Aldehydes and ketones both react with 2,4-dinitrophenylhydrazine to give

Explanation

This reagent, often called Brady's reagent, condenses with any carbonyl group to give a bright orange or yellow solid, so it detects the presence of the group without distinguishing aldehyde from ketone. The precipitate has a sharp melting point characteristic of the parent compound, which allows identification. Tollens or Fehling's is then used to decide which of the two classes it is.