🧪 2023 P5 — Solid A, solutions B & C (Version A: guided notebook)

📖 What's the chemistry?

(a) Solid A = zinc carbonate

Heating zinc carbonate decomposes it: ZnCO₃ → ZnO + CO₂. The zinc oxide left behind is yellow when hot and white when cold — a reversible colour change. The gas is carbon dioxide, which forms a white precipitate in limewater; adding acid to a carbonate also gives effervescence.

(b) Solutions B and C

• B = calcium chloride (Ca²⁺ + Cl⁻): silver nitrate → white ppt (chloride); NaOH → white ppt.
• C = copper(II) sulfate (Cu²⁺ + SO₄²⁻): barium nitrate → white ppt (sulfate); ammonia → blue ppt soluble in excess to a dark blue solution; NaOH → blue ppt insoluble in excess.

(c) Separation technique

Mixing C with potassium iodide gives a solid (copper(I) iodide) in a liquid. A solid suspended in a liquid is separated by filtration: the solid is the residue on the filter paper, the liquid is the filtrate that runs through.

Examiner tips (from the real report)

• Give the colour change on heating and on cooling — the reverse change is easy to forget.
• Say "soluble in excess" / "insoluble in excess" — it distinguishes the cations.
• Don't swap residue (stays on the paper) and filtrate (runs through).