13.10 - Qualitative Analysis - Identifying Unknowns
Qualitative analysis questions
- Qualitative analysis questions involve two unknown substances, tested in turn and identified from what the tests show.
- Predicting observations - When a substance is named, marks are awarded for predicting what each test would show using exact qualitative analysis terminology.
- Identifying an unknown - When test observations are provided in a table, marks are awarded for deducing the identity of the ions present.
- Questions are answered using the printed "Notes for use in qualitative analysis" sheet provided with the exam paper.
A typical question
A student tests two solutions, solution C and solution D.
Tests on solution C: solution C is aqueous calcium nitrate. Complete the expected observations.
(a) The student carries out a flame test on the first portion. [1]
(b) Aqueous sodium hydroxide is added dropwise until in excess to the second portion. [2]
Parts (c) and (d) are two more tests on solution C.
Tests on solution D: the tests and the student's observations are given in a table.
(e) Deduce the pH of solution D. [1]
(f) Identify the gas made when sodium carbonate is added to solution D. [1]
Reading the notes sheet
The notes sheet is printed on every paper. Read each row in full rather than relying on memory.
Tests for anions
| Anion | Test | Test result |
|---|---|---|
| carbonate, CO32- | add dilute acid, then test for carbon dioxide gas | effervescence, carbon dioxide produced |
| chloride, Cl- | acidify with dilute nitric acid, then add aqueous silver nitrate | white precipitate |
| bromide, Br- | acidify with dilute nitric acid, then add aqueous silver nitrate | cream precipitate |
| iodide, I- | acidify with dilute nitric acid, then add aqueous silver nitrate | yellow precipitate |
| nitrate, NO3- | add aqueous sodium hydroxide, then aluminium foil; warm carefully | ammonia produced |
| sulfate, SO42- | acidify with dilute nitric acid, then add aqueous barium nitrate | white precipitate |
| sulfite, SO32- | add a small volume of acidified aqueous potassium manganate(VII) | purple to colourless |
Tests for aqueous cations
| Cation | Effect of aqueous sodium hydroxide | Effect of aqueous ammonia |
|---|---|---|
| aluminium, Al3+ | white precipitate, soluble in excess, giving a colourless solution | white precipitate, insoluble in excess |
| ammonium, NH4+ | ammonia produced on warming | - |
| calcium, Ca2+ | white precipitate, insoluble in excess | no precipitate, or very slight white precipitate |
| chromium(III), Cr3+ | green precipitate, soluble in excess | green precipitate, insoluble in excess |
| copper(II), Cu2+ | light blue precipitate, insoluble in excess | light blue precipitate, soluble in excess, giving a dark blue solution |
| iron(II), Fe2+ | green precipitate, insoluble in excess, turning brown near the surface on standing | green precipitate, insoluble in excess, turning brown near the surface on standing |
| iron(III), Fe3+ | red-brown precipitate, insoluble in excess | red-brown precipitate, insoluble in excess |
| zinc, Zn2+ | white precipitate, soluble in excess, giving a colourless solution | white precipitate, soluble in excess, giving a colourless solution |
Tests for gases
| Gas | Test and test result |
|---|---|
| ammonia, NH3 | turns damp red litmus paper blue |
| carbon dioxide, CO2 | turns limewater milky |
| chlorine, Cl2 | bleaches damp litmus paper |
| hydrogen, H2 | pops with a lighted splint |
| oxygen, O2 | relights a glowing splint |
| sulfur dioxide, SO2 | turns acidified aqueous potassium manganate(VII) from purple to colourless |
Flame tests for metal ions
| Metal ion | Flame colour |
|---|---|
| lithium, Li+ | red |
| sodium, Na+ | yellow |
| potassium, K+ | lilac |
| calcium, Ca2+ | orange-red |
| barium, Ba2+ | light green |
| copper(II), Cu2+ | blue-green |
- Sodium hydroxide and ammonia are added dropwise then in excess.
- Aluminium, calcium, and zinc all form a white precipitate initially. Only testing in excess separates them: aluminium and zinc precipitates redissolve in excess sodium hydroxide, while calcium precipitate remains insoluble.
Recording observations in the words that score
Always give the observation, not the identity. For example, for sodium hydroxide added to iron(II) sulfate, write "a green precipitate forms, insoluble in excess", not "it shows iron(II) is present".
