Dry hydrogen chloride (HCl) gas does not change the colour of litmus paper because it does not undergo ionization in the absence of water. Litmus paper is a commonly used indicator for the presence of acids or bases in a solution. It contains a mixture of dyes that change colour in response to the acidity or basicity of their surroundings.

In order for litmus paper to change colour, the HCl gas must first dissolve in water to form hydrochloric acid (HCl(aq)). When this happens, the HCl molecules dissociate into hydrogen ions (H+) and chloride ions (Cl-). The hydrogen ions are responsible for the acidic properties of hydrochloric acid and can react with the dye molecules in litmus paper, causing them to change colour.

However, when HCl is in the form of a dry gas, it does not come into contact with water molecules and therefore does not dissociate into ions. As a result, it does not react with the dye molecules in litmus paper and does not change its colour.

In summary, dry HCl gas does not change the colour of litmus paper because it does not dissolve in water and does not undergo ionization. It is only when HCl is dissolved in water and forms hydrochloric acid that it can interact with litmus paper and cause a colour change.