Question

Why does dry HCL gas does not change the colour of blue litmus paper?

Answer 1

Because is ot a acid and they have no ionic radical

Answer 2

 Dry HCl gas does not contain any hydrogen ions in it, so it does not show acidic behaviour. In fact, dry HCl gas does not change the colour of dry litmus paper as it has no hydrogen ions [H+(aq) ions] in it.