Acid–alkali indicators
3 marks · NormalExplain why universal indicator is preferred over litmus paper when measuring the pH of a solution.
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Also accept: Universal indicator changes through many colours, litmus only gives red or blue
Common mistakes: Saying universal indicator is more accurate without explanation
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Marking points
B1 · 1 mark · Mentions universal indicator can show the full pH scale (0–14) or a wide range.
B2 · 1 mark · States that litmus paper only shows acid or alkali (red or blue).
B3 · 1 mark · States that universal indicator gives more precise pH information or specific pH value.
Worked solution
Universal indicator is preferred because it can show the entire range of pH values by changing to different colours depending on the pH. Litmus paper only tells you if a solution is acidic or alkaline (red or blue), not its exact pH. Universal indicator provides more precise information about how acidic or alkaline a solution is.