Question

why dry HCL not have any test with Litmus Paper but most HCL turns blue Litmus to red​

Answer 1

The colour of blue litmus paper changes only in the presence of ions like hydrogen (H+) or hydronium (H3O+) ions. HCl can produce these ions only when in the form of aqueous solution. Hence dry HCl gas does not change the colour of dry litmus paper.

Answer 2

Dry HCl does not dissociate into H+ ions. Blue litmus turns red only if acidic behaviour is shown and acidic behaviour is shown only if the acid dissociates into H+ ions. For H+ ions to form, the acid must be dissolved in water (i.e, in an aqueous state) so that it dissociates into H+ ions (which react with water to form hydronium ions).