Identifying Ions & Gases — study notes

A forensic scientist doesn't need to see a culprit to identify them — a fingerprint is enough, because every person's is unique. Qualitative analysis works the same way in chemistry: you don't need to *see* an ion to know it's there, you just need a test whose result is unique to that one ion. Learn the pattern behind these tests and a topic that looks like a long list of facts becomes one simple idea, repeated.

Reading a Precipitate Like a Fingerprint

Metal ions in solution (cations) are positively charged — think Al³⁺ or Cu²⁺. Add a base such as aqueous sodium hydroxide (NaOH), and its negative hydroxide ions (OH⁻) are pulled toward the positive metal ion, locking together to form a metal hydroxide. Almost every metal hydroxide is insoluble in water, so instead of dissolving it appears as a precipitate — a cloudy solid suddenly appearing in the liquid.

Here's the useful part: the colour of that precipitate is different for each metal, so it identifies the ion on sight. Two reagents do the identifying — aqueous sodium hydroxide and aqueous ammonia — and between them they distinguish every cation on the syllabus. The twist examiners love: some ions give the same colour with sodium hydroxide alone, so you also have to check what happens when you keep adding the reagent well past the point a precipitate first forms — some precipitates dissolve back into solution in excess, others just sit there unchanged.

Worked Example — Telling Zinc and Aluminium Apart

A colourless solution gives a white precipitate with NaOH, and the precipitate dissolves back to a colourless solution when NaOH is added in excess. Identify the ion.

1. A white precipitate that dissolves in excess NaOH narrows it down — but only to two possible ions, Al³⁺ or Zn²⁺, since both behave identically with this one reagent.
2. To split them, switch reagents and repeat the test with excess aqueous ammonia instead. Zinc's white precipitate dissolves in excess ammonia too, going colourless; aluminium's precipitate stays solid no matter how much ammonia is added.
3. Running the ammonia test on this sample: the precipitate dissolves, and the solution goes colourless.
4. Since it dissolved in excess of both reagents, the ion must be Zn²⁺.

The rest of the method — every worked example, a listen-along audio walkthrough and a practice worksheet — is in the full lesson below.