The phrases that score
| What you might write | What the mark scheme credits |
|---|---|
| "it goes fizzy" | effervescence |
| "it goes cloudy" or "milky" | (turns) milky, or a white precipitate forms |
| "it makes a noise" | pops (with a lighted splint) |
| "the splint gets bigger" | relights (a glowing splint) |
| "it changes colour" | the actual colour change, e.g. purple to colourless |
| "a solid forms" | a precipitate forms, with its colour named |
| "it dissolves again" | soluble in excess (with the colour of solution that remains) |
Give both halves of a two-part observation
- "White precipitate, insoluble in excess" contains two distinct pieces of information.
- Mark schemes frequently award one mark to each half. Omitting the result in excess loses a mark.
Naming the colour precisely
Name each colour exactly as the notes sheet does:
- Cream - Not "off-white" or "yellowish-white" (bromide precipitate).
- Light green - Not just "green" (barium flame).
- Red-brown - Not "brown" (iron(III) precipitate).
Deducing an identity and giving the reasoning
- Identifying an unknown compound usually awards one mark for the cation and one for the anion.
- Match each ion to the specific observation that identifies it.
Worked example - Identifying a solid
A solid gives: no useful flame colour; sodium hydroxide dropwise gives a green precipitate, insoluble in excess, turning brown at the surface on standing; dilute nitric acid then barium nitrate gives a white precipitate. Identify the solid. [2]
Step 1 - Identify the cation
Green precipitate with sodium hydroxide, insoluble in excess, turning brown on standing → iron(II).
Step 2 - Identify the anion
White precipitate with barium nitrate (after nitric acid) → sulfate.
Step 3 - State the compound
The solid is iron(II) sulfate.
Multi-step deductions
Work through multi-step tests one at a time:
- Example: solution D turns universal indicator red (pH 1 - 3, an acid); fizzes with sodium carbonate, producing a gas that turns limewater milky (carbon dioxide); no change with silver nitrate (no halide); white precipitate with barium nitrate (sulfate present).
- Combined identity: D is dilute sulfuric acid.
Identify-the-gas and name-the-test questions
Short questions frequently ask you to name a gas from a result or describe a specific test directly.
Naming a gas from its observation
Look up the observation directly on the gas-test table: Example: filter paper soaked in acidified aqueous potassium manganate(VII) turns from purple to colourless → sulfur dioxide.
Describing a test outright
Provide both the reagent/method and the expected observation:
- Test for chlorine - Hold damp litmus paper in the gas; the paper is bleached (turns white).
- Chemical test for water - Add the liquid to anhydrous copper(II) sulfate; the powder turns from white to blue.
Common exam questions
Question 1
A solution gives a white precipitate with aqueous barium nitrate after being acidified with dilute nitric acid. Which anion is present?
Sulfate.
Question 2
A solution gives a cream precipitate with acidified aqueous silver nitrate. Which anion is present?
Bromide (chloride gives white, and iodide gives yellow, so the colour is what separates the three).
Question 3
Aqueous sodium hydroxide is added dropwise and then in excess to a solution, giving a white precipitate that dissolves in the excess to leave a colourless solution. Which cation could this be?
Aluminium or zinc (both give this pattern with sodium hydroxide alone; aqueous ammonia is needed to tell them apart, since aluminium's precipitate stays insoluble in excess ammonia and zinc's dissolves).
Question 4
A solid gives a lilac flame colour. Which metal ion is present?
Potassium.
Question 5
Aqueous ammonia is added to a solution of an iron salt, giving a red-brown precipitate that is insoluble in excess. Which iron ion is present?
Iron(III) (iron(II) gives a green precipitate that turns brown at the surface on standing, not a red-brown precipitate throughout).
Question 6
A gas relights a glowing splint. Name the gas.
Oxygen.
Question 7
A gas turns damp red litmus paper blue. Name the gas.
Ammonia.
Question 8
A student tests a solid and finds: a yellow flame colour; no precipitate with aqueous sodium hydroxide; and a white precipitate with acidified silver nitrate. Identify the solid.
Sodium (from the yellow flame) and chloride (from the white precipitate with silver nitrate). The sodium hydroxide test gives no precipitate because sodium is not one of the cations the notes sheet's hydroxide test identifies, which rules it out of that table rather than pointing to a different ion; the solid is sodium chloride.
Question 9
Why is the flame-test result alone not always enough to identify a cation?
Because if the sample is a mixture of ions, one flame colour can mask or be masked by another, so a mixture does not necessarily show a clean single colour; the flame test also gives no information at all about the anion present.
Question 10
Describe a test that would show whether a gas is carbon dioxide or sulfur dioxide, given that both are acidic gases.
Bubble the gas through acidified aqueous potassium manganate(VII): sulfur dioxide turns it from purple to colourless, while carbon dioxide causes no change. Limewater alone will not distinguish them, since both gases can affect it.