Consider the following statements about the physical properties of carboxylic acids.
(A) Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature with unpleasant odours.
(B) The higher carboxylic acids are wax-like solids and are practically odourless due to their high volatility.
(C) Most carboxylic acids exist as dimers in the vapour phase or in aprotic solvents.
(D) Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → (A), (C) and (D) only
Carboxylic acid molecules are held together by intermolecular hydrogen bonds, more extensively than alcohols. This decides their boiling points, and the size of the molecule decides their physical state and odour.
(A) Aliphatic carboxylic acids up to nine carbon atoms are colourless liquids at room temperature with unpleasant odours. — True. The lower acids are liquids with unpleasant smells.
(B) The higher carboxylic acids are wax-like solids and are practically odourless due to their high volatility. — False. The higher acids are odourless because of their low volatility: very few molecules escape into the air to be smelt.
(C) Most carboxylic acids exist as dimers in the vapour phase or in aprotic solvents. — True. The hydrogen bonds are not broken completely even in the vapour, so two molecules stay joined as a dimer.
(D) Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. — True. This is due to the more extensive association of acid molecules through hydrogen bonding.
NCERT (2026-27 edition):
8.8 Physical Properties – “Aliphatic carboxylic acids upto nine carbon atoms are colourless liquids at room temperature with unpleasant odours. The higher acids are wax like solids and are practically odourless due to their low volatility. Carboxylic acids are higher boiling liquids than aldehydes, ketones and even alcohols of comparable molecular masses. This is due to more extensive association of carboxylic acid molecules through intermolecular hydrogen bonding. The hydrogen bonds are not broken completely even in the vapour phase. In fact, most carboxylic acids exist as dimer in the vapour phase or in the aprotic solvents.”