Q4 of 45 Page 2

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

The litmus paper changes colour on the basis of the concentration of aqueous hydrogen ion in the solution.
Dry HCl being gas is not able to dissociate into aqueous hydrogen ions (H+). Thus there is no hydrogen ions for litmus to react.
 
Therefore, the colour of litmus paper does not change.

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