Identifying an inorganic unknown (Core Practical 15)

A plan for an unknown salt

1. Look: a coloured solid or solution suggests a transition metal ion; crystals may be hydrated.

2. Cations: flame test; NaOH(aq) dropwise then excess (and warm, testing for ammonia with damp red litmus); NH3(aq) dropwise then excess.

3. Anions: dilute acid for carbonate; HCl then BaCl2 for sulfate; HNO3 then AgNO3 then ammonia for halides.

4. Extra clues: what happens on standing, with H2O2, or when two solutions are mixed (a brown colour with iodide means I- has been oxidised to I2).

5. Put it together: write a neutral formula by balancing charges, and add water of crystallisation if the solid is hydrated.

Writing observations and inferences

• Observation: colour and state: 'white precipitate', 'effervescence', 'gas turns damp red litmus blue', 'no visible change'.
• Inference: the ion, with its formula: 'SO4²- present', 'NH4+ present', 'Fe2+ present (not Fe3+)'.
• Name the compound if asked for the substance, not just the ion (October 2024: 'iodide' alone did not score when the identity of the compound was needed).
• 'No change' is an observation too, and it can rule ions out.

Double salts

Some unknowns contain two cations and one anion, such as ammonium iron(II) sulfate, (NH4)2Fe(SO4)2·6H2O (pale green crystals). Tests would show NH4+ (ammonia on warming with NaOH), Fe2+ (green precipitate turning brown) and SO4²- (white precipitate with HCl and BaCl2). Charges: 2(+1) + (+2) = +4, balanced by 2SO4²-.

Worked example

Green crystals X: warming with NaOH(aq) gives a gas that turns damp red litmus blue and a green precipitate that turns brown on standing. A solution of X with HCl and BaCl2 gives a white precipitate. Suggest a formula for X.

1. Ammonia gas: NH4+.
2. Green precipitate turning brown: Fe2+.
3. White precipitate with BaCl2 in acid: SO4²-.
4. Balance charges: 2NH4+ + Fe2+ = +4, so 2SO4²-. Crystals: hydrated.

Answer: (NH4)2Fe(SO4)2·6H2O, ammonium iron(II) sulfate

Check yourself

1. Which formula is correct for iron(III) sulfate?
   • FeSO4
   • Fe3(SO4)2
   • Fe(SO4)3
   • Fe2(SO4)3
   Show answer

   Fe2(SO4)3. 2 x (+3) = +6 balances 3 x (-2) = -6.

2. Calculate the molar mass of ammonium iron(II) sulfate, (NH4)2Fe(SO4)2·6H2O. (H 1.0, N 14.0, O 16.0, S 32.1, Fe 55.8)
   Show answer

   392.0 g mol^-1. 2(18.0) + 55.8 + 2(96.1) + 6(18.0) = 392.0 g mol^-1.

3. What is the percentage by mass of water in (NH4)2Fe(SO4)2·6H2O?
   Show answer

   27.6 %. 6 x 18.0 / 392.0 x 100 = 27.6%.

4. When solution P is added to potassium iodide solution, the mixture turns brown. What does this show about P?
   • It contains an oxidising agent that oxidises iodide ions to iodine
   • It contains a reducing agent
   • It contains silver ions
   • It is a strong acid
   Show answer

   It contains an oxidising agent that oxidises iodide ions to iodine. Brown is iodine, formed by oxidising I-. The October 2024 report credited the redox point.

5. Which is the best way to record the result when barium chloride is added to an acidified solution of a sulfate?
   • Barium sulfate
   • Sulfate present
   • White precipitate
   • White solid appears to be BaSO4
   Show answer

   White precipitate. The observation column needs what is seen.

6. A solution gives a yellow precipitate with acidified silver nitrate, which does not dissolve in concentrated ammonia. Which ion is present?
   • I-
   • Br-
   • Cl-
   • SO4²-
   Show answer

   I-. AgI is insoluble in concentrated ammonia.

7. Warming a green solid with NaOH(aq) gives ammonia, and the solid's solution gives a green precipitate that turns brown in air. What does the solid contain?
   • Only Fe3+ ions
   • Cu2+ and NH4+ ions
   • NH4+ and Fe2+ ions
   • Only Ni2+ ions
   Show answer

   NH4+ and Fe2+ ions. Ammonia shows NH4+; green turning brown shows Fe2+.

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Sources

• Pearson Edexcel IAL Chemistry (2018) specification
• Pearson IAL Chemistry teacher practical guide
• Pearson examiners' report, WCH16 October 2024
• Pearson WCH16 January 2022 question paper (unused)

Last updated 8 November 2026. Spotted a mistake? Tell us.