🧪 2017 P5 — mixing, filtering & salt Z (Version B: written answers)
📖 What's the chemistry?
The two unknown solutions
Solution X is aqueous sodium carbonate — colourless. Solution Y is
aqueous ammonium iron(II) sulfate — the Fe²⁺ makes it pale green.
Part (a) — making and filtering salt Z
Mixing them precipitates iron(II) carbonate — salt Z: a green precipitate forms, and on standing
an orange layer creeps over the surface (the air is already oxidising the iron(II)). Filtering leaves the
green residue Z on the paper; the colourless filtrate still holds unreacted sodium carbonate, so
when it runs into the dilute sulfuric acid it effervesces — the gas turns limewater milky → carbon
dioxide. The paper's trick: start filtering into the empty tube, and only once the filtrate runs
clear move the funnel to the acid tube.
Part (b) — what's in X and Y?
(b)(i) X + dilute sulfuric acid: effervescence, limewater → milky → CO₂ → carbonate in X.
(b)(ii) Y + NaOH (boiling tube): a green precipitate, insoluble in excess → Fe²⁺. Warming the
alkaline mixture drives off a pungent gas that turns damp red litmus blue → ammonia → NH₄⁺. On
standing the green precipitate turns brown — oxidation again.
(b)(iii) Y + aqueous ammonia: the same green precipitate, insoluble in excess.
Part (c) — the residue is the answer
Half the moist residue + tilted sulfuric acid (no shaking — watch the fizz around the solid):
effervescence → CO₂ → CO₃²⁻; shaking gives a colourless solution (the TYS allows pale green).
A 1 cm portion + NaOH: green precipitate, insoluble in excess → Fe²⁺. So Z = iron(II) carbonate.
Examiner tips
Three separate limewater tests are needed — actually perform each one.
The (d)(iii) point: ammonia appears in (b)(ii) but not in (c) because the ammonium ions stayed
dissolved and left in the filtrate — they never reach residue Z.