Tollens' test (Ag+ in NH3, ammoniacal AgNO3):

• Aldehyde reduces Ag+ → Ag0 (silver mirror on glass)
• RCHO + 2Ag(NH3)2+ + 2OH- → RCOO- + 2Ag↓ + 4NH3 + H2O
• Positive for ALL aldehydes (aliphatic and aromatic)
• Negative for ketones (including methyl ketones)

Fehling's test (CuSO4 + NaOH + sodium potassium tartrate):

• Aldehyde reduces Cu2+ (blue) → Cu2O (red precipitate)
• Positive for aliphatic aldehydes
• Aromatic aldehydes (benzaldehyde) give negative Fehling's test
• Negative for all ketones

JEE distinction: Tollens' works for ALL aldehydes; Fehling's fails for aromatic aldehydes.