On heating, the following trioxocarbonate (IV) salts decompose to give solid residue except

Analyze the question: We are asked which trioxocarbonate (IV) salt does NOT leave a solid residue when heated.

Concept: Metal carbonates (except those of Na and K) decompose on heating to give a metal oxide (a solid) and carbon(IV) oxide gas. Ammonium carbonate is different because it has no metal. It breaks down completely into gases.

Solution:

• Calcium carbonate: CaCO₃ → CaO + CO₂. The solid residue is CaO (white).
• Lead(II) carbonate: PbCO₃ → PbO + CO₂. The solid residue is PbO (yellow when cold, reddish-brown when hot).
• Zinc carbonate: ZnCO₃ → ZnO + CO₂. The solid residue is ZnO (yellow when hot, white when cold).
• Ammonium carbonate: (NH₄)₂CO₃ → 2NH₃ + H₂O + CO₂. All the products are gases or vapour, so no solid residue remains.

Answer: The salt that leaves no solid residue is ammonium trioxocarbonate (IV), which is option A.

Tip: Ammonium salts such as (NH₄)₂CO₃ and NH₄Cl usually vanish completely on heating because they have no metal to form a solid oxide. When a question says "except" and one option is an ammonium salt, check it first.