Question

why dry hcl gas will not affect litmus test​

Answer 1

dry HCl doesn't show acidic nature with out water

Answer 2

It is hydrogen ions (H+) that gives acidic properties.

To change the colour of blue litmus paper to red, there must be a presence of acid. Dry HCl gas is not acidic because water is required for the dissociation of HCl into H+ and Cl-.

Therefore, dry HCl gas is not acidic and will not change the colour of dry blue litmus paper.

If you hold litmus over a test tube with a reaction that is generating HCl, the HCl will react with trace water in the atmosphere and trace water on the paper to change colour.