Question

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

Answer 1

it is because substance behave as acids or bases only when they are moist or in aqueous solution because only then they would be able to release h+ or oh- ions

Answer 2

An acid shows its characteristics only when it is dissolved in water, because in water it gives H+ ion which is responsible for all its characteristics such as changing the colour of Litmus Paper.

So, dry HCl gas does not change the colour of dry litmus paper in the absence of H+ ion.