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.

Explanation

The gas evolved upon heating sulphur powder on a spatula is sulphur dioxide
`S(s) + O_(2)(g) overset("heat")rarr SO_(2)(g)`
(i) `SO_(2)(g)` has no action of dry litmus paper.
`SO_(2)(g)` dissolves in moisture (water) present in moist litmus paper to form sulphurous acid. In acidic solution, moist litmus paper will change to red.
`SO_(2)(g) + underset(("Moisture"))(H_(2)O(aq)) rarr underset("Sulphurous acid")(H_(2)SO_(3)(aq))`