Lassaigne's test converts covalent bonds to ionic forms: Na metal reduces and converts N, S, halogens into NaCN, Na2S, NaX respectively. The ionic extract is then tested by well-known ionic reactions.

The N+S trap is the most-tested concept: When both N and S are present, NaSCN (not NaCN + Na2S) forms. FeCl3 test gives blood-red (Fe(SCN)3), NOT Prussian blue. Students who don't know this answer wrongly.

Prussian blue requires only NaCN (no sulfur): The Prussian blue test (FeSO4/NaOH/H2SO4 → Fe4[Fe(CN)6]3) works only when nitrogen is present AND sulfur is absent.

Silver halide differentiation is purely visual + NH3 solubility: AgCl (white, soluble in dilute NH3) → AgBr (pale yellow, partially soluble in conc. NH3) → AgI (yellow, insoluble in NH3). Trend: color darkens, solubility in NH3 decreases from Cl to I.

Before AgNO3 test for halogens: If N or S present in the compound, the Lassaigne's extract MUST be boiled with dilute HNO3 to destroy NaCN and Na2S before adding AgNO3.

2,4-DNP detects any C=O (cannot distinguish): Tollens' test is needed to separate aldehyde (positive silver mirror) from ketone (no silver mirror). Fehling's further separates aliphatic aldehyde (positive) from aromatic aldehyde (negative).

Iodoform specificity: Only methyl ketones (CH3COR), acetaldehyde, and ethanol (and secondary methyl alcohols) give yellow CHI3. All other aldehydes and non-methyl ketones give negative results.

KMnO4 titration needs no indicator and needs heat: The persistent faint pink is the endpoint. Temperature 60–70 °C is essential — too cold → false early endpoint; too hot → oxalic acid decomposes.

Carbylamine test is primary amine specific: CHCl3 + KOH + heat gives foul isocyanide ONLY with primary amines (-NH2). Secondary and tertiary amines give no reaction.

NaHCO3 effervescence distinguishes -COOH from phenol: Carboxylic acids react with NaHCO3 to give CO2 gas. Phenols are acidic but do NOT react with NaHCO3 to give CO2.