Qualitative Analysis: Tests for Anions and Cations

Spec Mapping — OCR H432 Module 3.1.4 — Qualitative analysis, covering the chemical tests for the carbonate ion (CO₃²⁻), the sulfate ion (SO₄²⁻), the halide ions (Cl⁻, Br⁻, I⁻) using silver nitrate followed by aqueous ammonia, and the ammonium ion (NH₄⁺); the order in which these tests are performed to avoid false positives; and the writing of balanced ionic equations for each test (refer to the official OCR H432 specification document for exact wording). OCR does not require recall of flame test colours, in contrast to some other UK A-Level boards.

Qualitative analysis is the chemistry of "what is in this unknown sample?" — a battery of test-tube procedures that identify the cations and anions present in an ionic compound without measuring quantities. OCR Module 3.1.4 is the conclusion of the inorganic-chemistry block and is the direct PAG 4 anchor: every test described in this lesson is performed during PAG 4 (qualitative analysis of cations and anions) and is examined in Paper 1, Paper 2 and the Practical Endorsement. The four anions and one cation specified by OCR are: carbonate (CO₃²⁻), sulfate (SO₄²⁻), halides (Cl⁻ / Br⁻ / I⁻), and ammonium (NH₄⁺). The critical skill is performing the tests in the correct sequence — carbonate first, then sulfate, then halide — because each test can give false positives if a competing anion is still present. This lesson develops the full protocol, the ionic equations, the colour codes, and the confirmation steps with ammonia.

Key Insight: the sequence of tests is not arbitrary. It is chosen so that each test removes (or rules out) the species that would otherwise interfere with the next test in the chain. Memorise the sequence; never invert it.

Importance of sequence — avoiding false positives

When identifying unknown anions, the order matters because Group 2 cations like Ba²⁺ and Ag⁺ form insoluble white precipitates with multiple anions, not just the one you want to test for. The OCR-recommended order is:

flowchart TD
  A[Unknown solution / dissolved solid] --> B[1. Test for carbonate: add dilute HCl]
  B --> B1[Effervescence + limewater cloudy = CO3 2- present]
  B --> C[2. Test for sulfate: add BaCl2 in dilute HCl]
  C --> C1[White precipitate BaSO4 = SO4 2- present]
  C --> D[3. Test for halide: acidify with dilute HNO3 then add AgNO3]
  D --> D1[Coloured precipitate = halide present]
  D --> E[Confirm halide identity with aqueous ammonia]
  F[Separate test for ammonium NH4 +] --> F1[Warm with NaOH; damp red litmus turns blue]

Why this order?

Testing for sulfate before carbonate fails because BaCO₃ is also an insoluble white precipitate → false positive for sulfate. Dilute HCl in the sulfate-test step would dissolve any BaCO₃ but only after it has already formed.
Testing for halide before sulfate fails because Ag₂SO₄ is sparingly soluble and can precipitate as a pale white solid → false positive for halide (especially for Cl⁻).
Testing for halide before carbonate fails because Ag₂CO₃ is also insoluble (pale yellow) → false positive.

The mnemonic: "Carbonate, Sulfate, Halide" — CSH — always in that order.

Always acidify with the correct acid at each stage:

Dilute HCl for the sulfate test (removes any carbonate as CO₂ first, and the Cl⁻ does not interfere with BaSO₄ formation).
Dilute HNO₃ for the halide test (removes any remaining carbonate or sulfate, and NO₃⁻ does not precipitate with Ag⁺).

Using HCl in the halide test would be a disaster — Cl⁻ would react with AgNO₃ to give AgCl white precipitate, falsely indicating chloride in every unknown.

1. Test for carbonate ion (CO₃²⁻)

Test: Add a few drops of dilute hydrochloric acid (or any strong dilute acid) to the solid or solution.

Observation: Effervescence — bubbles of colourless, odourless gas. Collect the gas (e.g. on the end of a delivery tube) and bubble it through limewater. If the limewater turns cloudy/milky, the gas is CO₂ and the anion in the original sample is carbonate.

