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Acids, Bases and Salts — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceAcids, Bases and Salts1 Mark
Question
For question two statements are given-one labelled Assertion (A) and the other labelled Reason (R). Select the correct answer to these questions from the codes (a), (b), (c) and (d) as given below:
  1. Both A and R are true, and R is correct explanation of the assertion.
  2. Both A and R are true, but R is not the correct explanation of the assertion.
  3. A is true, but R is false.
  4. A is false, but R is true.
Assertion: Phenolphthalein is an acid-base indicator.
Reason: Phenolphthalein gives different colours in acidic and basic medium
✓

Answer

  1. Both A and R are true, and R is correct explanation of the assertion.

Explanation:

Phenolphthalein is a weak organic acid and may be represented as HPh.

$\text{HPh}\ \ \ \ \ \ \ \ \ \ \ \rightleftharpoons\ \ \ \ \ \ \ \ \text{H}^+\ \ \ \ \ +\ \ \ \ \ \ \ \text{Ph}^-\\\text{(Colourles)}\ \ \ \ \ \ \ \ \text{(Colourles)}\ \ \ \ \ \ \ \ \text{(Pink)}$

ln acidic medium, excess of H+ ions are present and so equilibrium is towards left and hence solution is colourless. While in basic medium, OH ions combine with H+ ions to form unionised water molecules and so equilibrium is towards right and hence solution has pink colour. Therefore, phenolphthalein is an acid acid-base indicator.

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