Question

Which amongst the following anions does not give effervescence with dilute sulphuric acid?

• A. $C{O}_3^{2-}$
• B. $S{O}_3^{2-}$
• C. $C{l}^{-}$
• D. $S^{2-}$

Answer 1

The correct option is D $S^{2-}$

Carbonate, sulphite and sulphide ions produce effervescence due to the release of different gases on reaction with dilute sulphuric acid. The reactions can be written as:

$C{O}_3^{2-}\left(aq\right)+2H^{+}\left(aq\right)\to H_{2}C{O}_3\left(aq\right)\to H_{2}O\left(l\right)+C{O}_2\left(g\right)\uparrow$

$S{O}_3^{2-}\left(aq\right)+2H^{+}\left(aq\right)\to H_{2}S{O}_3\left(aq\right)\to H_{2}O\left(l\right)+S{O}_2\left(g\right)\uparrow$

$S^{2-}\left(aq\right)+2H^{+}\left(aq\right)\to$ $H_{2}S\left(g\right)\uparrow$

Due to this property, these three anions are grouped together to form group I anions.

However, chloride ions does not efferverscence with dilute sulphuric acid. Instead they react with concentrated sulphuric acid to release $HCl$ gas which in turn produces effervescence.
$H^{+}\left(aq\right)+C{l}^{-}\left(aq\right)\to$ $HCl\left(g\right)\uparrow$