Answer the following: Dry HCl gas does not change the colour of dry blue litmus paper. Why ?
Generated by claude-sonnet-4-6 · 2026-06-15 06:45 · grounding rag
Model Answer
Dry HCl gas does not ionise in the absence of water and hence does not produce H⁺ ions. Since no H⁺ (hydronium) ions are formed, it shows no acidic character and cannot change the colour of dry blue litmus paper.
$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
H⁺ ions are produced only in the presence of water. Without water, HCl remains as molecules and cannot act as an acid.
Source: Chapter 2, Section 2.2.1
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Explanation
- The key concept is that acids show acidic properties only in aqueous solution — they must ionise to release H⁺ ions.
- Dry litmus paper has no moisture, so HCl gas cannot ionise → no H₃O⁺ → no colour change.
- Always mention: (1) no ionisation without water, (2) no H⁺ ions produced, (3) therefore no acidic behaviour. Writing the equation earns a mark in most marking schemes.