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

📖 What's the chemistry?

(a) Solid A

On strong heating the white solid A turns yellow and a colourless gas is given off; on cooling, the yellow solid turns pale yellow, then fades to white — a reversible colour change. (Not required: A was basic zinc carbonate, which breaks down to zinc oxide, carbon dioxide and water vapour; zinc oxide is yellow when hot and white when cold.) With acid, A gives effervescence, and the gas forms a white precipitate in limewater → carbon dioxide → A contains carbonate ions. Nothing tests for zinc, so the conclusion is carbonate, not "zinc carbonate".

(b) Solutions B and C

• B = calcium chloride (Ca²⁺ + Cl⁻): silver nitrate → white precipitate (chloride); NaOH → white precipitate, insoluble in excess; ammonia → no observable change.
• C = copper(II) sulfate (Cu²⁺ + SO₄²⁻): barium nitrate → white precipitate (sulfate); ammonia → light blue precipitate soluble in excess to a dark blue solution; NaOH → light blue precipitate insoluble in excess.

(c) Separation technique

Mixing C with potassium iodide gives a solid 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).