In a litmus test it is observed that an aqueous solution of acetic acid turns

Carbon and its Compounds

Properties of Ethanoic Acid

Options

(A)blue litmus colourless

(B)blue litmus red

(C)red litmus blue

(D)red litmus colourless

Official Answer

In a litmus test, an aqueous solution of acetic acid turns blue litmus red.

Acetic acidAcidic natureBlue litmus to redHydronium ions

Marking Scheme

11 mark for selecting the correct option (B).

Hint

Recall the general behavior of acidic substances when tested with blue and red litmus indicators.

Quick Oral Answer

An aqueous solution of acetic acid turns blue litmus red because it releases hydrogen ions ( $H^+$ ) in water, demonstrating its acidic nature.

Analysis & Explanation

Acetic acid ( $CH_3COOH$ ) is a weak carboxylic acid. In an aqueous solution, it partially dissociates to release hydrogen ions ( $H^+$ or hydronium ions, $H_3O^+$ ):

$CH_3COOH(aq) + H_2O(l) \rightleftharpoons CH_3COO^-(aq) + H_3O^+(aq)$

Because of the presence of these acidic hydronium ions, the solution behaves as an acid and turns blue litmus paper red.

Option A (blue litmus colourless) is incorrect because acids do not bleach or decolorize blue litmus.
Option C (red litmus blue) is incorrect because turning red litmus blue is a characteristic property of basic (alkaline) solutions, not acidic ones.
Option D (red litmus colourless) is incorrect because acids do not affect red litmus paper.

Common Mistakes

1Students sometimes confuse the behavior of weak acids with bases, mistakenly thinking that weak acids might turn red litmus blue or have no effect on blue litmus.

Interesting Facts

Although acetic acid is a weak acid (it does not fully ionize in water), it is strong enough to easily turn blue litmus red, unlike extremely weak non-acidic organic compounds like ethanol.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2012?

This question carries 1 mark in the CBSE Class 10 Science 2012 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Properties of Ethanoic Acid" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2012 paper?

This is a MCQ question from Section B in the CBSE Class 10 Science 2012 paper.