Question

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.

• A. Both A and R are true and R is the correct explanation of A
• B. Both A and R are true but R is NOT the correct explanation of A
• C. A is true but R is false
• D. A is false but R is true

Answer 1

The Correct Option is C

In this question, we need to evaluate the given assertion and reason regarding the behavior of phenolphthalein, a pH-dependent indicator. 

1. Assertion (A): Phenolphthalein is a pH dependent indicator, remains colourless in acidic solution and gives pink colour in basic medium.
   • Explanation: Phenolphthalein is an acid-base indicator. It is colorless in acidic solutions (pH < 7) and turns pink in basic solutions (pH > 8.2). This is because phenolphthalein's molecular structure changes depending on the pH, producing different colors.
   • Thus, Assertion A is true.
2. Reason (R): Phenolphthalein is a weak acid. It doesn’t dissociate in basic medium.
   • Explanation: The reason given here is partly incorrect. While phenolphthalein is indeed a weak acid, it does dissociate in basic mediums to form pink ions. In a basic solution, the phenolphthalein molecule loses its hydrogen ion (H⁺), leading to a structural change that allows it to exhibit a pink color.
   • Thus, Reason R is false.
3. Since Assertion A is true and Reason R is false, the correct answer is: A is true but R is false.

Conclusion: The interaction between phenolphthalein and solutions of different pH levels accurately demonstrates its function as a pH indicator, turning pink in basic solutions. However, the provided reason incorrectly describes its behavior in basic mediums.