Pratyush took sulphur powder on a spatula and heated it. He collected the gas evolved by inverting a test tube over it, as shown in figure below.

$(a)$ What will be the action of gas on

$(i)$ dry litmus paper ?

$(ii)$ moist litmus paper ?

$(b)$ Write a balanced chemical equation for the reaction taking place.

$(a)$ $(i)$ There will be no action on dry litmus paper.

$(ii)$ Since the gas is sulphur dioxide $(SO_2)$, it turns moist blue litmus paper to red because sulphur dioxide reacts with moisture to form sulphurous acid.

$(b)$ ${{S}_{(s)}}\,+\,{{O}_{2(g)}}\,\to \,\underset{\text{ Sulphur dioxide }}{\mathop{S{{O}_{2(g)}}}}\,$

$S{{O}_{2(g)}}\,+\,{{H}_{2}}{{O}_{(l)}}\,\to \,\underset{\text{ Sulphurous acid }}{\mathop{{{H}_{2}}S{{O}_{3(aq)}}}}\,$