Phenolphthalein turns pink in sodium hydroxide solution. The colour disappears when acid is added slowly. What is the reason?
Answer and explanation
Correct answer: The base gets neutralised and the solution becomes less basic
Phenolphthalein is a base-sensitive indicator: it is pink in a sufficiently basic solution and colourless when the solution is neutral or acidic. Sodium hydroxide supplies hydroxide ions, but added acid reacts with them in a neutralisation reaction to form water. As the hydroxide concentration and pH fall, the pink colour disappears. Therefore A is correct; acid does not create the colour, and neither solid sodium hydroxide nor gaseous decomposition is involved.
Frequently asked questions
What is the correct answer to this question?
The base gets neutralised and the solution becomes less basic
Why is this the correct answer?
Phenolphthalein is a base-sensitive indicator: it is pink in a sufficiently basic solution and colourless when the solution is neutral or acidic. Sodium hydroxide supplies hydroxide ions, but added acid reacts with them in a neutralisation reaction to form water. As the hydroxide concentration and pH fall, the pink colour disappears. Therefore A is correct; acid does not create the colour, and neither solid sodium hydroxide nor gaseous decomposition is involved.
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.
