Several lower alcohols are miscible with water in all proportions, but the solubility of alcohols in water decreases on going to higher members. Which of the following explains this decrease?
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → The size of the hydrophobic alkyl part of the molecule increases.
Alcohols dissolve in water because their –OH group forms hydrogen bonds with water molecules. Every monohydric alcohol has just one such group, while the alkyl part, which is hydrophobic (water-repelling), grows with each extra carbon. As this part becomes larger, the alcohol becomes less soluble.
Why the other options are incorrect
Option (1) The O–H bond becomes more polar as the alkyl group gets larger. — Incorrect. Alkyl groups are electron-releasing; they increase the electron density on oxygen and so decrease the polarity of the O–H bond.
Option (2) The higher alcohols form intramolecular hydrogen bonds instead of hydrogen bonds with water. — Incorrect. A simple alcohol has no second group in the molecule to which its –OH could bond; it forms intermolecular hydrogen bonds, with water or with other alcohol molecules.
Option (4) The larger alkyl group releases electrons and makes the alcohol a stronger acid. — Incorrect. An electron-releasing group decreases the acid strength of an alcohol, so the alcohol does not become a stronger acid.
NCERT (2026-27 edition):
7.4.3 Physical Properties – “Solubility of alcohols and phenols in water is due to their ability to form hydrogen bonds with water molecules … The solubility decreases with increase in size of alkyl/aryl (hydrophobic) groups. Several of the lower molecular mass alcohols are miscible with water in all proportions.”