Science — AI-generated practice question

Observation: Dry HCl gas does not change the colour of dry blue litmus paper, but HCl dissolved in water (hydrochloric acid) turns blue litmus red.

Ionic-level explanation:

• Litmus (an acid-base indicator) changes colour only in the presence of H⁺ (aq) ions.
• In dry HCl gas, there is no water. HCl molecules remain as molecules and do not ionise — no H⁺ ions are produced. Hence, no colour change occurs.
• When HCl dissolves in water, water molecules cause the separation of H⁺ ions from HCl:

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

• These H⁺ (aq) ions are responsible for the acidic properties, so blue litmus turns red.

Support for the statement: Since HCl shows acidic behaviour only when dissolved in water (i.e., only when H⁺ ions are produced), this proves that acids produce H⁺ ions only in the presence of water.

How H⁺ ions actually exist in aqueous solution:
H⁺ ions cannot exist alone; they combine with water to form the hydronium ion:

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

Thus, H⁺ ions always exist as H⁺(aq) or H₃O⁺ in aqueous solution.

Source: Chapter 2, Section 2.2.1

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• Key examiner expectation: Students must clearly distinguish dry HCl (no ionisation, no H⁺) from aqueous HCl (ionises to give H⁺), and link this to why the litmus test fails for dry gas.
• Both equations — HCl + H₂O → H₃O⁺ + Cl⁻, and H⁺ + H₂O → H₃O⁺ — are likely to fetch dedicated marks; write both.
• Use the term hydronium ion (H₃O⁺) — examiners look for it.
• The statement "acids produce H⁺ ions only in the presence of water" is directly from the textbook conclusion of Activity 2.9; connect your explanation explicitly to it.