pH and indicators

2 marks · Hard

State the colours that phenolphthalein and methyl orange turn in a strongly alkaline solution.

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Phenolphthalein: pink; Methyl orange: yellow

Also accept: Phenolphthalein: bright pink; Methyl orange: orange-yellow

Common mistakes: Phenolphthalein: colourless (wrong for alkali); Methyl orange: red (wrong for alkali)

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Marking points

B1 · 1 mark · Correct colour for phenolphthalein: pink (accept bright pink)

B2 · 1 mark · Correct colour for methyl orange: yellow (accept orange-yellow)

Worked solution

Phenolphthalein turns pink in a strongly alkaline solution.

Methyl orange turns yellow in a strongly alkaline solution.