Why does dry HCl gas not change the colour of the dry litmus paper?

Dry hydrogen chloride (HCl) gas does not change the color of dry litmus paper because the reaction between HCl and litmus paper requires the presence of water. Litmus paper is a common indicator that changes color in response to the acidity or basicity of a solution.

The reaction between hydrogen chloride gas and water is as follows:

HCl (g) +H₂O (l) → H₃O+ (aq)+ Cl- (aq)

Hydrogen chloride gas (HCl) reacts with water to form hydronium ions (H₃O+) and chloride ions (Cl−) in solution. The presence of these ions in solution is what allows litmus paper to change color.

When dry HCl gas comes into contact with dry litmus paper, there is no water available to facilitate this reaction. As a result, the litmus paper does not change color. In general, many chemical reactions, especially those involving ions in solution, require water as a medium for the reaction to occur. In the absence of water, certain reactions may not take place, and the expected changes, such as color changes in indicators like litmus paper, may not occur.