Osazone

Osazones are a class of carbohydrate derivatives found in organic chemistry formed when reducing sugars are reacted with excess of phenylhydrazine at boiling temperatures.
Formation.
Osazone formation was developed by Emil Fischer, who used the reaction as a test to identify monosaccharides.
The formation of a pair of hydrazone functionalities involves both oxidation and condensation reactions. Since the reaction requires a free carbonyl group, only "reducing sugars" participate. Sucrose, which is nonreducing, does not form an osazone. 
Appearance.
Osazones are highly coloured and crystalline compounds. Osazones are readily distinguished.