🧪 2023 P5 — Solid A, solutions B & C (Version B: written answers, auto-marked)

📖 What's the chemistry?

(a) Solid A

White solid → yellow on strong heating (a colourless gas is given off) → on cooling pale yellow, then white (reversible). With acid: effervescence; the gas forms a white precipitate in limewater (carbon dioxide) → A contains carbonate ions (zinc is never tested, so not "zinc carbonate").

(b) Solutions B and C

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