When ethanoic acid is tested with both litmus paper and a universal indicator, litmus turns red but the universal indicator shows a colour corresponding to a weak acid — not as strong as hydrochloric acid of the same concentration. Why does ethanoic acid behave as a weaker acid than HCl even though both release H⁺ ions in solution?
Generated by claude-sonnet-4-6 · 2026-06-26 01:11 · grounding rag
Model Answer
Ethanoic acid is a weak acid because it is partially (incompletely) ionised in water — only a few H⁺ ions are released. HCl, being a mineral (strong) acid, is completely ionised, releasing far more H⁺ ions at the same concentration. Since the universal indicator responds to H⁺ ion concentration (pH), it shows ethanoic acid at a higher pH than HCl, indicating weaker acidity. Litmus only detects whether a substance is acidic or basic, so both turn it red.
Source: Chapter 4, Section 4.4.2 — Properties of Ethanoic Acid
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Explanation
- The key concept is degree of ionisation: strong acids ionise completely; weak acids ionise partially.
- Litmus is a qualitative test (acid/base only); universal indicator is quantitative — it reflects actual H⁺ concentration, so it distinguishes strong from weak acids.
- Examiners award 1 mark for "partial/incomplete ionisation of ethanoic acid" and 1 mark for the comparison with complete ionisation of HCl. Don't just say "weak acid" — explain why (partial ionisation = fewer H⁺ ions).