Dry HCl gas does not change the colour of dry litmus paper because, in the absence of water, it cannot produce hydrogen ions, and litmus only works when these ions are present in aqueous solution.

Quick Scoop: Core Idea

  • Litmus paper is an indicator that shows colour change only when ions (like hydrogen ions H+H^+H+) are present in solution.
  • HCl becomes an acid (in the practical sense) only when dissolved in water, where it ionizes to give H+H^+H+ and Cl−Cl^-Cl− ions.
  • Dry HCl gas has no water around it, so it does not ionize and cannot supply free hydrogen ions to react with the litmus dye.
  • Dry litmus paper also has no moisture, so there is no aqueous medium for the reaction; as a result, no colour change happens.

In simple words: No water → No ions → Litmus can’t “see” the acid → No colour change.

If you first moisten the litmus paper and then expose it to HCl gas, some HCl dissolves in the water on the paper, forms H+H^+H+ ions, and the blue litmus will turn red.

Mini Explanation (with a tiny story)

Imagine litmus as a tester strip that only works when dipped in a liquid; if you just wave it in dry air, it stays the same. Dry HCl gas is like “sleeping acid” that only wakes up and shows its acidic nature when it meets water and breaks into ions. When both the gas and the paper are dry, nothing really happens between them, so the colour stays unchanged.

Key points to remember

  • Acids show their acidic properties through H+H^+H+ (or H3O+H_3O^+H3​O+) ions in water.
  • Litmus reacts with these ions in solution, not just with neutral gas molecules.
  • Therefore, dry HCl gas + dry litmus paper → no reaction, no colour change.

TL;DR: Dry HCl gas cannot ionize without water, and litmus only changes colour in the presence of ions in an aqueous medium, so dry HCl does not affect dry litmus paper.

Information gathered from public forums or data available on the internet and portrayed here.