Ionic equation (acid step): $\text{CO}_3^{2-}\text{(s/aq)} + 2\text{H}^+\text{(aq)} \rightarrow \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$

Limewater test for CO₂: $\text{Ca(OH)}_2\text{(aq)} + \text{CO}_2\text{(g)} \rightarrow \text{CaCO}_3\text{(s)} + \text{H}_2\text{O(l)}$ (white insoluble CaCO₃ is what makes the limewater cloudy)

Example with sodium carbonate: $\text{Na}_2\text{CO}_3\text{(s)} + 2\text{HCl(aq)} \rightarrow 2\text{NaCl(aq)} + \text{CO}_2\text{(g)} + \text{H}_2\text{O(l)}$

The carbonate test works equally well on solid samples (which fizz vigorously when acid is added) or aqueous solutions of carbonate salts. Hydrogencarbonate ions (HCO₃⁻) also give CO₂ on acidification — they are functionally indistinguishable from carbonate in this test.

2. Test for sulfate ion (SO₄²⁻)

Test: Add a few drops of dilute hydrochloric acid first (to acidify and destroy any carbonate as CO₂), then add a few drops of aqueous barium chloride BaCl₂ (or equivalently barium nitrate Ba(NO₃)₂).

Observation: A white precipitate of BaSO₄.

Ionic equation: $\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}$

The HCl-acidification step is critical: without it, any carbonate present would form an indistinguishable white precipitate of BaCO₃, giving a false positive. With HCl, carbonate is destroyed (as CO₂(g)) before BaCl₂ is added, so any white precipitate seen on adding BaCl₂ can only be BaSO₄.

BaSO₄ is one of the most insoluble salts known ( $K_{sp} \sim 10^{-10}$ ), and the white precipitate is a clean, unambiguous confirmation. The same chemistry is exploited in barium meals (previous lesson) — the lack of solubility makes BaSO₄ both a safe contrast agent in medicine and an unambiguous diagnostic test for sulfate.

Important alternative acid: dilute HNO₃ can be used instead of HCl, but HCl is the OCR-preferred choice because nitrate is the spectator ion in the silver-nitrate halide test that follows, and using HCl first avoids confusing chloride contamination later. Choose HCl for the sulfate step, HNO₃ for the halide step.

3. Test for halide ions (Cl⁻, Br⁻, I⁻)

Test: Acidify the unknown solution with a few drops of dilute nitric acid HNO₃ (to destroy any remaining carbonate or sulfate). Then add a few drops of aqueous silver nitrate AgNO₃.

Observation — colour of precipitate:

Halide	Ionic equation	Precipitate colour
Cl⁻	$\text{Ag}^+\text{(aq)} + \text{Cl}^-\text{(aq)} \rightarrow \text{AgCl(s)}$	White
Br⁻	$\text{Ag}^+\text{(aq)} + \text{Br}^-\text{(aq)} \rightarrow \text{AgBr(s)}$	Cream
I⁻	$\text{Ag}^+\text{(aq)} + \text{I}^-\text{(aq)} \rightarrow \text{AgI(s)}$	Yellow

The colours are subtle — white, cream and yellow can be hard to distinguish in dim light or against a coloured test-tube background. To confirm which halide is present, follow up with aqueous ammonia.

Confirming with aqueous ammonia

Precipitate	Dilute NH₃	Concentrated NH₃
AgCl (white)	Dissolves	Dissolves
AgBr (cream)	Insoluble	Dissolves
AgI (yellow)	Insoluble	Insoluble

The silver halide dissolves by forming a soluble diamminesilver(I) complex ion:

$\text{AgCl(s)} + 2\text{NH}_3\text{(aq)} \rightarrow [\text{Ag(NH}_3\text{)}_2]^+\text{(aq)} + \text{Cl}^-\text{(aq)}$

The solubility trend in ammonia mirrors the lattice-energy trend of the silver halides — AgCl is the most ionic and most soluble; AgI is the most covalent (highly polarised by Ag⁺, with I⁻ being large and polarisable) and the least soluble, so even concentrated NH₃ cannot pull Ag⁺ out of the AgI lattice. The differential solubility is therefore a clean diagnostic:

AgCl + dilute NH₃ → dissolves = chloride confirmed.
AgBr + dilute NH₃ → no change; + concentrated NH₃ → dissolves = bromide confirmed.
AgI + dilute NH₃ → no change; + concentrated NH₃ → no change = iodide confirmed.

Light-stability is another diagnostic: AgCl darkens slowly in sunlight (the photochemistry behind black-and-white film); AgBr darkens more rapidly; AgI most rapidly of all. This is a useful confirmation if ammonia is not available, but it is not the primary OCR test.

4. Test for ammonium ion (NH₄⁺)

Test: Add a few drops of aqueous sodium hydroxide to the unknown solid or solution and warm gently. Hold a piece of damp red litmus paper at the mouth of the test tube.

