Misconception 1: "Prussian blue is always formed when nitrogen is present"

Reality: Prussian blue is formed ONLY when nitrogen is present WITHOUT sulfur. When both N and S are present, NaSCN forms instead of NaCN, and the FeCl3 test gives blood-red (not Prussian blue). Many students miss this and lose a guaranteed mark.

Misconception 2: "Fehling's test is positive for all aldehydes"

Reality: Fehling's test is NEGATIVE for aromatic aldehydes like benzaldehyde (C6H5CHO). Only aliphatic aldehydes and certain reducing sugars (containing -CHO or alpha-hydroxy ketone groups) reduce Fehling's solution. Aromatic aldehydes are not reducing enough to reduce Cu2+ in Fehling's.

Misconception 3: "KMnO4 titration requires methyl orange or phenolphthalein"

Reality: KMnO4 titrations are self-indicating — no external indicator is needed. KMnO4 itself (purple) is decolorized during the reaction, and the endpoint is the first persistent faint pink color. Adding phenolphthalein or methyl orange would be incorrect and would interfere with the observation.

Misconception 4: "Iodoform test detects all ketones"

Reality: Iodoform test is specific for methyl ketones (CH3CO-R structure) only. 3-Pentanone (C2H5-CO-C2H5) has no methyl adjacent to C=O and gives NEGATIVE iodoform test. Students often assume any ketone gives iodoform.

Misconception 5: "AgF can be detected by AgNO3 in Lassaigne's test like other halides"

Reality: AgF is water-soluble (high hydration enthalpy of F- overcomes lattice enthalpy). So no precipitate forms with AgNO3 for fluorine. Lassaigne's test CANNOT detect fluorine by precipitation.

Misconception 6: "Carbylamine test can be used for any amine"

Reality: Carbylamine test is SPECIFIC for primary amines (-NH2). Secondary amines (R2NH) and tertiary amines (R3N) do not react with CHCl3/KOH to form isocyanides. Many students apply this test indiscriminately.