. Different reagents (Ag⁺ vs Ba²⁺), different ions tested.
• Dilute vs concentrated ammonia. Dilute dissolves only AgCl; concentrated dissolves AgCl and AgBr.
• Ammonium ion vs ammonia. NH₄⁺ is the ion in the salt; NH₃ is the gas released when OH⁻ takes an H⁺ from it.
• Same sample vs fresh sample. The anion tests run in sequence on one acidified solution; the NH₄⁺ test needs a fresh portion, since NaOH would precipitate or neutralise what the other tests rely on.
• Observation vs inference. “Yellow precipitate” is seen; “iodide present” is deduced.

Quick self-test

1. Name the reagent, and the observation, that confirm the gas from a carbonate test is CO₂.
2. Write the ionic equation, with state symbols, for the sulfate test.
3. Why must a solution be acidified before barium ions are added?
4. Give the colour of the silver halide precipitate formed by bromide ions.
5. Which silver halide is insoluble in concentrated ammonia?
6. A student acidifies with dilute hydrochloric acid and then adds silver nitrate. Why is a white precipitate meaningless here?
7. Give two reasons the specification’s order is carbonate, then sulfate, then halide.
8. Why is the sodium hydroxide warmed in the ammonium test?
9. Why must the red litmus paper be damp?
10. A carbonate sample releases 30.0 cm³ of CO₂ at RTP. How many moles of carbonate ions were present? (Molar gas volume 24.0 dm³ mol⁻¹.)
11. Write the full equation for ammonium sulfate reacting with sodium hydroxide on warming.
12. A solution gives a yellow precipitate with acidified silver nitrate. Name the precipitate and state what you would add to confirm the halide.

Answers

1. Limewater (calcium hydroxide solution); it turns milky (white precipitate of CaCO₃).
2. Ba²⁺(aq) + SO₄²⁻(aq) → BaSO₄(s)
3. Acid destroys any carbonate; otherwise Ba²⁺ would form white BaCO₃ and mimic sulfate.
4. Cream.
5. AgI (silver iodide).
6. The acid itself supplies Cl⁻ ions, so AgCl forms whether or not the unknown contains chloride.
7. Carbonate precipitates with both Ba²⁺ and Ag⁺, so it must go first; Ag₂SO₄ is insoluble, so sulfate must be removed before the silver nitrate test.
8. Ammonia is very soluble in water; warming drives the gas out of solution so it can reach the litmus.
9. Ammonia must dissolve in water to release OH⁻ ions before it turns litmus blue.
10. 30.0 cm³ = 0.0300 dm³; n = 0.0300 ÷ 24.0 = 0.00125 mol (1.25 × 10⁻³ mol).
11. (NH₄)₂SO₄ + 2NaOH → Na₂SO₄ + 2NH₃ + 2H₂O
12. Silver iodide, AgI. Add concentrated aqueous ammonia: the precipitate does not dissolve, which confirms iodide rather than bromide.

Where marks are usually lost

• Writing “goes cloudy” or “turns white” with no mention of a precipitate.
• Giving fizzing as the full carbonate test without the limewater result.
• Acidifying with HCl or H₂SO₄ before a halide or sulfate test in the same solution.
• Using barium chloride and then testing that solution for halides.
• Mixing up which silver halides dissolve in dilute and in concentrated ammonia.
• Forgetting to warm, using blue litmus, or letting the litmus touch the tube in the NH₄⁺ test.
• State symbols wrong in ionic equations, especially (aq) on a precipitate.
• Balancing slips: one H⁺ in the carbonate equation, or one NaOH for (NH₄)₂SO₄.
• Stating a conclusion (“contains sulfate”) when the question asked for an observation.

Official syllabus

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