Question

why does dry HCL gas not change the colour of the dry litmus paper

Answer 1

The HCL gas does not turn the Litmus colour in dry state because there is no exchange of ion in the hcl when the Litmus Paper is dry but then there is a exchange of ion in wet state than the Litmus change it colour

Answer 2

We know that the colour of litmus paper is changed by H + ions because of acid.

Dry HCL it does not dissociate to give H+ ions.

Acids dissociate to give ions only in the aqueous solution.

In this HCL is not in aqueous form nor litmus paper is wet, so the colour of litmus paper does not change.