Why does dry HCl gas not change the colour of the dry litmus.

Explanation

Litmus paper changes colour only in the presence of ions like hydrogen `(H^(+))` or hydronium `(H_(3)O^(+))` ions.
HCl gas is a dry gas. Hence it doesnot release `H^(+)` ions.
If litmus paper was wet, HCl gas will dissolve in the water in the paper and release `H^(+)` ions. But as litmys paper is dry, this doesn.t happen. Hence the litmus paper does not change colour.