Carbonyl compounds such as aldehydes and ketones reacts readily with derivatives of ammonia containing a hetero atom attached to the ammonia nitrogen to give stable solid derivatives with sharp melting points. A well known examples of those derivatives of ammonia containing a hetero atom attached to the ammonia are hodroxyl amine ($\ce{H2N-OH}$), semicarbazide ($\ce{H2N-NH-C(=O)-NH2}$), and phenylhydrazine ($\ce{Ph-NH-NH2}$).
The most common phenylhydrazine used is 2,4-dinitrophenylhydrazine (2,4-DNP), because it gives hydrazone derivative, which can be used as qualitative test for these carbonyl compounds.
Traditionally, carbonyl compounds can be identified using sharp melting points of two of their nitrogen derivatives, e.g., corresponding semicarbozones and 2,4-dinitrophenylhydrazones. For example, one of the laboratory experiment for sophomore undergraduates taking Organic Chemistry II course at Texas A&M University is Reactions of Aldehydes and Ketones. During this lab experiments, students needs to make two derivatives (semicarbozones and 2,4-dinitrophenylhydrazones) of their unknown compound, and identify them by their melting points and other classification tests (to judge the unknown is aldehyde or ketone, and is the type of $\ce{CH3C(=O)-R}$). I listed the given values of several melting point values in the corresponding Laboratory Manual:
$$
\begin{array}{ccc} \\\hline
\text{Name of Unknown} & \text{Boiling point} & \text{M.p. of semicarbazone} & \text{M.p. of 2,4-DNP}\\\hline
\text{2-Propanone} & \pu{56 ^\circ C} & \pu{190 ^\circ C} & \pu{126 ^\circ C} \\
\text{2-Butanone} & \pu{80 ^\circ C} & \pu{145 ^\circ C} & \pu{118 ^\circ C} \\
\text{3-Methyl-2-butanone} & \pu{94 ^\circ C} & \pu{113 ^\circ C} & \pu{124 ^\circ C} \\
\text{2-Pentanone} & \pu{102 ^\circ C} & \pu{112 ^\circ C} & \pu{144 ^\circ C} \\
\text{3-Pentanone} & \pu{102 ^\circ C} & \pu{139 ^\circ C} & \pu{156 ^\circ C} \\
\text{2,4-Dimethyl-3-pentanone} & \pu{124 ^\circ C} & \pu{160 ^\circ C} & \pu{88 ^\circ C} \\
\text{2-Hexanone} & \pu{128 ^\circ C} & \pu{125 ^\circ C} & \pu{110 ^\circ C} \\
\text{4-Methyl-3-penten-2-one} & \pu{130 ^\circ C} & \pu{164 ^\circ C} & \pu{205 ^\circ C} \\\hline
\end{array}
$$
The first two is two compounds you want to identify: 2-propanone and 2-butanone. By the values given,, you'd be able to identify them exclusively by chemical and physical means. Needless to say that preparation of corresponding semicarbozones and 2,4-dinitrophenylhydrazones of given carbonyl compounds are so simple (simplest possible) that even undergraduate students with no prior experience in organic chemistry can also do it.
Then again, we have now introduced simple analytical method to identify unknown carbonyl compounds: $\ce{^1H}$-$\mathrm{NMR}$, aiming students to have well understand of spectroscopy. Following is two $\ce{^1H}$-$\mathrm{NMR}$ spectra of propanone (acetone) and butanone (ethylmethylketone). As you see, you can easily distinguish the two compounds in minutes: Propanone gives only one resonance for the methyl group, a singlet. Butanone, on the other hand, gives three signals - a quartet ($\ce{-CH2 -}$), a singlet ($\ce{-CH3}$), and a triplet ($\ce{-CH3}$).
