Question

Assertion: Phenolphthalein is an acid-base indicator.
Reason: In an acid-base titration, this indicator is used.

• A. Assertion and reason are true and the reason is the correct explanation of assertion
• B. Both assertion and reason are true but the reason is not the correct explanation of assertion
• C. The assertion is true but the reason is false.
• D. Both assertion and reason are false.

Answer 1

The correct option is B

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.
$\underset{\left(Colourless\right)}{HPh}\rightleftharpoons \underset{\left(colourless\right)}{H^{+}}+\underset{\left(Pink\right)}{P{h}^{-}}$

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, $O{H}^{-}$ 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.