Qualitative analysis

In a nutshell

Qualitative analysis means working out which ions are in an unknown compound from what you see in a test tube: fizzing, a coloured precipitate, a gas that changes an indicator.

At AS you need four tests: carbonate, sulfate and halide (anions) and ammonium (a cation). You also need the ionic equation for each, and the order to run the anion tests in so that one ion cannot fake another's result.

Assumed knowledge: Compounds, formulae and equations; Acids; The halogens.

Core content

Tests for ions at a glance

An observation is what you see: a colour, a state, fizzing. "BaSO₄ forms" is an inference, not an observation, and does not score as one.

A precipitate is an insoluble solid that forms when two solutions react. Always give its colour and the word precipitate.

Testing for carbonate ions

Add a dilute acid. A carbonate gives effervescence (fizzing), because carbon dioxide gas is made.

Confirm the gas is CO₂ by bubbling it through limewater, which turns milky (cloudy).

The equations, using sodium carbonate and nitric acid:

• Full: Na₂CO₃(aq) + 2HNO₃(aq) → 2NaNO₃(aq) + H₂O(l) + CO₂(g)
• Ionic: 2H⁺(aq) + CO₃^2-(aq) → H₂O(l) + CO₂(g)
• Limewater going milky: Ca(OH)₂(aq) + CO₂(g) → CaCO₃(s) + H₂O(l)

On its own, any dilute acid works. When you are going on to test for sulfate and halide ions, use dilute nitric acid: HCl would add Cl^- ions and H₂SO₄ would add SO₄^2- ions, both of which the later tests would detect.

Testing for sulfate ions

Add a solution containing Ba²⁺(aq) ions, such as barium nitrate or barium chloride. A sulfate gives a white precipitate of barium sulfate.

• Full: Ba(NO₃)₂(aq) + MgSO₄(aq) → BaSO₄(s) + Mg(NO₃)₂(aq)
• Ionic: Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)

The catch: barium carbonate is also a white, insoluble solid. If carbonate ions are still present, Ba²⁺ gives a white precipitate even with no sulfate there:

Ba²⁺(aq) + CO₃^2-(aq) → BaCO₃(s)

So the carbonate test (with excess dilute nitric acid) comes first, and removes any carbonate as CO₂ before the barium is added.

In the full sequence, choose barium nitrate rather than barium chloride. Barium chloride adds Cl^- ions, which would give a white precipitate in the halide test that follows.

Testing for halide ions

Add dilute nitric acid, then silver nitrate solution, AgNO₃(aq). Each halide gives a silver halide precipitate of a different colour:

• Chloride: white precipitate of AgCl
• Bromide: cream precipitate of AgBr
• Iodide: yellow precipitate of AgI

The ionic equations all have the same shape: Ag⁺(aq) + X^-(aq) → AgX(s). For example:

• Full: AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)
• Ionic: Ag⁺(aq) + Cl^-(aq) → AgCl(s)

White, cream and yellow can look alike in a real test tube, so you add aqueous ammonia to the precipitate to confirm:

The pattern: the solubility of the silver halide in ammonia decreases from AgCl to AgI.

For OCR you only need the observations with ammonia. The equations for the silver-ammonia complexes are not required.

The nitric acid is there to remove carbonate ions, which would otherwise react with Ag⁺ to give a precipitate of silver carbonate and confuse the result. Never use hydrochloric acid here: it adds Cl^- ions and every sample would test positive for chloride.

Testing for ammonium ions

Add sodium hydroxide solution and warm gently. An ammonium compound releases ammonia gas, which turns moist red litmus paper blue.

• Full: NH₄Cl(aq) + NaOH(aq) → NaCl(aq) + NH₃(g) + H₂O(l)
• Ionic: NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)

Why each detail matters:

• Warm: ammonia is very soluble in water, so without heating most of it stays dissolved and little gas escapes.
• Moist litmus: ammonia has to dissolve in water on the paper to make it alkaline. Dry paper may not change.
• Red to blue: ammonia is an alkaline gas. Blue litmus would show no change.

Ammonia also has a sharp, pungent smell, but the litmus result is the test to quote.

The order of tests: carbonate, sulfate, then halide

When an unknown might contain more than one anion, OCR expects the tests in this order: carbonate, then sulfate, then halide.

