Phenolphthalein turns pink in sodium hydroxide. When hydrochloric acid is added slowly, when will the colour disappear?
Answer and explanation
Correct answer: When the solution becomes acidic or nearly neutral
Phenolphthalein is pink only in a sufficiently basic solution. Adding hydrochloric acid supplies hydrogen ions, which neutralise hydroxide ions from sodium hydroxide: HCl + NaOH → NaCl + H2O. As the excess alkalinity is removed and the solution approaches neutral or becomes acidic, phenolphthalein loses its pink colour. The other choices do not describe the indicator transition.
Frequently asked questions
What is the correct answer to this question?
When the solution becomes acidic or nearly neutral
Why is this the correct answer?
Phenolphthalein is pink only in a sufficiently basic solution. Adding hydrochloric acid supplies hydrogen ions, which neutralise hydroxide ions from sodium hydroxide: HCl + NaOH → NaCl + H2O. As the excess alkalinity is removed and the solution approaches neutral or becomes acidic, phenolphthalein loses its pink colour. The other choices do not describe the indicator transition.
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.
