If phenolphthalein is added to an acidic solution and then a base is added slowly, when will the pink colour appear?
Answer and explanation
Correct answer: When the solution becomes sufficiently basic
The governing concept is the colour range of an acid–base indicator. Phenolphthalein is colourless in acidic and approximately neutral conditions, but it changes to pink in a sufficiently basic medium, generally around pH 8.2–10.0. As base is added to the original acidic solution, hydrogen ions are neutralised and the pH rises. Once the indicator’s transition range is reached, pink appears. Therefore option A is correct; increasing acidity would keep it colourless.
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What is the correct answer to this question?
When the solution becomes sufficiently basic
Why is this the correct answer?
The governing concept is the colour range of an acid–base indicator. Phenolphthalein is colourless in acidic and approximately neutral conditions, but it changes to pink in a sufficiently basic medium, generally around pH 8.2–10.0. As base is added to the original acidic solution, hydrogen ions are neutralised and the pH rises. Once the indicator’s transition range is reached, pink appears. Therefore option A is correct; increasing acidity would keep it colourless.
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.
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