The reason is that the later reagents are less selective, so an earlier ion can give a false positive:

Run on one sample of the unknown in solution:

1. Add dilute nitric acid until no more fizzing. Fizzing (gas turns limewater milky) means carbonate. The excess acid destroys all the carbonate.
2. Add barium nitrate solution. A white precipitate means sulfate. If there is one, add Ba(NO₃)₂(aq) in excess so all the sulfate is removed, then filter and keep the filtrate.
3. Add silver nitrate solution to the filtrate. A precipitate means a halide; use its colour and then aqueous ammonia to say which.

Test for the ammonium ion on a fresh sample, since it uses a different reagent (NaOH) and does not interfere with the anion tests.

Still don't get it? · why the order is carbonate, sulfate, then halide

Picture three fishing nets. The first net only ever catches one kind of fish. The second catches two kinds. The third catches almost everything. If you want to know exactly which fish are in a pond, you use the fussy net first, lift out what it caught, and only then use the less fussy ones. Use the "catch everything" net first and you cannot tell what you caught.

Now map that onto the ions:

1. Acid is the fussy net. Of these anions, only carbonate reacts with acid (it fizzes). So fizzing can only mean carbonate. Adding acid until the fizzing stops also turns all the carbonate into CO₂, which escapes. The carbonate is now gone.
2. Barium ions catch two kinds: sulfate and carbonate. But carbonate has already gone, so a white precipitate now can only mean sulfate. Adding excess barium nitrate and filtering takes all the sulfate out too.
3. Silver ions catch the most: halides, carbonate and sulfate. With carbonate and sulfate already removed, a precipitate now can only mean a halide.

Reverse the order and silver nitrate would give a precipitate with a plain carbonate, and you would wrongly report a halide.

In the exam, say it like this: BaCO₃ and Ag₂SO₄ are also insoluble, so carbonate must be tested for (and removed) before sulfate, and sulfate before halide, to avoid a false positive. Use nitric acid because it adds neither Cl^- nor SO₄^2-.

PAG4: qualitative analysis of ions

OCR links this subtopic to PAG4: identifying the anions and cations in a mixture of Group 2 salts, on a test-tube scale. The skills being assessed are using the apparatus for the tests and making and recording qualitative observations.

Good practice that turns into marks:

• Use small volumes in test tubes and add reagents dropwise, so you see the first sign of a precipitate.
• Record results in a table with three columns: test, observation, inference.
• Write observations as colour + state: "white precipitate", "colourless solution", "effervescence". If nothing happens, write "no visible change", not "nothing".
• Hold litmus paper at the mouth of the tube, not touching the glass or the solution (the solution itself contains NaOH, which would turn it blue anyway).
• Waft gases towards you rather than sniffing directly; use concentrated ammonia in a fume cupboard.

Worked examples

Example 1: identify an unknown from its results

A white solid, Y, dissolves in water. A student records:

Model answer, one inference per observation:

1. No effervescence with acid, so no carbonate ions.
2. No white precipitate with Ba²⁺, so no sulfate ions.
3. Cream precipitate with Ag⁺, insoluble in dilute but soluble in concentrated ammonia, so bromide ions: Ag⁺(aq) + Br^-(aq) → AgBr(s)
4. Alkaline gas (ammonia) released on warming with NaOH, so ammonium ions: NH₄⁺(aq) + OH^-(aq) → NH₃(g) + H₂O(l)
5. Y is ammonium bromide, NH₄Br.

Where marks go: each inference must be linked to its observation. "Y is NH₄Br" on its own does not show how you got there.

Example 2: plan the tests for a mixture

A solution is made from a mixture of magnesium sulfate and magnesium chloride. Plan tests on one sample that would show which anions are present, with the expected observations.

1. Add dilute nitric acid. No effervescence, so no carbonate. (The acid also makes sure any carbonate would have been removed.)
2. Add barium nitrate solution in excess. White precipitate of BaSO₄, so sulfate is present: Ba²⁺(aq) + SO₄^2-(aq) → BaSO₄(s)
3. Filter off the precipitate, so no sulfate is left to react with silver ions.
4. Add silver nitrate solution to the filtrate. White precipitate of AgCl: Ag⁺(aq) + Cl^-(aq) → AgCl(s)
5. Add dilute aqueous ammonia: the precipitate dissolves, confirming chloride.

Where marks go: choosing barium nitrate, not barium chloride (which would add the very Cl^- you are testing for), and keeping the carbonate, sulfate, halide order.

