Generated by claude-sonnet-4-6 · 2026-06-15 06:43 · grounding rag
Model Answer
(a)
(i) Acids generate H⁺ (hydrogen ions / hydronium ions, H₃O⁺) in water.
(ii) Bases generate OH⁻ (hydroxide ions) in water.
(b)
Dry HCl gas does not ionise in the absence of water and thus produces no H⁺ ions. Since litmus changes colour only in the presence of H⁺ ions (in solution), dry HCl cannot affect dry litmus paper.
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Explanation
- For (a), examiners expect the correct ionic formulae: H⁺/H₃O⁺ for acids and OH⁻ for bases. Either H⁺ or H₃O⁺ is accepted.
- For (b), the key concept is that ionisation requires water as a medium. Without water, HCl remains molecular and cannot produce H⁺ ions, so no colour change occurs. Always link the absence of water → no ions → no effect on litmus.