Chemicals:

2,4-dinitrophenyl hydrazine
acetone
benzaldehyde
conc HCl
2 N HCl
Apparatus and glass wares:
2 conical measures, 350 mL
porcelain dish
beaker 800 mL
2 glass stirring rods
2 snap-cap vials 10 mL
Reagent solution:
1.2 g of 2,4-dinitrophenyl hydrazine are doused with 12 mL of conc. HCl in a porcelain dish (fume hood!). The formed light yellow hydrochloride is mixed to produce a slurry that is poured into 600 mL of 2 N HCl while stirring. The hydrochloride should be perfectly dissolved.
Experimental procedure:
Two conical measures are each filled with 300 mL of the hydrochloric acid solution of 2,4-dinitrophenyl hydrazine. 10 mL of acetone and benzaldehyde, respectively, are slowly added to the dinitrophenyl hydrazine solutions while stirring.
Results:
Copious crystalline precipitates are formed. Acetone gives a yellow precipitate and benzaldehyde gives a orange yellow precipitate.
Discussion:
The formation of this precipitate is a positive test for the carbonyl group of ketones and aldehydes. The ketone or aldehyde is converted to its hydrazone by reaction with dinitrophenyl hydrazine.
Apositive test is a yellow, orange, or red precipitate. Small crystals made from unconjugated aldehydes and ketones give precipitates toward the yellow end of the scale. Large crystals made from conjugated compounds tend to be more red.
Hydrazones have a sharp melting point and can assist in identifying carbonyl compounds.