NDA (II) 2022 General Ability Test · Question 72 of 150

Correct — C, Silver oxide. Silver(I) oxide breaks down on heating into metallic silver and oxygen, 2Ag₂O → 4Ag + O₂; reference data list it as decomposing from about 350 °C, with substantial reduction to silver above 380 °C. Silver is a very unreactive metal, so its hold on oxygen is weak and moderate heat is enough to release it. Zinc oxide and magnesium oxide, the oxides of far more reactive metals, are thermally stable and do not decompose on ordinary heating.

• (a)Zinc oxide — Zinc oxide does not decompose on heating. What it does do is change colour reversibly — yellow when hot, white again when cold — and candidates who remember that demonstration often mistake a colour change for a chemical breakdown.
• (b)Silver oxide and zinc oxide — Half right, and that is what makes it dangerous. Silver oxide does decompose, but zinc oxide does not, so pairing them over-reaches. It tempts anyone who knows the silver-oxide fact and adds zinc oxide on the strength of its hot-cold colour change.
• (d)Magnesium oxide — Magnesium has a powerful affinity for oxygen — it burns in air with a brilliant flame — so its oxide is one of the most stable known. Magnesium oxide is used as a refractory furnace lining precisely because heat does not break it down.

Thermal decomposition is a reaction in which one compound is broken by heat into two or more simpler substances. How easily a metal oxide decomposes tracks the metal's position in the reactivity series — oxides of unreactive metals such as silver, mercury and gold give up their oxygen readily on heating, while oxides of reactive metals such as magnesium, aluminium, zinc and calcium hold on to it and stay intact.

The stem's 'undergoes/undergo' is a deliberate signal that the answer may be one compound or two, which is why option (b) exists. Solve it by ranking the three metals rather than memorising decomposition temperatures — silver sits near the bottom of the reactivity series, zinc and magnesium well above it. Only the silver compound is therefore a candidate, and options (a), (b) and (d) all fail on the same point. The same reasoning explains extraction chemistry — ores of low-reactivity metals can be broken down by heating alone, whereas reactive metals must be won by electrolysis.

• Thermal decomposition is one compound broken by heat into two or more simpler substances.
• Silver(I) oxide decomposes to silver and oxygen on heating, from roughly 350 °C.
• Oxides of unreactive metals (silver, mercury, gold) decompose easily; oxides of reactive metals (magnesium, zinc, aluminium, calcium) are thermally stable.
• Mercury(II) oxide is the classic partner example — heating it yields mercury and oxygen, the reaction by which oxygen was first isolated.
• Other standard thermal decompositions are calcium carbonate to quicklime and carbon dioxide, and sodium bicarbonate to sodium carbonate, water and carbon dioxide.

• Reading zinc oxide's reversible yellow-when-hot colour change as decomposition.
• Picking the 'both' option because it looks safely inclusive.
• Assuming every metal oxide breaks down if heated hard enough.

As a pick-the-compound item like this, as a classify-the-reaction item given an equation, or through a matching set pairing reactions with their types.

• practice — not a real PYQ

  Heating lead nitrate gives lead oxide, nitrogen dioxide and oxygen. This reaction is an example of

  • (a)combination
  • (b)displacement
  • (c)thermal decomposition
  • (d)double displacement

  Answer(c) thermal decomposition — one compound is broken by heat into simpler products.

• practice — not a real PYQ

  Which one of the following oxides is thermally the most stable?

  • (a)Silver oxide
  • (b)Mercury(II) oxide
  • (c)Magnesium oxide
  • (d)Silver chloride

  Answer(c) Magnesium oxide — magnesium's strong affinity for oxygen makes its oxide highly resistant to heat, while the silver and mercury compounds break down comparatively easily.