Example 3: explain a false result

A student adds barium chloride solution directly to a solution of sodium carbonate and sees a white precipitate. They conclude that sulfate is present. Explain the error and how to avoid it.

1. Barium ions also react with carbonate ions to form barium carbonate, a white insoluble precipitate: Ba²⁺(aq) + CO₃^2-(aq) → BaCO₃(s)
2. So the white precipitate is a false positive; it does not show sulfate.
3. Fix: first add dilute nitric acid until effervescence stops. This removes the carbonate as CO₂: 2H⁺(aq) + CO₃^2-(aq) → H₂O(l) + CO₂(g)
4. Then add the barium solution. Only sulfate can now give a white precipitate.

Common exam mistakes

• Giving an inference instead of an observation. "CO₂ is produced" or "BaSO₄ forms" is not what you see. Write "effervescence" and "white precipitate".
• Leaving out the colour or the word "precipitate". "Goes white" or "turns cream" is not enough; the result is a cream precipitate.
• Acidifying with the wrong acid. HCl before the halide test adds Cl^-, so every sample "contains chloride". H₂SO₄ before the sulfate test adds SO₄^2-. Use dilute nitric acid.
• Using barium chloride in a full sequence, then reporting chloride when silver nitrate gives a white precipitate. The Cl^- came from your own reagent; use barium nitrate.
• Running the tests in the wrong order. Silver nitrate added to a carbonate gives a precipitate, which is misread as a halide. The order is carbonate, sulfate, halide, because BaCO₃ and Ag₂SO₄ are insoluble.
• Mixing up the ammonia results. AgCl dissolves in dilute ammonia; AgBr only in concentrated; AgI does not dissolve, even in concentrated.
• Missing a detail in the ammonium test. The NaOH must be warmed, the litmus must be moist (or damp), and it turns red to blue. Say moist red litmus turns blue, not just "litmus changes colour".
• Calling the gas "ammonium". The ion in the compound is ammonium, NH₄⁺; the gas given off is ammonia, NH₃.
• Writing "chlorine" for "chloride". A white precipitate with silver nitrate shows chloride ions, Cl^-, not chlorine, Cl₂.
• Wrong ionic equations. Common slips: writing AgCO₃ or AgSO₄ instead of Ag₂CO₃ or Ag₂SO₄; leaving spectator ions (Na⁺, NO₃^-) in; missing the (s) on the precipitate. Check atoms and charges balance.

Key definitions

• Qualitative analysis: identifying which ions (or substances) are present in a sample, rather than how much of each.
• Precipitate: an insoluble solid formed when two solutions react.
• Effervescence: fizzing caused by a gas being produced in a liquid.
• Spectator ions: ions that are present in the reaction mixture but do not take part in the reaction; they are left out of the ionic equation.
• False positive: a positive result caused by an ion other than the one being tested for (e.g. BaCO₃ precipitating in the sulfate test).

Specification

• I can describe the test for carbonate ions: add a dilute acid, see effervescence, and show the CO₂ turns limewater milky, with the ionic equation 2H⁺(aq) + CO₃^2-(aq) → H₂O(l) + CO₂(g). (3.1.4 a i; see 2.1.4 c)
• I can describe the test for sulfate ions with Ba²⁺(aq), giving a white precipitate, with the ionic equation for BaSO₄. (3.1.4 a i)
• I can describe the test for Cl^-, Br^- and I^- with acidified silver nitrate, state the precipitate colours and the observations with dilute and concentrated ammonia, and write the ionic equations. (3.1.4 a i; see 3.1.3 g)
• I can describe the test for ammonium ions by warming with NaOH(aq) to form NH₃, which turns moist red litmus blue, with the ionic equation. (3.1.4 a ii)
• I can explain why the anion tests must be done in the order carbonate, sulfate, then halide, using the insolubility of BaCO₃ and Ag₂SO₄. (3.1.4 a, additional guidance)
• I can carry out the tests on a test-tube scale, record observations clearly, and identify the ions in an unknown compound or a mixture. (3.1.4 a, PAG4)

Related notes

• Compounds, formulae and equations: writing ionic equations with state symbols
• Acids: acids reacting with carbonates
• The halogens: the silver nitrate test for halide ions in the context of Group 17
• Group 2: the Group 2 compounds behind the PAG4 mixture
• Qualitative analysis (A level): the same anion and ammonium tests, plus transition metal cations (Cu²⁺, Fe²⁺, Fe³⁺, Mn²⁺, Cr³⁺)