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Science · 2025 · 2 marks
CBSE 2025 · Region 5 · Set 2 · Q21
A gas 'X' is found in the upper layer of atmosphere. It is a deadly poison, but still essential for all life forms on earth. (a) Identify the gas and…
A gas 'X' is found in the upper layer of atmosphere. It is a deadly poison, but still essential for all life forms on earth.
(a)
Identify the gas and state the main factor for its depletion in the atmosphere.
(b)
How is this gas formed in the upper atmosphere ?
Marking-scheme solution
(a)
Ozone \(\displaystyle \left(\mathrm{O}_{3}\right)\)
Excessive use of chlorofluorocarbons (CFC's) /Freons
(b)
The higher energy ultra violet radiations split apart molecular oxygen \(\displaystyle \left(\mathrm{O}_{2}\right)\) into free oxygen (O)atoms. These atoms then combine with the molecular oxygen to form ozone. /
\[\mathrm{O}_{2} \xrightarrow{\mathrm{tV}} \mathrm{O}+\mathrm{O}
\]
\[\begin{gathered}
\mathrm{O}+\mathrm{O}_{2} \rightarrow \mathrm{O}_{3} \\
\text { (Ozzone) }
\end{gathered}
\]
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