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Acids, Bases and Salts — Science STD 10 — Question

CBSE BoardEnglish MediumSTD 10ScienceAcids, Bases and Salts1 Mark
MCQ
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:
Assertion: Phenolphthalein is an acid-base indicator.
Reason: Phenolphthalein gives different colours in acidic and basic medium
  • ✓
    Both $A$ and $R$ are true, and $R$ is correct explanation of the assertion.
  • B
    Both $A$ and $R$ are true, but $R$ is not the correct explanation of the assertion.
  • C
    $A$ is true, but $R$ is false.
  • D
    $A$ is false, but $R$ is true.
✓

Answer

Correct option: A.
Both $A$ and $R$ are true, and $R$ is correct explanation of the assertion.

Phenolphthalein is a weak organic acid and may be represented as $HPh.$
$\text{HPh}\ \ \ \ \ \ \ \ \ \ \ \rightleftharpoons\ \ \ \ \ \ \ \ \text{H}^+\ \ \ \ \ +\ \ \ \ \ \ \ \text{Ph}^-\\\text{(Colourles)}\ \ \ \ \ \ \ \ \text{(Colourles)}\ \ \ \ \ \ \ \ \text{(Pink)}$
In 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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