Under which condition does dry hydrogen chloride gas turn blue litmus paper red?

The correct option is When the litmus paper is wet.

Explanation

The acidic character of a substance is determined by its ability to release hydrogen ions ($H^+$) or hydronium ions ($H_3O^+$). According to the Arrhenius theory of acids, this dissociation or ionization process occurs only in the presence of water.

Detailed Analysis

• Dry HCl Gas and Dry Litmus Paper: Hydrogen chloride gas is a covalent compound. In a completely dry state, it does not dissociate into ions. Therefore, no free hydrogen ions ($H^+$) are present. Without these ions, the acidic property cannot be manifested, and the dry blue litmus paper does not change color.
• Dry HCl Gas and Wet Litmus Paper: When dry HCl gas comes into contact with wet litmus paper, the water present on the paper acts as a solvent. The HCl molecules dissolve in the water and dissociate to form hydrogen ions ($H^+$) and chloride ions ($Cl^-$).
  $$HCl + H_2O \rightarrow H_3O^+ + Cl^-$$
  It is the presence of these hydronium ions ($H_3O^+$) that causes the blue litmus dye to turn red, indicating acidity.
• Temperature Factors: The temperature of the gas (hot or cold) is not the primary condition required for the indicator to function; the presence of an aqueous medium for ionization is the decisive factor.

Key Takeaway: Acids demonstrate acidic properties and affect indicators only in the presence of water, as water is essential for the ionization of the acid to produce $H^+$ ions.