🧪 2023 P5 — Solid A, solutions B & C(Version B: written answers, auto-marked)
📖 What's the chemistry?
(a) Solid A = zinc carbonate
ZnCO₃ → ZnO + CO₂. Zinc oxide is yellow when hot, white when cold (reversible). The gas
carbon dioxide forms a white precipitate in limewater; acid on a carbonate gives
effervescence.
(b) Solutions B and C
B = calcium chloride (Ca²⁺ + Cl⁻): silver nitrate → white ppt; NaOH → white ppt.
C = copper(II) sulfate (Cu²⁺ + SO₄²⁻): barium nitrate → white ppt; ammonia → blue ppt
soluble in excess (dark blue); NaOH → blue ppt insoluble in excess.
(c) Separation technique
A solid in a liquid is separated by filtration: solid = residue on the
paper, liquid = filtrate below.
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", "colorless" = "colourless". "Milky", "chalky" and "cloudy" are not credited — write a white precipitate is formed). Each point
you make earns its marks; the feedback tells you what's still missing so you can re-mark.
Examiner tips
Give the colour change on heating and cooling; use "soluble/insoluble in excess".
Don't swap residue and filtrate; write "no observable change" when nothing happens.