Identification of ions and gases
Flame tests, tests for cations, anions and gases.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 12.5.1Describe tests for common cations, anions and gases and state the observations.
7 minute read
Identification of ions and gases
This subtopic is pure recall dressed up as chemistry — a fixed list of tests, reagents and observations that comes up in almost every paper. Learning it as a table pays off directly.
Flame tests (for some metal cations)
A clean wire loop dipped in the sample and held in a blue Bunsen flame gives a characteristic colour: lithium — red, sodium — yellow, potassium — lilac, calcium — orange-red, copper — blue-green.
Tests for cations with sodium hydroxide
Adding aqueous sodium hydroxide, drop by drop then in excess, to a solution gives a coloured precipitate that can identify the metal ion, and sometimes tells you more from how it behaves in excess:
- Copper(II): blue precipitate, insoluble in excess.
- Iron(II): green precipitate, insoluble in excess.
- Iron(III): red-brown precipitate, insoluble in excess.
- Aluminium: white precipitate, dissolves in excess NaOH to give a colourless solution.
- Ammonium ion (test is different — warm with NaOH): releases ammonia gas, detected by damp red litmus turning blue.
Tests for anions
- Carbonate: add dilute acid → effervescence, gas turns limewater cloudy (CO₂).
- Chloride: add dilute nitric acid then aqueous silver nitrate → white precipitate.
- Sulfate: add dilute hydrochloric acid then aqueous barium chloride → white precipitate. (Acid added first specifically to rule out a false-positive from carbonate ions, which would also give a white precipitate with barium chloride.)
Tests for gases
- Hydrogen: lit splint → "pop".
- Oxygen: glowing splint → relights.
- Carbon dioxide: limewater → turns milky/cloudy.
- Ammonia: damp red litmus paper → turns blue.
- Chlorine: damp blue litmus paper → turns red then bleaches white.
Think of it like this
This whole subtopic is a "lock and key" catalogue: each ion has one specific reagent that produces one specific, recognisable observation, so the skill being tested is matching the right key (reagent) to the right lock (ion), not reasoning from first principles.
Worked examples
Method, step by step
A solution is thought to contain aluminium ions. Describe a test using sodium hydroxide that would confirm this, distinguishing aluminium from a metal like iron(III).
- 1Add aqueous sodium hydroxide dropwise to the solution.
- 2A white precipitate forms — this alone would not distinguish aluminium from some other metal ions.
- 3Continue adding sodium hydroxide in excess: if the precipitate dissolves to give a colourless solution, the ion is aluminium (iron(III) gives a red-brown precipitate that stays insoluble in excess).
Add NaOH dropwise: a white precipitate forms. Add excess NaOH: the precipitate dissolves to give a colourless solution, confirming aluminium ions (unlike iron(III), whose red-brown precipitate remains insoluble in excess).
Common misconceptions
- Forgetting to add dilute acid before testing for sulfate ions with barium chloride — without it, carbonate ions would also give a white precipitate and produce a false positive.
- Mixing up aluminium and other precipitates — aluminium hydroxide is unusual in that it is white AND dissolves in excess sodium hydroxide, unlike copper, iron(II) and iron(III) precipitates which stay insoluble in excess.
- Confusing the tests for ammonia (damp red litmus → blue) and chlorine (damp blue litmus → red then bleached white) — they are opposite colour changes and it is easy to swap them under exam pressure.
In the exam
- Always state both the reagent AND the exact observation (colour, precipitate/gas, and any colour change) — a reagent name alone rarely earns full marks.
- For carbonate vs sulfate vs chloride tests, the acid used and precipitate colour are all easy to confuse — write them as a fixed table and drill it, since these three come up constantly.