A student adds phenolphthalein to a colourless solution and observes a pink colour. Then dilute hydrochloric acid is added slowly and the pink colour disappears. Which conclusion is most correct?
Answer and explanation
Correct answer: The solution was basic and moved toward neutralisation after adding acid
Phenolphthalein is pink in a basic solution and colourless when the solution is acidic or sufficiently near neutral. The initial pink colour therefore shows that the sample was basic. Adding hydrochloric acid supplies H⁺ ions, which react with OH⁻ ions and reduce the basicity; as the mixture approaches neutralisation, the pink colour disappears. Hence option A is correct, while the other choices reverse indicator behaviour or misread the change.
Frequently asked questions
What is the correct answer to this question?
The solution was basic and moved toward neutralisation after adding acid
Why is this the correct answer?
Phenolphthalein is pink in a basic solution and colourless when the solution is acidic or sufficiently near neutral. The initial pink colour therefore shows that the sample was basic. Adding hydrochloric acid supplies H⁺ ions, which react with OH⁻ ions and reduce the basicity; as the mixture approaches neutralisation, the pink colour disappears. Hence option A is correct, while the other choices reverse indicator behaviour or misread the change.
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.
