. Why does dry HCl gas not change the colour of dry litmus paper
Ans. The colour of litmus paper is changed by the hydrogen ions. Dry HCl does not contain hydrogen ions (H+). Acid (For example; HCl) dissociates only in aqueous solution to give ions. since, in this case, neither HCl is in the aqueous form nor litmus paper is wet. Therefore, the colour of litmus paper does not change.