Which of the following oxides decomposes on heating?
A
ZnO
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B
HgO
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C
Al2O3
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D
Na2O
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Solution
The correct option is BHgO Metal oxides are highly stable, and many decompose only at very high temperatures.
Clearly, the one with the burning splint has O2 present, so it must decompose at a lower temperature than the other. The other oxide didn't decompose to the elements at all.
HgO decomposes to Hg and O2 gas at about 4500C well below the temperature of a laboratory burn flame, but ZnO does not reduce at 10000C and requires carbon reduction to produce the metal and oxygen gas. Aluminium oxide does not decompose even at 20000C and requires chemical intervention to reduce it to the elements.