Two beakers have been left standing on the front bench of the laboratory. One beaker holds solution E and the other holds solution F. Both solutions are col...
Question 1 Report
Two beakers have been left standing on the front bench of the laboratory. One beaker holds solution E and the other holds solution F. Both solutions are colourless. You are told that one solution contains zinc ions and the other contains aluminium ions, but you are not told which is which. Aqueous sodium hydroxide alone will not separate these two solutions, because each gives a white precipitate that dissolves again in excess. Aqueous ammonia will separate them.
Pour about 2 cm depth of solution E into each of two boiling tubes. Pour about 2 cm depth of solution F into each of two further boiling tubes. Stand all four boiling tubes in a rack. To the first boiling tube of solution E, add aqueous sodium hydroxide drop by drop, shaking after every few drops, until no further change is seen; then keep adding the aqueous sodium hydroxide until it is present in large excess. Repeat this with the first boiling tube of solution F. To the second boiling tube of solution E, add aqueous ammonia drop by drop in the same way, then add aqueous ammonia until it is in large excess. Repeat this with the second boiling tube of solution F. Record everything you see, including what happens to each precipitate when the reagent is in excess.
| test | observations with solution E | observations with solution F |
|---|---|---|
| aqueous sodium hydroxide added drop by drop | ||
| aqueous sodium hydroxide added in excess | ||
| aqueous ammonia added drop by drop | ||
| aqueous ammonia added in excess |
(a) Record your observations in Table 2.1. [8]
(b) Deduce which of solution E and solution F contains aluminium ions. Use your observations to justify your answer. [3]
(c) A candidate adds the aqueous ammonia too quickly and misses the first change. Describe what that candidate should do to obtain a valid result. [2]
(d) Give one reason why the same two boiling tubes must not be used for both reagents without washing. [2]
Answer Details
This question tests the qualitative analysis of two similar cations. Zinc and aluminium ions both give a white precipitate with sodium hydroxide that redissolves in excess, so that reagent cannot tell them apart. Aqueous ammonia separates them because the zinc precipitate dissolves in excess ammonia (forming a soluble complex) while the aluminium precipitate does not.
(a) The completed Table 2.1 (marked consequentially as a consistent set, one mark per cell) is:
| test | observations with solution E | observations with solution F |
|---|---|---|
| aqueous sodium hydroxide, drop by drop | white precipitate [1] | white precipitate [1] |
| aqueous sodium hydroxide in excess | precipitate dissolves to a colourless solution [1] | precipitate dissolves to a colourless solution [1] |
| aqueous ammonia, drop by drop | white precipitate [1] | white precipitate [1] |
| aqueous ammonia in excess | the white precipitate remains / is insoluble (aluminium) | the white precipitate dissolves to a colourless solution (zinc) |
The final row carries the two distinguishing marks [1][1]: the precipitate that stays in excess ammonia is aluminium hydroxide, the one that dissolves is zinc hydroxide. All eight cells score in part (a).
(b) Solution E contains the aluminium ions [3]: it is the solution whose precipitate remained when the ammonia was in excess [1], because aluminium hydroxide does not dissolve in excess ammonia [1], whereas zinc hydroxide does dissolve in excess ammonia to form a soluble complex [1].
(c) A candidate who adds the ammonia too quickly and misses the first change should (any two [2]): rinse out and repeat with a fresh portion of the solution; add the ammonia one drop at a time, shaking between drops; watch against a white background or white tile so the faint precipitate is seen.
(d) The same tubes must not be reused without washing (any two [2]): reagent left in the tube reacts with the next reagent added; the precipitate from the first test would be mistaken for a new one; ammonia left in a tube would dissolve a zinc precipitate and give a false negative.
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