Identifying Unknowns Exam Questions

• Lilac flame indicates potassium ions (K+) [1]
• Fizzing with HCl and gas turning limewater milky indicates carbonate ions (CO3 2-) — the gas is CO2 [1]
• The solid is potassium carbonate (K2CO3) [1]
• Soluble in water and colourless solution is consistent with K2CO3 (a soluble potassium salt that does not contain a coloured transition metal ion) [1]

Examiner note: Work through each test logically. The flame test gives the cation. The acid test gives the anion. Combine them. The solubility observation confirms the identification (all potassium salts are soluble). This systematic approach works for most identification questions.

Test 1 — Dissolve each solid in water:

• Copper sulfate forms a blue solution; the other two form colourless solutions [1]
• This identifies the copper sulfate [1]

Test 2 — Add dilute hydrochloric acid to the remaining two solids:

• Calcium carbonate fizzes/effervesces (produces CO2 which turns limewater milky) [1]
• Sodium chloride does not fizz — it simply dissolves [1]
• This identifies calcium carbonate and sodium chloride [1]

Examiner note: Start with the simplest test that eliminates the most options. The blue colour of copper sulfate solution is the quickest identifier. Then the carbonate test (acid + limewater) distinguishes calcium carbonate from sodium chloride. You could also use a flame test: Na gives yellow, Ca gives orange-red. Either approach is acceptable.

(a) The cation is iron(II) / Fe2+ [1]

Wait — the green precipitate with NaOH and blue-green flame suggest Cu2+ or Fe2+. A blue-green flame is characteristic of Cu2+, but Cu2+ gives a blue precipitate with NaOH, not green. A green precipitate with NaOH that is insoluble in excess is Fe2+. However, iron compounds do not give a flame colour. Re-evaluating:

Green solution and green precipitate with NaOH (insoluble in excess) = Fe2+ [1] Blue-green flame = possibly Cu contamination or the question tests whether candidates prioritise the NaOH result over the flame test

Corrected answer: The cation is Fe2+ (iron(II)) based on the green precipitate with NaOH (insoluble in excess) [1]. The green solution supports this [1]. The blue-green flame is anomalous for iron but may indicate a trace of copper [1].

Note: If the examiner intends Cu2+, then: blue-green flame = Cu2+ [1], but the green precipitate contradicts this (Cu2+ gives blue precipitate). Accept Fe2+ with green precipitate evidence.

(b) No precipitate with silver nitrate = no halide ion present [1]. White precipitate with barium chloride = sulfate ion (SO4 2-) present [1]

(c) Iron(II) sulfate / FeSO4 [1]

Examiner note: In real exam questions, all test results should be consistent. If you encounter conflicting results, go with the most reliable test. The NaOH precipitation test is more diagnostic than flame tests for transition metals. Fe2+ gives green solutions and green precipitates; Fe3+ gives yellow/brown solutions and red-brown precipitates; Cu2+ gives blue solutions and blue precipitates.

Test 1 — Hold damp red litmus paper in the gas:

• If it turns blue, the gas is ammonia [1]

Test 2 — If litmus does not turn blue, bubble the gas through limewater:

• If the limewater turns milky/cloudy, the gas is carbon dioxide [1]

Test 3 — If limewater does not change, hold a burning splint to the gas:

• If the gas burns with a squeaky pop, it is hydrogen [1]

Order: test for ammonia first (it is the only alkaline gas), then CO2, then H2 [1]

Examiner note: The order of testing matters for efficiency, though marks are typically awarded for each correct test-and-result pair regardless of order. Ammonia is distinctive because it is the only common alkaline gas. Start with the test that gives the most unique result.

(a) The cation is aluminium (Al3+) or zinc (Zn2+) [1] — both give white precipitates with NaOH that dissolve in excess (amphoteric hydroxides). Since no further test is described, either answer with justification is acceptable [1].

(b) The anion is bromide (Br-) [1] because the cream-coloured precipitate with silver nitrate is silver bromide (AgBr) [1]. White precipitate = chloride, cream = bromide, yellow = iodide.

Examiner note: To distinguish Al3+ from Zn2+, add excess ammonia solution (NH3): zinc hydroxide dissolves in excess ammonia (forming a complex ion), but aluminium hydroxide does not. Alternatively, Al3+ compounds are colourless in solution, while Zn2+ compounds are also colourless — so a flame test (no colour for both) or further chemical test is needed. This is a common extension question.

(a) Carbon dioxide (CO2) [1]

(b) A carbonate (contains CO3 2- ions) [1]

(c) Calcium carbonate (CaCO3) [1] — orange-red flame indicates calcium (Ca2+)

Examiner note: The reaction is: CaCO3 + 2HCl → CaCl2 + H2O + CO2. Calcium carbonate is insoluble in water (it is the compound in marble and chalk), so the flame test would need to be done on the solid directly or on the solution formed after reacting with acid. The question assumes the student can perform the flame test on the original solid.

Test 1 — Add dilute NaOH solution to each:

• Iron(II) sulfate: green precipitate (iron(II) hydroxide) [1]
• Iron(III) chloride: red-brown precipitate (iron(III) hydroxide) [1]

Test 2 — Add dilute nitric acid then silver nitrate solution to each:

• Iron(II) sulfate: no precipitate (sulfate ions do not react with silver nitrate to form an insoluble product) [1]
• Iron(III) chloride: white precipitate (silver chloride, AgCl) [1]

Examiner note: Test 1 distinguishes the cations: Fe2+ gives green, Fe3+ gives red-brown. Test 2 distinguishes the anions: sulfate gives no precipitate with AgNO3, chloride gives a white precipitate. Either test alone is sufficient to identify each solution, but using both confirms the identification. Note: barium chloride with sulfate gives a white precipitate too, but the question asks you to distinguish, so focusing on the difference in results is key.