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

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

Dry HCl gas doesn’t change the color of dry litmus paper because it’s not acidic until it dissolves in water. Acidity comes from the HCl dissolving and forming hydronium ions (H3O+), which can’t happen without water.

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