Why does dry HCl gas not change the colour of the dry litmus paper?
Litmus paper detects the presence of H+and OH–. Dry HCl cannot split into its constituent H+ and Cl– by itself; a polar medium like water splits acids into its constituent.
HA + H2O —> H3O+ + A–
Now, litmus paper detects H+ or H3O+ and changes the colour accordingly.
Fact: Litmus paper consists of dyes obtained from some lichens adsorbed onto a paper. These dye are naturally pink and changes to red and blue as per the medium (acidic and basic respectively).
Next: While diluting an acid, why is it recommended that the acid should be added to water and not water to the acid?
See also: Why does an aqueous solution of acid conduct electricity?