Functional Group Test (carboxylic Acids)
Which simple laboratory observation reliably distinguishes a carboxylic acid from a phenol when a small amount of solid sodium bicarbonate is added to each compound?
Select the correct option:
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About This Question
- Subject
- chemistry
- Chapter
- principles related to practical chemistry
- Topic
- functional group test (carboxylic acids)
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
The carboxylic acid produces brisk effervescence of carbon dioxide while the phenol does not
Sodium bicarbonate is a weak base whose conjugate acid, carbonic acid, has a pK_a of about 6.4. A compound can liberate carbon dioxide from bicarbonate only if it is more acidic than carbonic acid. Carboxylic acids have pK_a values near 4 to 5, making them strong enough to protonate bicarbonate and release carbon dioxide as brisk effervescence. Phenols, with pK_a values near 10, are far weaker acids and cannot displace carbonic acid, so they give no effervescence. This difference makes the bicarbonate test a clean way to distinguish the two. Option B is wrong because the phenol does not react, so the gas evolution is not equal. Option C reverses the true acid strengths, incorrectly attributing reactivity to the phenol. Option D is incorrect because the carboxylic acid clearly does react. This relative-acidity reasoning is exactly the NCERT criterion used to order carboxylic acids, carbonic acid, and phenols. Plausibility check: only an acid stronger than carbonic acid can drive the equilibrium toward carbon dioxide release, consistent with the carboxylic acid being the lower-pK_a species.
This easy difficulty chemistry question is from the chapter principles related to practical chemistry, covering the topic of functional group test (carboxylic acids). It appeared in the 2025 exam.
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