Why does dry HCl gas not exhibit acidic properties, even though HCl is a well-known acid?
Generated by claude-sonnet-4-6 · 2026-06-26 01:04 · grounding rag
Model Answer
Acidic properties of HCl are due to the production of H⁺(aq) or hydronium ions (H₃O⁺) in solution. In dry HCl gas, water is absent, so ionisation cannot occur:
$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$
Since H⁺ ions are not produced without water, dry HCl shows no acidic properties.
Source: Chapter 2, Section 2.2.1
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Explanation
- The key concept is that acids produce H⁺(aq) ions only in the presence of water — this is what the examiner is looking for.
- State clearly that H⁺ cannot separate from HCl in the absence of water.
- Writing the equation earns a mark; don't skip it.
- The Activity 2.9 observation (dry litmus doesn't change colour, wet litmus does) is the experimental basis — mention it if the question asks for an activity, but here the conceptual explanation + equation is sufficient for 2 marks.