Solution Z is the aqueous solution of a single sodium salt. A student carried out three tests on separate portions of solution Z. The apparatus used for the...
Question 1 Report
Solution Z is the aqueous solution of a single sodium salt. A student carried out three tests on separate portions of solution Z. The apparatus used for the second test is shown, and the results are recorded in the table.
| test on solution Z | observation |
|---|---|
| flame test | yellow flame |
| aqueous sodium hydroxide and aluminium foil added, then warmed | a gas is given off that turns damp red litmus paper blue |
| dilute nitric acid then aqueous silver nitrate added to a fresh portion | no precipitate |
(a) Use the flame test result to name the metal ion in solution Z. [1]
(b) The second test is the test for the nitrate ion. Name the gas that turns the damp red litmus paper blue. [1]
(c) Explain how the third test result shows that solution Z does not contain a chloride ion. [2]
(d) Deduce the full name of the salt in solution Z. [1]
(e) Complete the table for a fresh portion of solution Z tested with dilute acid: state the observation expected if a carbonate ion were present. [1]
(f) Describe how you would use the second test to distinguish between a solution of a nitrate and a solution of an ammonium salt. [3]
Answer Details
This is an ion-analysis question on a single sodium salt. Each test targets one ion: the flame test finds the metal, the sodium hydroxide and aluminium test finds nitrate, and silver nitrate checks for chloride.
(a) A yellow flame identifies the sodium ion, \(\text{Na}^+\) [1].
(b) In the nitrate test the gas that turns damp red litmus blue is ammonia [1] (the aluminium reduces nitrate to ammonia in alkaline solution).
(c) If a chloride ion were present, acidified silver nitrate would give a white precipitate of silver chloride [1]; because no precipitate forms, no chloride ion is present [1].
(d) The metal ion is sodium and the anion shown by the second test is nitrate, so the salt is sodium nitrate [1].
(e) If a carbonate ion were present, adding dilute acid to a fresh portion would give effervescence (bubbles) of a gas that turns limewater milky (carbon dioxide) [1].
(f) To distinguish a nitrate from an ammonium salt using this warm-with-hydroxide test:
- a nitrate only releases ammonia if you add aluminium foil with the sodium hydroxide and warm [1];
- an ammonium salt releases ammonia with sodium hydroxide alone on warming, with no foil needed [1];
- so test with sodium hydroxide only (no foil): ammonia given off means an ammonium salt, while no gas means a nitrate. This tells them apart [1].
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