Why does dry HCl gas not change the colour of the dry litmus paper?
Dry HCl gas does not change the colour of dry litmus paper because hydrogen ions (H⁺) are produced only in the presence of water. The separation of H⁺ ion from HCl molecules cannot occur in the absence of water.
Acids show their acidic character only when they dissociate to produce hydrogen ions in aqueous solution. Since dry HCl gas has no water present, it cannot dissociate into ions and therefore cannot exhibit its acidic properties to change the colour of litmus paper.
Explanation
The textbook context from Activity 2.9 clearly explains that hydrogen ions in HCl are produced in the presence of water. The separation of H⁺ ion from HCl molecules cannot occur in the absence of water. This is the fundamental reason why dry HCl gas fails to show acidic character and cannot change the colour of dry litmus paper. The acidic property depends entirely on the dissociation of HCl into H⁺ ions, which requires water as a medium.