Question

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

Answer 1

because HCl gas in its dry state does not exhibit acidic character but as it comes in contact with moisture it exhibit acidic character

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.