Acid is slowly added to a basic solution. The pink colour of phenolphthalein finally disappears. What point can this indicate?
Answer and explanation
Correct answer: Reaching near neutralisation
Phenolphthalein is pink in a sufficiently basic medium and becomes colourless when the pH falls below its transition range, roughly pH 8.2–10. As acid is added to the base, hydroxide ions are consumed in the neutralisation reaction H⁺ + OH⁻ → H₂O. Therefore disappearance of the pink colour indicates that the mixture has approached the indicator endpoint, usually near neutralisation. It does not prove metal formation, compulsory CO₂ production or salt vapour. Option A is correct.
Frequently asked questions
What is the correct answer to this question?
Reaching near neutralisation
Why is this the correct answer?
Phenolphthalein is pink in a sufficiently basic medium and becomes colourless when the pH falls below its transition range, roughly pH 8.2–10. As acid is added to the base, hydroxide ions are consumed in the neutralisation reaction H⁺ + OH⁻ → H₂O. Therefore disappearance of the pink colour indicates that the mixture has approached the indicator endpoint, usually near neutralisation. It does not prove metal formation, compulsory CO₂ production or salt vapour. Option A is correct.
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.
