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.

Solution

Chemistry ma’am · 4 steps

Write the indicator equilibrium first: base pulls it towards the ions, so R has it backwards.

1. Indicator equilibrium. Phenolphthalein is a weak acid, written HIn. The undissociated HIn is colourless and its anion In⁻ is pink.

   HIn ⇌ H⁺ + In⁻

2. Check Assertion A. In acid, the extra H⁺ pushes the equilibrium left, so HIn dominates and the solution stays colourless. In base, OH⁻ removes H⁺ as water, the equilibrium shifts right, and pink In⁻ builds up (roughly above pH 8.3). A is true.
3. Check Reason R. The first half is right: phenolphthalein is a weak acid. But in basic medium it dissociates more, not less, and that is exactly why the pink colour appears. R is false.
4. So. A is true and R is false, which is option (c).

The idea to keep. For any acid indicator HIn, base removes H⁺ and drives dissociation forward. The colour you see in base is the colour of the anion In⁻.

Answer from the NTA final answer key. Solution by PrepWiser.