Becoz.. dry hcl gas does not contain hydrogen ions so..it does not show acidic behaviour

Introduction:
Dry HCl gas does not change the color of dry litmus paper because of its non-reactive nature with litmus paper. The reason behind this lies in the unique properties of both HCl gas and litmus paper.

Properties of HCl Gas:
- HCl gas is a molecular compound composed of hydrogen (H) and chlorine (Cl) atoms.
- It is a colorless gas with a pungent odor.
- HCl gas is highly soluble in water, forming hydrochloric acid.
- In its gaseous state, HCl does not readily undergo any chemical reactions with most substances.

Properties of Litmus Paper:
- Litmus paper is a pH indicator made from a natural dye extracted from lichens.
- It is usually blue in its basic form and turns red in an acidic environment.
- Litmus paper works based on the principle of acid-base reactions, where the dye in the paper changes its structure and color depending on the pH of the solution it comes into contact with.
- The change in color occurs due to the presence of hydrogen ions (H+) or hydroxide ions (OH-) in the solution.

Explanation:
When dry HCl gas comes into contact with dry litmus paper, no acid-base reaction occurs, and hence no color change is observed. This can be explained by the following factors:

1. Non-Ionization of HCl Gas:
- Dry HCl gas does not readily ionize or dissociate into hydrogen ions (H+) and chloride ions (Cl-) in the absence of water.
- Since the litmus paper requires the presence of hydrogen ions to change its color, the lack of ionization of HCl gas prevents any acid-base reaction from taking place.

2. Absence of Water:
- Litmus paper requires the presence of water to facilitate the ionization of acidic or basic substances.
- Dry litmus paper lacks the necessary moisture to enable an acid-base reaction with dry HCl gas.

3. Lack of pH Change:
- Dry HCl gas does not alter the pH of the surroundings significantly.
- The lack of a pH change means that there are no hydrogen ions available to react with the litmus paper and cause a color change.

Conclusion:
Dry HCl gas does not change the color of dry litmus paper due to its non-ionization in the absence of water and the absence of a pH change. This lack of reactivity between HCl gas and litmus paper highlights the importance of specific conditions, such as the presence of moisture, in order for certain chemical reactions to occur.