Assertion: Phenolphthalein is an acid-base indicator.
Reason: In an acid-base titration, this indicator is used.
Both assertion and reason are true but the reason is not the correct explanation of assertion
Phenolphthalein is a weak organic acid and may be represented as HPh.
HPh(Colourless) ⇌ H+(colourless) + Ph−(Pink)
In acidic medium, an excess of H+ ions are present and so equilibrium is towards left and hence the solution is colourless. While in the basic medium, OH− ions combine with H+ ions to form unionised water molecules and so equilibrium is towards the right and hence solution has a pink colour. Therefore, phenolphthalein is an acid-base indicator.