Consider the following statements about the physical properties of aldehydes and ketones.
(A) Ethanal is a volatile liquid at room temperature.
(B) The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bonds with water.
(C) Aldehydes and ketones are insoluble in organic solvents like benzene, ether, methanol and chloroform.
(D) As the size of the molecule increases, the odour of aldehydes becomes more pungent and less fragrant.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (A) and (B) only
The physical properties of aldehydes and ketones come from their polar carbonyl group and from the size of the alkyl part attached to it.
(A) Ethanal is a volatile liquid at room temperature. — True. Methanal is a gas, ethanal is a volatile liquid, and the higher members are liquids or solids.
(B) The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bonds with water. — True. The carbonyl oxygen forms hydrogen bonds with water molecules, and in the small molecules the alkyl part is too short to oppose this.
(C) Aldehydes and ketones are insoluble in organic solvents like benzene, ether, methanol and chloroform. — False. All aldehydes and ketones are fairly soluble in these organic solvents.
(D) As the size of the molecule increases, the odour of aldehydes becomes more pungent and less fragrant. — False. It is the other way round: the lower aldehydes have sharp pungent odours, and larger molecules become less pungent and more fragrant, which is why many are used in perfumes and flavours.
NCERT (2026-27 edition):
8.3 Physical Properties – “Methanal is a gas at room temperature. Ethanal is a volatile liquid. Other aldehydes and ketones are liquid or solid at room temperature.”
“The lower members of aldehydes and ketones such as methanal, ethanal and propanone are miscible with water in all proportions, because they form hydrogen bond with water.”
“…All aldehydes and ketones are fairly soluble in organic solvents like benzene, ether, methanol, chloroform, etc. The lower aldehydes have sharp pungent odours. As the size of the molecule increases, the odour becomes less pungent and more fragrant.”