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Q. Rain is called acid rain when its pH is below

Punjab PMETPunjab PMET 2008

Solution:

Acid rain results from air pollution by oxides of nitrogen $\left( NO _{ x }\right)$ and sulphur $\left( SO _{ x }\right)$. These gases react with water and form acids.
$SO _{2}+ H _{2} O \longrightarrow H _{2} SO _{3}$
$SO _{3}+ H _{2} O \longrightarrow H _{2} SO _{4}$
$N _{2} O _{5}+ H _{2} O \longrightarrow HNO _{3}$
When the $pH$ of rain is below $5.6$, it is called acid rain.