Question

Why the lower members of aldehyde and ketone are soluble in water while higher members are sparingly soluble or insoluble in water.

Answer 1

Aldehydes and ketones are both carbonyl compounds. They are organic compounds in which the carbonyl carbon is connected to C or H atoms on either side. An aldehyde has one or both vacancies of the carbonyl carbon satisfied by a H atom, while a ketone has both its vacancies satisfied by carbon.

Complete step by step answer:
Aldehydes and ketones are carbonyl compounds. They contain carbonyl group as a functional group which is highly polar. Thus, when dissolved in water, lower aldehydes and ketones can form hydrogen bonds with water. As the carbon chain increases in length, solubility decreases. All the aldehydes and ketones are soluble in organic solvents and in general are less dense than water.
Moreover, the haloalkanes are less soluble in water. Some type of energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bond between the water molecules to make them soluble in water.
Although, the reason for the solubility is that aldehydes and ketones can’t hydrogen bond with themselves, they can further hydrogen bond with water molecules. The small aldehydes and ketones are miscible in all proportions. They have to break the strong hydrogen bonds between water molecules without replacing them. This makes the process energetically less favorable and hence, the solubility decreases.
However, both lower and higher member contain a highly polar carbonyl group $- CO$, that forms reasonably strong bond with water molecules and lower members of aldehydes and ketones are miscible with water in all proportions but, the solubility decreases as the hydrocarbon chain length increases as hydrocarbon contribute to non-polar part.

Note:
Formaldehyde is the simplest aldehyde whereas acetone is the smallest ketone. Formaldehyde is used in preparing glues and polymeric products such as germicides, insecticides and fungicides for plants. It is also used in drug testing and photography whereas acetone is an excellent solvent for a number of plastics and synthetic fibers. It is also used as a nail-paint remover and a paint thinner.