Dry HCl gas does not change the color of dry litmus paper because it does not produce free hydrogen ions (H⁺ ions) in the absence of water. The litmus paper changes color only in the presence of H₃O⁺ (hydronium) ions, which can form only when HCl dissolves in water. Since dry HCl gas is not in an aqueous state and no water is present on the dry litmus paper, the gas does not dissociate to release hydrogen ions. Hence, there is no acidic effect on the dry litmus paper, and its color remains unchanged.
