Question

Why do dry HCl gas not change the colour of litmus ?

Answer 1

dry hcl which we use does not contain any hydrogen ions in it .so due to lack of hydrogen ions it does not show acidic behaviour. That"s the reason why dry hcl not change the colour of litmus.
and we know that the change in colour of litmus is only in the presence of ions.so it not change colour at all

Answer 2

Litmus react with hydrogen ion of aqueous solution and turn their colour but gas cannot react with litmus paper. HCl is an acid because it dissociates to give H+ ions but only aqueous solution. Dry HCl does not give H+ ions