When treated with excess phenylhydrazine (3 equivalents), sugars form osazones at C1 and C2:

• 1st equivalent: reacts with C1 (CHO of glucose or C2 keto of fructose) → phenylhydrazone
• 2nd equivalent: oxidizes adjacent CHOH to C=O (acting as oxidant)
• 3rd equivalent: reacts with the new C=O → second phenylhydrazone linkage

Since only C1 and C2 are involved, and glucose and fructose differ ONLY at C1 and C2 (identical configuration at C3, C4, C5), they produce the SAME osazone (glucosazone = fructosazone). Similarly, mannose (C2 epimer of glucose) also gives the same osazone.

Different osazones: Glucose/fructose/mannose → identical needles, mp 205 degC. Galactose → different osazone $\frac{different C3}{C4 configuration}$, mp 201 degC. Lactose → different osazone (different overall structure).

Shape of osazone crystals is used as an identification tool: glucosazone forms needle-like crystals, lactosazone forms powder-puff shapes.