To change the colour of blue litmus paper to red, there must be a presence of acid.Dry HCL gas is not acidic because water is required for the dissociation of HCL into H+ and Cl-. Therefore, dry HCL gas will not change the colour of blue litmus paper.

Introduction:
HCl gas, or hydrogen chloride gas, is a strong acid that is widely used in various chemical processes. Litmus paper is a common indicator used to determine whether a solution is acidic or basic. It changes color when exposed to acidic or basic substances. However, in the case of dry litmus paper, HCl gas does not change its color. This phenomenon can be explained by several factors.

Properties of Litmus Paper:
Litmus paper is made from a natural dye extracted from lichens. It contains a mixture of different chemical compounds that act as indicators. These compounds undergo chemical reactions when they come into contact with acidic or basic substances, resulting in a change in color. Litmus paper is typically red in an acidic solution and blue in a basic solution.

Hydrogen Chloride (HCl) Gas:
HCl gas is a molecular compound composed of hydrogen and chlorine atoms. It is highly soluble in water and dissociates into H+ and Cl- ions. In aqueous solutions, HCl is a strong acid, meaning it completely ionizes to release hydrogen ions. However, in its gaseous form, HCl does not readily dissociate into ions.

Dissociation of Acids:
When acids dissolve in water, they produce hydrogen ions (H+) that are responsible for their acidic properties. These hydrogen ions are responsible for the color change observed with litmus paper. However, HCl gas does not dissociate into hydrogen ions in the absence of water, and therefore, it cannot exhibit its acidic properties.

Interaction with Dry Litmus Paper:
Litmus paper requires moisture to initiate the chemical reactions that result in a color change. In the absence of moisture, litmus paper remains dry and cannot interact with the HCl gas. As a result, there is no change in color observed when dry litmus paper is exposed to HCl gas.

Conclusion:
In summary, HCl gas does not change the color of dry litmus paper because it does not dissociate into hydrogen ions in the absence of water. Litmus paper requires moisture to initiate the chemical reactions responsible for the color change. Therefore, without the presence of water, dry litmus paper remains unaffected by the acidic properties of HCl gas.