Acid–alkali indicators

2 marks · Hard

State the colours shown by phenolphthalein in (i) a strongly acidic solution, and (ii) a strongly alkaline solution.

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(i) Colourless; (ii) Pink

Also accept: (i) Colourless, (ii) Pink; (i) Colourless; (ii) pink

Common mistakes: Red for acidic; Blue for alkaline

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Marking points

B1 · 1 mark · Colourless for phenolphthalein in strong acid

B2 · 1 mark · Pink for phenolphthalein in strong alkali

Worked solution

Phenolphthalein is colourless in strongly acidic solutions and pink in strongly alkaline solutions.