IGCSE Chemistry 0620: Testing for Ammonium Ions in Fertiliser Granules

A garden centre stores two unlabelled sacks of white fertiliser granules. Records show one sack should contain ammonium sulfate and the other sodium sulfate, but both fertilisers look identical and dissolve in water to give colourless solutions. A horticultural technician needs a chemical test to work out which sack is which before restocking the shelves.

(a) Describe what would be observed when aqueous sodium hydroxide is added to a solution of each fertiliser and the mixture is warmed gently. Use your answer to explain how the technician could use this test to identify which sack contains the ammonium sulfate. [3]

(b) Suggest why simply smelling the dry granules straight from each sack, without carrying out any chemical test, would not reliably show which sack contains the ammonium sulfate. [1]

Part (a): Testing each fertiliser with sodium hydroxide

Ammonium ions, $\text{NH}_4^+$ , react with hydroxide ions when warmed:

$\text{NH}_4^+(aq) + \text{OH}^-(aq) \xrightarrow{\text{warm}} \text{NH}_3(g) + \text{H}_2\text{O}(l)$

For the ammonium sulfate sample: warming with aqueous sodium hydroxide releases a gas with a pungent smell. Holding a piece of damp red litmus paper at the mouth of the tube confirms this gas is ammonia, because the paper turns blue. No precipitate forms, since ammonium is not a metal ion.

For the sodium sulfate sample: sodium ions, $\text{Na}^+$ , do not react with aqueous sodium hydroxide at all. No gas is released and no precipitate forms. There is no visible change.

The technician can therefore identify the sacks: whichever sample releases ammonia gas (confirmed by the damp red litmus paper turning blue) on warming with sodium hydroxide is the ammonium sulfate; the sample that shows no change at all is the sodium sulfate.

Part (b): Why smelling the dry granules is unreliable

Ammonia gas is only produced when ammonium ions react with a base such as sodium hydroxide, and the mixture is warmed. Solid ammonium sulfate granules, sitting in a sack at room temperature with no base added, do not release ammonia gas by themselves. So smelling the dry granules directly would not reveal any ammonia smell from the ammonium sulfate sack, and this method could not distinguish it from the sodium sulfate sack.

Part (c): Confirming sodium ions by flame test

A flame test on a sample from the sodium sulfate sack would show a yellow flame, confirming the presence of sodium ions, $\text{Na}^+$ .

Final answers

(a) Ammonium sulfate: warming with sodium hydroxide releases ammonia gas (turns damp red litmus paper $\boxed{\text{blue}}$ ), no precipitate. Sodium sulfate: no gas, no precipitate. The sample that releases ammonia is the ammonium sulfate.
(b) Solid ammonium sulfate does not release ammonia at room temperature without reacting with a base first, so no smell would be detected from either sack.
(c) $\boxed{\text{Yellow}}$ .