Q.iii. Name two gases which deplete ozone layer.

Ozone oxidises mercury to HgO, causing the classic 'tailing of mercury'.

Ozone is a powerful oxidising agent. It oxidises mercury to mercuric oxide:

2 Hg + 2 O3 -> 2 HgO + 2 O2 (i.e. Hg + O3 -> HgO + O2)

The formation of HgO on the surface makes mercury lose its clean, mobile meniscus and stick to glass (the 'tailing of mercury'), a standard test for ozone. The product is HgO, not HgO2 or Hg2O.

NO catalytically depletes ozone: $NO+O_3\rightarrow NO_2+O_2$, then $NO_2+O\rightarrow NO+O_2$ (NO regenerated).

Nitric oxide (NO) reaching the stratosphere (from supersonic aircraft exhaust, or naturally from $N_2O$) catalytically destroys ozone in a chain cycle:

$NO + O_3 \longrightarrow NO_2 + O_2$

$NO_2 + O \longrightarrow NO + O_2$

The $NO$ is regenerated at the end of the cycle, so a small amount of it can destroy a very large number of ozone molecules, making it a significant ozone-depleting species among the options given.

$O_3$ decomposes to release highly reactive nascent oxygen, making it a strong oxidising agent.

Ozone is thermodynamically unstable and readily decomposes: $O_3 \rightarrow O_2 + [O]$ (nascent/atomic oxygen). This nascent oxygen atom is extremely reactive and readily oxidises other substances, accepting electrons easily. Because one of the three oxygen atoms is very loosely bound and is easily given up as nascent oxygen, ozone acts as a powerful oxidising agent, e.g. it oxidises moist iodine to iodine and turns KI-starch paper blue: $O_3 + 2KI + H_2O \rightarrow 2KOH + O_2 + I_2$.