Phenolphthalein is added to a colourless solution and it turns pink. On adding acid, the colour disappears. What does this show?
Answer and explanation
Correct answer: The original solution was basic and became neutralised by acid
Phenolphthalein is colourless in an acidic or neutral medium and pink in a sufficiently basic medium. The initial pink colour therefore shows that the original solution was basic. Adding acid consumes hydroxide ions through neutralisation, lowering the pH; when the solution leaves the indicator’s pink transition range, the colour disappears. Thus A is correct. The observation does not show that the original solution was strongly acidic.
Frequently asked questions
What is the correct answer to this question?
The original solution was basic and became neutralised by acid
Why is this the correct answer?
Phenolphthalein is colourless in an acidic or neutral medium and pink in a sufficiently basic medium. The initial pink colour therefore shows that the original solution was basic. Adding acid consumes hydroxide ions through neutralisation, lowering the pH; when the solution leaves the indicator’s pink transition range, the colour disappears. Thus A is correct. The observation does not show that the original solution was strongly acidic.
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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