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The reaction with nitrous acid (HNO2 from NaNO2 + HCl) is the most informative distinction reaction for amines.

Primary aliphatic amine: RNH2 + HNO2 -> ROH + N2 (bubbles). The aliphatic diazonium decomposes instantly.

Primary aromatic amine (0-5°C): ArNH2 + HNO2 -> ArN2+Cl- (stable diazonium salt, clear solution). This is diazotisation — the gateway to numerous synthetic transformations.

Secondary amine (any): R2NH + HNO2 -> R2N-N=O (N-nitrosoamine, yellow oily liquid). Both aliphatic and aromatic secondary amines give this result.

Tertiary aliphatic amine: R3N + HNO2 -> R3NH+NO2- (salt formation only, no visible change).

Tertiary aromatic amine: C6H5NR2 + HNO2 -> p-ON-C6H4-NR2 (C-nitrosation on activated ring, green colour).

The Hinsberg test provides an alternative systematic distinction: 1° product dissolves in NaOH (acidic N-H), 2° product doesn't dissolve (no N-H), 3° gives no reaction. The carbylamine test $\frac{CHCl3}{KOH}$ specifically confirms primary amines via foul-smelling isocyanide formation.