📖 What's the chemistry?
The unknown salt S
Salt S is ammonium iron(III) sulfate — it contains NH₄⁺, Fe³⁺
and SO₄²⁻. The Fe³⁺ makes the solution a pale yellow-brown.
The anion tests (a)
- Barium nitrate → white precipitate (barium sulfate) → sulfate.
- Silver nitrate → no change → no chloride.
Reducing Fe³⁺ with zinc (b)
Acidified S + zinc: hydrogen is given off (squeaky pop) and Fe³⁺ is
reduced to Fe²⁺ (yellow → colourless), leaving a grey solid; the tube warms. The Fe²⁺ solution is a
reducing agent.
The cation tests (c)–(e)
- NaOH / ammonia on Fe³⁺ → reddish-brown precipitate, insoluble in excess.
- Warming S with NaOH gives ammonia (damp red litmus → blue) → NH₄⁺.
- NaOH on the Fe²⁺ from tube X → green precipitate.
- The Fe²⁺ solution decolourises purple manganate(VII).
How the auto-marking works
Your written answers are checked offline against the mark scheme's required points using keyword and
synonym patterns (e.g. "ppt" = "precipitate", "pop" = hydrogen test). 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".