📖 What's the chemistry?
The unknown salt S
Salt S is ammonium iron(III) sulfate, so it contains three ions to find:
NH₄⁺, Fe³⁺ and SO₄²⁻. The Fe³⁺ makes the solution a pale yellow-brown.
The anion tests (a)
- Barium nitrate → white precipitate (barium sulfate) → the anion is sulfate.
- Silver nitrate → no change → there is no chloride present.
Reducing Fe³⁺ with zinc (b)
Acidified S plus zinc does two things at once: the zinc–acid reaction gives off
hydrogen (a squeaky pop with a lighted splint), and the zinc reduces Fe³⁺ to Fe²⁺, so the
yellow solution turns colourless. A grey solid is left and 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 off ammonia (turns damp red litmus blue) → NH₄⁺.
- NaOH on the Fe²⁺ from tube X → green precipitate.
- The Fe²⁺ solution decolourises purple manganate(VII) → it is 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".