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Given below are two statements: One is labelled as Assertion A and the other is labelled as Reason R. Assertion A: Phenolphthalein is a pH dependent indicator, remains colourless in acidic solution and gives pink colour in basic medium. Reason R: Phenolphthalein is a weak acid. It doesn’t dissociate in basic medium. In the light of the above statements, choose the most appropriate answer from the options given below.

1. Both A and R are true and R is the correct explanation of A
2. Both A and R are true but R is NOT the correct explanation of A
3. A is true but R is false
4. A is false but R is true

Correct answer: A is true but R is false

Solution

Phenolphthalein is colourless in acid and pink in base, so A is true. But it is precisely its dissociation (and ring-opening to the pink quinoid form) in basic medium that produces the colour, so R, claiming it does not dissociate in base, is false. Hence A is true but R is false.

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