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  4. A sample of air turns lime water milky and also turns acidified potassium dichromate green in aqueous solution has low p H. This is due to the presence of pollutants.

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Q. A sample of air turns lime water milky and also turns acidified potassium dichromate green in aqueous solution has low $p H$. This is due to the presence of pollutants.

MHT CETMHT CET 2021

Solution:

$SO _2$ turns lime water milky.
$SO _2$ in aqueous solution gives $H _2 SO _3$.
$H _2 SO _3 \rightleftharpoons H ^{+}+ HSO _3^{-}$
$CO _2$ also turns lime water milky.
$CO _2$ in aqueous solution gives $H _2 CO _3$.
$H _2 CO _3 \rightleftharpoons H ^{+}+ HCO _3^{-}$
$H _2 SO _3$ is stronger acid than $H _2 CO _3$. Thus, ionisation of $H _2 SO _3$ is larger than $H _2 CO _3$. Thus, $p H$ of $H _2 SO _3$ is lower than that of $H _2 SO _2$.