🧪 2021 P5 — Qualitative analysis of salt S(Version B: written answers, auto-marked)
📖 What's the chemistry?
The unknown salt S
Salt S is never named — your tests find its ions: NH₄⁺, Fe³⁺
and SO₄²⁻. The Fe³⁺ makes the solution yellow-brown.
The anion tests (a)
Barium nitrate → white precipitate (barium sulfate) → sulfate.
Silver nitrate → no observable change → no chloride.
Acid, then zinc (b)
Dilute sulfuric acid: S turns paler, to yellow. Then zinc: effervescence of
hydrogen (it extinguishes a lighted splint with a 'pop') and Fe³⁺ is reduced to Fe²⁺ (the solution
turns colourless; pale green accepted), leaving a grey solid; the tube becomes warm. 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 ammonia (turns damp red litmus paper blue) → NH₄⁺.
NaOH on the solution from tube X → green precipitate — iron(II) made by the zinc, not an ion of S.
The solution from X turns purple manganate(VII) colourless.
How the auto-marking works
Your written answers are checked offline against the mark scheme's required points using keyword and
synonym patterns, so you need not match its wording exactly (but write words like "precipitate" in full,
as in the exam). Each point you make
earns its marks; the feedback tells you which points are still missing so you can improve and re-mark.
Examiner tips
Let a precipitate settle before judging its colour; always test the gas.
Write "no observable change" when nothing happens; use "colourless", not "clear".