Science (086) — AI-generated practice question

Acidic behaviour is due to the formation of H⁺(aq) ions in solution. Dry HCl gas has no water, so it cannot ionise — the separation of H⁺ from HCl cannot occur in the absence of water. Therefore, dry HCl produces no H⁺ ions and does not affect dry litmus paper.

When HCl dissolves in water, the reaction occurs:

$$\text{HCl} + \text{H}_2\text{O} \rightarrow \text{H}_3\text{O}^+ + \text{Cl}^-$$

H⁺ ions (as hydronium ions, H₃O⁺) are produced, which turn blue litmus red.

Since HCl shows acidic properties only in the presence of water, it is incorrect to call HCl itself an acid. It is the aqueous solution of HCl that is the acid.

Source: Chapter 2, Section 2.2.1

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• The key concept examiners look for: ionisation requires water; without it, no H⁺(aq) forms.
• Always include the equation HCl + H₂O → H₃O⁺ + Cl⁻ for full marks.
• Conclude clearly that acidic behaviour belongs to the solution, not the pure compound — this directly answers the "why incorrect" part.
• The term hydronium ion (H₃O⁺) should be used; writing only H⁺ is acceptable but mentioning both is better.