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Class 10ScienceChemical Substances – Nature and BehaviourAcids, Bases, and SaltsMedium
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If phenolphthalein is added to an acidic solution and then a base is added slowly, when will the pink colour appear?

Answer and explanation

Correct answer: When the solution becomes sufficiently basic

The governing concept is the colour range of an acid–base indicator. Phenolphthalein is colourless in acidic and approximately neutral conditions, but it changes to pink in a sufficiently basic medium, generally around pH 8.2–10.0. As base is added to the original acidic solution, hydrogen ions are neutralised and the pH rises. Once the indicator’s transition range is reached, pink appears. Therefore option A is correct; increasing acidity would keep it colourless.

Related tags

PhenolphthaleinIndicatorNeutralisationAcidsBasesAnd SaltsAcids Bases And SaltsChemical Substances – Nature And BehaviourChemical Substances Nature And BehaviourScience
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10 more MCQs
  1. 1What type of substance turns blue litmus paper red?
  2. 2What type of substance turns red litmus paper blue?
  3. 3Which of the following substances acts as a natural indicator?
  4. 4What is the general nature of an aqueous soap solution?
  5. 5Which of the following substances turns red litmus paper blue?
  6. 6What is the nature of a solution whose pH is less than 7?
  7. 7At 25°C, a solution with a pH greater than 7 is of which nature?
  8. 8A student dips red litmus paper into a soap solution. What change will occur in the red litmus paper?
  9. 9Which gas is generally produced when dilute acids react with active metals?
  10. 10A student dips red litmus paper into a soap solution. What will be the colour of the litmus paper?
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