Observation: If NH₄⁺ is present, ammonia gas (NH₃) is released, which turns the damp red litmus paper blue. NH₃ has a characteristic pungent smell (the smell of household ammonia / urine).

Ionic equation: $\text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}$

Or, full equation for ammonium chloride: $\text{NH}_4\text{Cl(aq)} + \text{NaOH(aq)} \rightarrow \text{NaCl(aq)} + \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}$

Why warm? At room temperature, the ammonia produced stays dissolved as NH₃(aq) (or as NH₄OH equilibrium); warming drives off the gas so it reaches the litmus paper and registers a positive result.

Why red litmus, not blue? Red litmus turns blue in alkali — an unambiguous positive test. Using blue litmus would give no colour change (it is already blue), making the test useless.

The pungent smell of NH₃ is itself a diagnostic, but always confirm with damp red litmus turning blue for a definitive result. NH₃ is the only common alkaline gas produced from ionic compounds, so the test is highly specific.

5. Flame tests (OCR note — recall not required)

OCR does not require recall of flame test colours — a distinction from some other A-Level boards (AQA does require these). However, you should know:

A flame test involves dipping a clean (acid-rinsed) nichrome wire into the solid or solution and holding the loaded wire in a non-luminous Bunsen flame.
Different cations produce characteristic colours because electrons are excited to higher energy levels by the flame and emit light of specific wavelengths as they drop back down.
The colour observed corresponds to the dominant emission wavelength of the cation's electronic transitions.

If you are given a flame-test colour in a question, you may be expected to interpret it (e.g. "the green flame suggests Ba²⁺"), but you do not need to memorise the colour list for the OCR exam. Save your revision time for the ionic-equation work that does carry marks.

A short reference for general chemistry knowledge:

Cation	Flame colour
Li⁺	Crimson red
Na⁺	Persistent yellow
K⁺	Lilac
Ca²⁺	Brick red
Sr²⁺	Crimson
Ba²⁺	Apple green
Cu²⁺	Blue-green

Combined worked example

Q: An unknown white solid is thought to contain one anion and one cation. Describe how you would identify both if the solid is ammonium sulfate, (NH₄)₂SO₄.

Answer: Dissolve a small sample in distilled water for the solution tests, and reserve some solid for the ammonium test.

Carbonate test: Add dilute HCl to a portion of the solution. Observation: no effervescence → no carbonate present. (Or if there were carbonate, CO₂ would fizz and turn limewater cloudy.)

Sulfate test: To another portion, add dilute HCl (already done), then a few drops of aqueous BaCl₂. Observation: white precipitate of BaSO₄. Ionic equation: $\text{Ba}^{2+}\text{(aq)} + \text{SO}_4^{2-}\text{(aq)} \rightarrow \text{BaSO}_4\text{(s)}$ . → Sulfate confirmed.

Halide test (to rule out): Take a fresh portion, acidify with dilute HNO₃, add a few drops of AgNO₃. Observation: no precipitate → no halide present.

Ammonium test: Take a fresh portion of the solid (or solution), add aqueous NaOH, and warm gently. Hold damp red litmus paper at the mouth of the test tube. Observation: pungent smell, red litmus turns blue → NH₃ released. Ionic equation: $\text{NH}_4^+\text{(aq)} + \text{OH}^-\text{(aq)} \rightarrow \text{NH}_3\text{(g)} + \text{H}_2\text{O(l)}$ . → Ammonium confirmed.

Conclusion: the solid is (NH₄)₂SO₄, ammonium sulfate.

Worked examples

Example 1 — distinguishing NaCl from NaI

Q: Two white solid samples, P and Q, are known to be either sodium chloride or sodium iodide. Describe how you would distinguish them using a single test, including the observations expected for each.

Answer: Add a few drops of dilute HNO₃ (acidification), then AgNO₃ to a small sample of each. P: white precipitate of AgCl forms — adding dilute NH₃ to this precipitate dissolves it, confirming chloride; P is NaCl. Q: yellow precipitate of AgI forms — even with concentrated NH₃ this does not dissolve, confirming iodide; Q is NaI.

Example 2 — sequence-of-tests rationale

Q: A student tests an unknown sample with BaCl₂ in HCl first and observes a white precipitate; she concludes that sulfate is present. Explain why this conclusion may be premature, and describe the correct procedure.

Qualitative Analysis