Which of the following compounds undergoes/undergo thermal decomposition?
- A.Zinc oxide
- B.Silver oxide and zinc oxide
- C.Silver oxide
- D.Magnesium oxide
Correct answer
Explanation
The correct answer is C, Silver oxide. On heating, silver oxide breaks down into silver metal and oxygen, while zinc oxide and magnesium oxide stay unchanged.
The reaction is 2Ag₂O → 4Ag + O₂. Silver is a noble, unreactive metal near the bottom of the reactivity series, so its bond with oxygen is weak and moderate heat is enough to break it. Mercury(II) oxide behaves the same way, and Joseph Priestley discovered oxygen in 1774 by heating it. Oxides of more reactive metals such as zinc, magnesium, aluminium and calcium are very stable and do not decompose on ordinary heating; they have to be reduced with carbon or by electrolysis to get the metal.
A is wrong because zinc oxide only turns yellow when hot and white again on cooling; it does not decompose.
B is wrong because zinc oxide in this pair does not decompose on heating.
D is wrong because magnesium oxide is extremely stable, with a melting point near 2800 °C.
Exam tip: oxides of metals low in the reactivity series (Ag, Hg) decompose on heating; those of Zn, Mg and Al do not.
Practice Questions
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