Three aqueous solutions, A, B and C, were placed in a rack as shown. Aqueous ammonia was added drop by drop to a portion of each, and then an excess of aque...
Question 1 Report
Three aqueous solutions, A, B and C, were placed in a rack as shown. Aqueous ammonia was added drop by drop to a portion of each, and then an excess of aqueous ammonia was added. The results are shown in the table.
| solution | with a few drops of aqueous ammonia | with excess aqueous ammonia |
|---|---|---|
| solution A | light blue precipitate | dissolves to a dark blue solution |
| solution B | white precipitate | ...... |
| solution C | white precipitate | precipitate remains (insoluble) |
(a) Name the metal ion present in solution A. [1]
(b) Solution B contains either zinc ions or aluminium ions. Complete the table with the observation for solution B that would show it contains zinc ions. [1]
(c) Deduce the metal ion in solution C, given that it also gives a white precipitate with aqueous sodium hydroxide that is insoluble in excess and a brick-red flame test. [1]
(d) Describe how a flame test could be used to confirm the identity of the ion in solution A. Give the method and the expected result. [3]
(e) State why only a few drops of ammonia are added first, before the excess. [2]
Answer Details
This uses the aqueous ammonia test for cations. Adding ammonia drop by drop gives a coloured hydroxide precipitate; adding excess then shows whether the precipitate redissolves, and a flame test can confirm the metal.
(a) A light blue precipitate that dissolves in excess ammonia to a deep (dark) blue solution is the result for the copper(II) ion, \(\text{Cu}^{2+}\) [1].
(b) Zinc gives a white precipitate that dissolves in excess to give a colourless solution [1]. (Aluminium would stay insoluble in excess, so dissolving is what shows zinc.) The completed table row for solution B is:
| solution | with a few drops of aqueous ammonia | with excess aqueous ammonia |
|---|---|---|
| solution B | white precipitate | dissolves to a colourless solution (zinc) |
(c) A white precipitate with both ammonia and sodium hydroxide that is insoluble in excess, together with a brick-red flame, identifies the calcium ion, \(\text{Ca}^{2+}\) [1].
(d) To confirm the ion in solution A by a flame test: clean a nichrome wire in dilute acid, dip it in the solution and hold it in a hot Bunsen flame [1]; observe the flame colour [1]; a blue-green flame confirms copper(II) [1].
(e) Only a few drops of ammonia are added first so that the colour of the first precipitate can be seen [1], before testing separately whether it dissolves in excess [1]. These are two distinct observations, and adding excess straight away could hide the first one.
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