Question

Which of the following oxides decomposes on heating?

• A. $ZnO$
• B. $HgO$
• C. $A{l}_2{O}_3$
• D. $N{a}_{2}O$

Answer 1

Answer: (B)

Metal oxides are highly stable, and many decompose only at very high temperatures.

Clearly, the one with the burning splint has $O_2$ 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 $O_2$ gas at about ${450}^{0}C$ well below the temperature of a laboratory burn flame, but $ZnO$ does not reduce at ${1000}^{0}C$ and requires carbon reduction to produce the metal and oxygen gas. Aluminium oxide does not decompose even at ${2000}^{0}C$ and requires chemical intervention to reduce it to the elements.

Hence, option $B$ is correct.