Which of the following trioxonitrate(V) salts would decompose on heating to form a metal?

Analyze the question: We need the nitrate whose thermal decomposition leaves the free metal as the residue. This depends on where the metal sits in the reactivity series.

Concept: thermal decomposition of nitrates

1. Very reactive metals (K, Na): the nitrate decomposes to the nitrite and oxygen. 2KNO₃ → 2KNO₂ + O₂

2. Moderately reactive metals (Mg, Al, Zn, Fe, Pb, Cu): the nitrate decomposes to the metal oxide, nitrogen dioxide and oxygen. 2Pb(NO₃)₂ → 2PbO + 4NO₂ + O₂ 2Cu(NO₃)₂ → 2CuO + 4NO₂ + O₂

3. Very unreactive metals (Ag, Au, Hg): the metal oxide is itself unstable to heat, so the free metal is formed. 2AgNO₃ → 2Ag + 2NO₂ + O₂

Applying this to the options:

• A. Cu(NO₃)₂ gives CuO (black solid), which is an oxide, not the metal.
• B. AgNO₃ gives silver metal, along with brown NO₂ gas and oxygen.
• C. Pb(NO₃)₂ gives PbO (yellow when cold, brown when hot), which is an oxide.
• D. KNO₃ gives KNO₂, which is a nitrite.

Only silver nitrate leaves the metal behind.

Answer: B. AgNO₃

Exam tip: Remember the pattern "K, Na → nitrite; the middle metals → oxide; Ag, Hg, Au → metal". Silver is below hydrogen in the reactivity series, so it is the one you should link with "decomposes to the metal."