IGCSE Chemistry 0620: Distinguishing Aluminium and Zinc Ions with Ammonia

A school science technician finds two unlabelled bottles of colourless solution, P and Q, on a stock shelf. The technician's records show that the two bottles should contain aluminium sulfate solution and zinc sulfate solution, prepared for different classes, but do not say which bottle is which.

(a) The technician adds aqueous sodium hydroxide dropwise, then in excess, to a sample from bottle P. Describe what would be observed, and explain why this result alone cannot show whether bottle P contains aluminium sulfate solution or zinc sulfate solution. [3]

(b) The technician then adds aqueous ammonia dropwise, then in excess, to fresh samples from each bottle.

In bottle P, the white precipitate formed does not dissolve when excess aqueous ammonia is added.
In bottle Q, the white precipitate formed dissolves when excess aqueous ammonia is added, giving a colourless solution.

Identify the metal ion present in bottle P and the metal ion present in bottle Q. [2]

(c) Explain why testing with aqueous ammonia, rather than aqueous sodium hydroxide, allows the technician to tell the two solutions apart. [2]

Part (a): Testing bottle P with sodium hydroxide

Adding aqueous sodium hydroxide dropwise gives a white precipitate. Adding more sodium hydroxide, in excess, causes this white precipitate to dissolve, giving a colourless solution.

This result cannot identify the ion on its own, because both aluminium ions, $\text{Al}^{3+}$ , and zinc ions, $\text{Zn}^{2+}$ , behave identically with sodium hydroxide: each forms a white hydroxide precipitate that redissolves in excess sodium hydroxide to give a colourless solution. Bottle P could therefore contain either ion based on this test alone.

Part (b): Testing with aqueous ammonia

Aqueous ammonia behaves differently from sodium hydroxide for these two ions:

Cation	With excess aqueous ammonia
aluminium, $\text{Al}^{3+}$	white precipitate, insoluble in excess
zinc, $\text{Zn}^{2+}$	white precipitate, soluble in excess (colourless solution)

Bottle P: the precipitate stays insoluble in excess ammonia, so bottle P contains aluminium ions, $\text{Al}^{3+}$ .
Bottle Q: the precipitate dissolves in excess ammonia, so bottle Q contains zinc ions, $\text{Zn}^{2+}$ .

Part (c): Why ammonia distinguishes the two ions

Sodium hydroxide cannot distinguish aluminium ions from zinc ions because both precipitates dissolve in excess sodium hydroxide. The results are identical.

Aqueous ammonia does not behave the same way for both ions:

the aluminium hydroxide precipitate remains insoluble in excess ammonia, but
the zinc hydroxide precipitate still dissolves in excess ammonia.

Because the two ions now give different results with ammonia (one insoluble, one soluble), rather than the same result as with sodium hydroxide, ammonia is the reagent that lets the technician tell bottle P and bottle Q apart.

Final answers

(a) White precipitate that dissolves in excess sodium hydroxide; this result is the same for both $\text{Al}^{3+}$ and $\text{Zn}^{2+}$ , so it cannot identify which ion is present.
(b) Bottle P: $\boxed{\text{aluminium ions, Al}^{3+}}$ . Bottle Q: $\boxed{\text{zinc ions, Zn}^{2+}}$ .
(c) Ammonia gives different results for the two ions (insoluble vs soluble in excess), while sodium hydroxide gives the same result for both.