How many of the following carboxylic acids are miscible in water?
Methanoic acid, Ethanoic acid, Propanoic acid, Butanoic acid, Hexadecanoic acid, Benzoic acid
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 4
The –COOH group forms hydrogen bonds with water, while the hydrocarbon part of the molecule does not. Simple aliphatic acids with up to four carbon atoms are miscible in water; the higher acids and benzoic acid are not.
Miscible:
Methanoic acid (one carbon atom)
Ethanoic acid (two carbon atoms)
Propanoic acid (three carbon atoms)
Butanoic acid (four carbon atoms)
Not miscible:
Hexadecanoic acid — a C16 higher fatty acid; practically insoluble because of the large hydrophobic hydrocarbon part.
Benzoic acid — nearly insoluble in cold water, although it is the simplest aromatic acid.
Miscible in water: methanoic, ethanoic, propanoic and butanoic acids = 4.
NCERT (2026-27 edition):
8.8 Physical Properties – “Simple aliphatic carboxylic acids having upto four carbon atoms are miscible in water due to the formation of hydrogen bonds with water. … Higher carboxylic acids are practically insoluble in water due to the increased hydrophobic interaction of hydrocarbon part. Benzoic acid, the simplest aromatic carboxylic acid is nearly insoluble in cold water.”
Carboxylic Acids – “…Some higher members of aliphatic carboxylic acids (C12 – C18) known as fatty acids, occur in natural fats as esters of glycerol.”