📖 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 turns
limewater milky; 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).