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

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?

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