Both litmus and turmeric can distinguish between acidic and basic solutions, yet neither is used when the exact strength of acidity or basicity needs to be determined. Identify one key limitation shared by these natural indicators compared to a universal indicator, and explain how this limitation affects their practical usefulness in a chemistry laboratory.
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Model Answer
Key limitation: Both litmus and turmeric can only tell whether a solution is acidic or basic (qualitative result); they cannot indicate the strength (degree) of acidity or basicity.
Effect on practical usefulness: In a laboratory, knowing merely that a solution is acidic is often insufficient. A universal indicator, by contrast, shows a range of colours corresponding to pH values (0–14), allowing the chemist to determine whether an acid is weak or strong and to compare the strengths of different solutions accurately.
Source: Chapter 2, Section 2.3 — How Strong Are Acid or Base Solutions?
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Explanation
- Examiners expect you to name the limitation clearly: natural indicators give only a yes/no (acid/base) answer, not a pH value or strength.
- The contrast with the universal indicator (which shows different colours at different H⁺ concentrations on the pH scale 0–14) is what earns the second mark.
- Activity 4.7 (Chapter 4) directly illustrates this: litmus cannot distinguish dilute acetic acid from dilute HCl, but the universal indicator can, because it shows their different pH values.
- Keep the answer focused — one limitation + one practical consequence = 2 marks.