Which of the following statements best explains why dry HCl gas does NOT change the colour of dry litmus paper, but aqueous HCl does?
HCl only releases H⁺ ions when dissolved in water.
Acidity requires the presence of hydrogen ions (H⁺). Dry HCl gas consists of discrete HCl molecules and does not ionise — there are no free H⁺ ions to turn litmus red. Without H⁺, there is no acidic character and dry litmus shows no colour change.
When HCl dissolves in water: HCl(g) + H₂O(l) → H₃O⁺(aq) + Cl⁻(aq). The water molecules facilitate the dissociation, producing H⁺ (hydronium) ions. These H⁺ ions are what turn blue litmus red.
The correct answer is : HCl only releases H⁺ ions when dissolved in water.
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