In which condition will phenolphthalein change from colourless to pink?
Answer and explanation
Correct answer: When added to a basic solution
The governing concept is the colour range of an acid–base indicator. Phenolphthalein remains colourless in acidic and approximately neutral conditions, but its structure changes in a sufficiently basic aqueous medium, producing a pink colour. Thus option A is correct. Acidic solutions do not turn it pink, and a neutral salt solution normally gives no pink colour; the dry-acid choice is not a valid indicator test.
Frequently asked questions
What is the correct answer to this question?
When added to a basic solution
Why is this the correct answer?
The governing concept is the colour range of an acid–base indicator. Phenolphthalein remains colourless in acidic and approximately neutral conditions, but its structure changes in a sufficiently basic aqueous medium, producing a pink colour. Thus option A is correct. Acidic solutions do not turn it pink, and a neutral salt solution normally gives no pink colour; the dry-acid choice is not a valid indicator test.
Which subject and chapter does this question cover?
This is a Class 10 Science question. Chapter: Chemical Substances – Nature and Behaviour. Topic: Acids, Bases, and Salts.
