Which of the following solids would not decompose on heating?

Analyzing the question: We need the solid that is thermally stable, meaning it does not break down when heated with a Bunsen burner.

Concept: Thermal stability of salts depends on the metal present. Salts of very reactive metals (K, Na) are the most stable. Their carbonates resist decomposition under ordinary laboratory heating. The exception is that hydrogen carbonates decompose easily.

Checking each option:

• A. Ammonium chloride (NH₄Cl): It decomposes (sublimes and dissociates) on heating: NH₄Cl(s) ⇌ NH₃(g) + HCl(g) It decomposes.

• B. Lead(II) trioxonitrate(V), Pb(NO₃)₂: Heavy-metal nitrates decompose, giving the oxide, nitrogen dioxide and oxygen: 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ It decomposes.

• C. Potassium trioxocarbonate(IV), K₂CO₃: Potassium is highly electropositive, so its carbonate is very stable. It does not decompose at ordinary laboratory temperatures. It does not decompose.

• D. Sodium hydrogen trioxocarbonate(IV), NaHCO₃: It decomposes easily on heating: 2NaHCO₃ → Na₂CO₃ + H₂O + CO₂ It decomposes.

Answer: Option C.

Exam tip: Remember that Group 1 carbonates (except lithium) are stable to heat. Hydrogen carbonates, nitrates, and ammonium salts almost always decompose. Think "K and Na carbonates are tough, everything else breaks down."