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

solution:
Acid do not ionize in the absence of water to liberate $\mathrm{H}_{3} \mathrm{O}^{+}$which shows acidic nature of a solution.

Therefore, dry $\mathrm{HCl}$ gas does not change the colour of the dry litmus paper because it does not contain $\mathrm{H}^{+}$ions.
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