A student is given a single unknown salt solution and must plan a complete scheme of tests to identify both the positive ion and the negative ion. The salt ...

This is a full planning question covering both the positive and negative ion of an unknown salt. It rewards a logical order of tests, the correct reagents, and clear positive observations, plus the reasoning for why ammonia is used alongside sodium hydroxide and why the order of the anion tests matters.

(a) Positive observations [5]:

(b) Plan for the positive ion [6]: add a few drops of aqueous sodium hydroxide, then excess, noting the precipitate colour and whether it dissolves [1]; the coloured precipitates identify the transition metals, copper(II) light blue, iron(II) green, iron(III) red-brown [1]; a white precipitate could be zinc, aluminium or calcium [1]; repeat on a fresh portion with aqueous ammonia [1]; zinc dissolves in excess ammonia, aluminium does not, and calcium gives no precipitate at all with ammonia [1]; a flame test supports the result, for example calcium gives an orange-red flame [1]. Ammonia is used as well as sodium hydroxide precisely because it separates zinc, aluminium and calcium that all look the same with sodium hydroxide.

(c) Plan for the negative ion [4]: test for carbonate first by adding dilute acid and watching for effervescence / limewater going milky [1]; then, on a fresh acidified portion, add barium nitrate to test for sulfate (white precipitate) [1]; then, on a fresh acidified portion, add silver nitrate to test for a halide [1]; the order matters because carbonate would fizz and interfere, and the acid must be added first to destroy any carbonate before the barium and silver tests, otherwise carbonate would give false precipitates [1].

(d) Once a halide is shown, distinguish them with acidified silver nitrate on a fresh portion [1]: chloride gives a white precipitate, bromide a cream precipitate [1], and iodide a yellow precipitate [1]. The colour deepens down the group, which is the diagnostic feature.

(e) Two ways to make the work reliable and safe (any two) [2]: use a fresh portion for each test [1]; work in a fume cupboard for toxic gases, wear eye protection, or use only small amounts [1].

(f) The nitrate test is left until last because it also releases ammonia (when warmed with aluminium foil and sodium hydroxide), which could be confused with the ammonium test, so doing it last avoids that confusion [1].