Question

dry hydrogen chloride gas does not change the colour of blue litmus

Answer 1

Dry hydrogen chloride gas does not change the colour of blue litmus paper because it doesn't generate H+ ions.
Whereas hydrochloric acid ionises in aqueous solution to form H+ ions along with chloride ions. Hence, it shows acidic nature.


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Answer 2

We know that the colour of litmus paper is changed by H + ions because of acid.

Dry HCL it does not dissociate to give H+ ions.

Acids dissociate to give ions only in the aqueous solution.

In this HCL is not in aqueous form nor litmus paper is wet, so the colour of litmus paper does not change.