Four Unlabelled Bottles
During a practical class, four reagent bottles have lost their labels. The teacher knows the bottles contain sodium carbonate, aluminium sulphate, potassium chloride and copper(II) sulphate. A student obtains these results. A: flame test gives a persistent yellow colour; adding dilute hydrochloric acid produces a gas that turns limewater milky. B: adding sodium hydroxide solution gives a white precipitate that dissolves in excess; adding barium chloride solution gives a white precipitate that is insoluble in dilute hydrochloric acid. C: flame test gives a lilac colour; adding silver nitrate solution gives a white precipitate that dissolves in excess ammonia solution. D: adding sodium hydroxide gives a blue precipitate; adding excess ammonia gives a deep blue solution. The bottles are stored together on an open shelf near a sink, and bottle A has a cracked stopper.
1. Identify each solution A, B, C and D, justifying each identification with the observations given.
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A is sodium carbonate. The persistent yellow flame indicates sodium ions. Adding dilute hydrochloric acid produces carbon dioxide, which turns limewater milky:
Na₂CO₃ + 2HCl → 2NaCl + CO₂ + H₂O
B is aluminium sulphate. Sodium hydroxide gives a white precipitate of aluminium hydroxide, which dissolves in excess NaOH to form a soluble aluminate. Barium chloride gives a white precipitate of barium sulphate, which is insoluble in dilute hydrochloric acid, confirming sulphate ions.
C is potassium chloride. The lilac flame indicates potassium ions. Silver nitrate gives a white precipitate of silver chloride, which dissolves in excess ammonia solution, confirming chloride ions.
D is copper(II) sulphate. Sodium hydroxide gives a pale blue precipitate of copper(II) hydroxide. Excess ammonia dissolves it to form a deep blue copper–ammonia complex, which is characteristic of copper(II) ions.
2. Explain why the barium chloride test on B should be carried out after adding dilute hydrochloric acid.
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Adding dilute hydrochloric acid acidifies the solution and removes carbonate or sulphite ions by converting them to carbon dioxide or sulphur dioxide. Without acidification, barium chloride could give a white precipitate of barium carbonate or barium sulphite, which would be mistaken for barium sulphate. Barium sulphate is insoluble in dilute hydrochloric acid, so a white precipitate that remains after acidification is good evidence for sulphate ions. The acid therefore makes the test more selective.
3. Identify two safety or storage problems in the way the bottles are kept and state how each should be corrected.
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The bottles are unlabelled. This is dangerous because a student could use the wrong chemical. Each bottle should be clearly labelled with the chemical name and hazard symbol, and unidentified bottles should be removed from use until identified by a teacher.
They are stored on an open shelf near a sink. Moisture, splashes and accidental knocking can cause contamination or spills. They should be stored in a locked chemical cupboard or on a labelled tray, away from sinks and direct sunlight.
Bottle A has a cracked stopper. Sodium carbonate is not highly dangerous, but a cracked stopper allows moisture and carbon dioxide from the air to enter, and it may allow spillage. The bottle should be transferred to a sound container or the stopper replaced. Copper(II) sulphate is harmful if swallowed, so contact with skin and ingestion must be avoided.