Which statement best distinguishes an aldehyde from a ketone?

• A. Both have carbonyl groups, but their positions differ
• B. Aldehydes are always liquid, while ketones are solid
• C. Ketones contain two functional groups; aldehydes contain one
• D. Aldehydes are used in energy storage, ketones in structure

Answer: (A)

The correct choice highlights an important structural distinction between aldehydes and ketones. Both functional groups feature a carbonyl group (a carbon double-bonded to an oxygen), but they are placed differently within the molecular structure. In aldehydes, the carbonyl group is located at the terminal end of the carbon chain, which means it is bound to at least one hydrogen atom. In contrast, ketones have their carbonyl group situated within the carbon chain, between two other carbon atoms. This position difference impacts their chemical behavior, reactivity, and the types of reactions they undergo.

While the other statements present various assertions about these compounds, they do not accurately capture the primary distinguishing feature. Aldehydes and ketones can both be liquids or solids depending on their size and the presence of substituents; they each have one carbonyl group rather than differing numbers of functional groups; and their roles in biological systems do not align precisely with the claims made in the other options. Hence, focusing on the positioning of the carbonyl group is the most accurate way to distinguish between these two classes of compounds.