🧪 2021 P5 — Qualitative analysis of salt S(Version A: guided notebook)
📖 What's the chemistry?
The unknown salt S
The paper never names salt S — your tests find its three ions:
NH₄⁺, Fe³⁺ and SO₄²⁻. The Fe³⁺ makes the solution yellow-brown.
The anion tests (a)
Barium nitrate → white precipitate (barium sulfate) → the anion is sulfate.
Silver nitrate → no observable change → there is no chloride present.
Acid, then zinc (b)
Dilute sulfuric acid makes S paler, to yellow. Then zinc does two things at once:
the zinc–acid reaction gives effervescence of hydrogen (the gas extinguishes a lighted splint with a
'pop'), and the zinc reduces Fe³⁺ to Fe²⁺, so the solution turns colourless (pale green is accepted). A grey
solid settles and the tube becomes warm. The solution in tube X now contains a
reducing agent.
The cation tests (c)–(e)
NaOH / ammonia on S → red-brown precipitate, insoluble in excess → Fe³⁺.
Heating S with NaOH gives off ammonia (turns damp red litmus paper blue) → NH₄⁺.
NaOH on the solution from tube X → green precipitate, insoluble in excess (some white precipitate may
also be seen). This iron(II) was made by the zinc — it is not an ion of S.
The solution from X turns purple manganate(VII) colourless → it contains a reducing agent.
Examiner tips (from the real report)
Let a precipitate settle before you judge its colour (the barium sulfate looks orange in the brown solution until it settles white).
Always test the gas — don't just say "a gas is given off".
Write "no observable change" when nothing happens; never call a solution "clear" — use "colourless".