Why identifying aldehydes by boiling is not as good as identifying them by melting temperature

In presence of 2,4-DNP

The -C=O of aldehydes or ketone reacts with -NH2 of DNP*, forming crystalline -C=N- compounds, called hydrazones. As the mol. wt increases, the organic compound has a clear MP, so the aim of this reaction is to have a 'heavy mol. wt' derivative , with which it can be easily identified.Lower aldehydes & ketones form oily hydrazones with phenyl hydrazine, so *2,4 dinitro phenyl hydrazine is used to derivatize them.
Almost all the hydrazones decompose at boilng point, so it is usually conenient to go after the MP of the hydrazones [or osazones in case of reducing sugars], to identify the original carbonyl compound.

They probably decompose at the boiling point.