Reaction of 2,4-DNPH with aromatic aldehydes
2,4-DNPH is a derivatization reagent for the identification of ketones and aldehydes. 
This was extensively used in organic analysis before instrumental methods were readily available. 
You determined the boiling point of the problem, which in the case of benzaldehyde is $179\ ^{\circ}\mathrm{C}$ , then you made a solid derivative and determined it's melting point. 
In the case of benzaldehyde and 2,4-DNPH that would be $237\ ^{\circ}\mathrm{C}$ . 
Then you go to the tables, cross check both values and had benzaldehyde identified. 

For aldehydes and ketones there are different options: oxime, semicarbazone, phenylhydrazone, p-nitrophenylhydrazone and 2,4-dinitrophenylhydrazone. 
The latter one was usualy preferred because they have higher melting points and so it was easier to get a solid even if not totally pure.