12b Identifying ions and gases | IGCSE Chemistry 0620

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🎯What you need to be able to do

Describe the tests for the anions carbonate, chloride, bromide, iodide, nitrate, sulfate and sulfite.
Describe the tests with aqueous sodium hydroxide and aqueous ammonia for Al³⁺, NH₄⁺, Ca²⁺, Cr³⁺, Cu²⁺, Fe²⁺, Fe³⁺ and Zn²⁺.
Describe the tests for ammonia, carbon dioxide, chlorine, hydrogen, oxygen and sulfur dioxide.
Describe flame tests for Li⁺, Na⁺, K⁺, Ca²⁺, Ba²⁺ and Cu²⁺.

📚The chemistry

All of this topic is Core, and the same tests are printed as the “Notes for use in qualitative analysis” in the practical papers (5 and 6). In the theory papers you must know them. Learn the wording as well as the colour: examiners want “white precipitate” (or “ppt.”), not “goes cloudy”.

Tests for anions

Carbonate, CO₃²⁻: add dilute acid — effervescence; the gas (carbon dioxide) turns limewater milky.
Chloride, bromide, iodide (in solution): acidify with dilute nitric acid, then add aqueous silver nitrate — white, cream and yellow precipitates respectively.
Nitrate, NO₃⁻ (in solution): add aqueous sodium hydroxide, then aluminium foil; warm carefully — ammonia is produced (damp red litmus turns blue).
Sulfate, SO₄²⁻ (in solution): acidify with dilute nitric acid, then add aqueous barium nitrate — white precipitate.
Sulfite, SO₃²⁻: add a small volume of acidified aqueous potassium manganate(VII) — it changes from purple to colourless.

Three test tubes after acidifying with dilute nitric acid and adding aqueous silver nitrate. Chloride gives a white precipitate, bromide a cream precipitate and iodide a yellow precipitate. — The silver halides. The nitric acid is added first to remove carbonate ions, which would also give a precipitate.

Why nitric acid? Hydrochloric acid would add chloride ions (a false positive for chloride), and sulfuric acid would add sulfate ions (a false positive for sulfate).

Tests for aqueous cations

Add aqueous sodium hydroxide a few drops at a time, then in excess; repeat with aqueous ammonia where you need to decide between two ions.

A flow chart for aqueous sodium hydroxide. No precipitate: warm; ammonia given off means ammonium, NH4+. White precipitate insoluble in excess: calcium, Ca2+. White precipitate that dissolves in excess to a colourless solution: then add aqueous ammonia in excess; white precipitate insoluble means aluminium, Al3+; white precipitate that dissolves means zinc, Zn2+. Coloured precipitates: light blue, insoluble in excess, copper(II); green, soluble in excess, chromium(III); green, insoluble, turns brown at the surface on standing, iron(II); red-brown, insoluble, iron(III). With aqueous ammonia, copper's precipitate dissolves in excess to a dark blue solution and calcium gives no precipitate or a very slight one. — Sodium hydroxide first; ammonia separates aluminium from zinc. Every result here matches the syllabus’s qualitative analysis notes.

Flame tests

Dip a clean wire (nichrome or platinum), moistened with acid, into the solid and hold it in a blue Bunsen flame.

Six flame colours: lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green. — Lithium red, sodium yellow, potassium lilac, calcium orange-red, barium light green, copper(II) blue-green.

Tests for gases

Six gas tests. Hydrogen: a lighted splint gives a squeaky pop. Oxygen: a glowing splint relights. Carbon dioxide: bubbled through limewater, which turns milky. Ammonia: damp red litmus paper turns blue. Chlorine: damp litmus paper is bleached. Sulfur dioxide: acidified potassium manganate(VII) turns from purple to colourless. — Hydrogen pops, oxygen relights, CO₂ turns limewater milky, ammonia turns damp red litmus blue, chlorine bleaches damp litmus, SO₂ decolourises acidified KMnO₄.

✏️Worked example

Solid Z dissolves in water to give a blue solution. Test 1: aqueous sodium hydroxide gives a light blue precipitate, insoluble in excess. Test 2: aqueous ammonia gives a light blue precipitate that dissolves in excess to give a dark blue solution. Test 3: aqueous sodium hydroxide and aluminium foil are added and warmed; a gas turns damp red litmus paper blue. (a) Identify the cation and the anion, and name Z. [3] (b) Name the gas in test 3. [1] (c) Predict the flame test colour of Z. [1]

(a) Tests 1 and 2: copper(II), Cu²⁺. Test 3: nitrate, NO₃⁻. Z is copper(II) nitrate.

(b) Ammonia.

(c) Blue-green.

Check it. Every observation is explained by the answer: blue solution (Cu²⁺ compounds are coloured, topic 8), dark blue with excess ammonia, and ammonia from the nitrate. If one result is left unexplained, the identification is probably wrong.

Ammonia from test 3 does not mean ammonium. The ammonium test uses sodium hydroxide without aluminium. With aluminium foil, the ammonia comes from reducing nitrate. Ammonium would already give ammonia when warmed with NaOH before the foil went in.

📝Practise

In the style of the multiple-choice, theory and practical papers.

1. (Multiple choice.) Which gas relights a glowing splint? A: carbon dioxide. B: hydrogen. C: oxygen. D: chlorine.

C. Hydrogen pops with a lighted splint.

2. (Theory.) Describe how to show that a white powder is a carbonate. [3]

Add dilute acid (e.g. hydrochloric): effervescence. Bubble the gas through limewater: it turns milky, so the gas is carbon dioxide.

3. (Theory.) Two white solids give flame colours of orange-red and light green. Identify the metal ions. [2]

Orange-red: calcium, Ca²⁺. Light green: barium, Ba²⁺.

4. (Theory.) Solutions of aluminium sulfate and zinc sulfate both give a white precipitate with aqueous sodium hydroxide that dissolves in excess. Describe a test to tell them apart. [2]

Add aqueous ammonia in excess. Zinc: the white precipitate dissolves. Aluminium: the white precipitate does not dissolve.

5. (Practical.) Describe the test for sulfate ions, and explain why dilute hydrochloric acid would not be used if you then tested the same sample for chloride. [3]

Acidify with dilute nitric acid, add aqueous barium nitrate: a white precipitate. Hydrochloric acid would add chloride ions, so the later chloride test would be positive whatever the sample contained.

6. (Theory.) Describe how aqueous sodium hydroxide distinguishes iron(II) ions from iron(III) ions. [2]

Iron(II): green precipitate (insoluble in excess, turns brown near the surface on standing). Iron(III): red-brown precipitate.

7. (Theory.) A gas bleaches damp litmus paper. Name the gas. [1]

Chlorine.

8. (Practical.) A solution of X is warmed with aqueous sodium hydroxide. A gas is given off that turns damp red litmus blue. Name the gas and the ion present in X. [2]

Ammonia; the ammonium ion, NH₄⁺.

9. (Theory.) A solution decolourises a small volume of acidified aqueous potassium manganate(VII), and gives no precipitate with acidified barium nitrate. Which anion from the list could it contain: sulfate, sulfite or carbonate? [1]

Sulfite: it turns acidified potassium manganate(VII) from purple to colourless. Sulfate is ruled out by the barium nitrate test.

🔗Go deeper — other people’s work

These are external resources, not mine. If one stops working, tell me and everything above it on this page still stands.

Royal Society of Chemistry — flame tests, with photographs of each colour
Royal Society of Chemistry — identifying ions, a microscale practical