give one chemical test to distinguish between the following pairs of compounds
a)methylamine and dimethylamine
b)ethylamine and aniline

(a)
 Methylamine and dimethylamine can be distinguished by the carbylamine test.

Carbylamine test: Aliphatic and aromatic primary amines on heating with chloroform and ethanolic potassium hydroxide form foul-smelling isocyanides or carbylamines. Methylamine (being an aliphatic primary amine) gives a positive carbylamine test, but dimethylamine does not.

(b) 

Ethylamine and aniline can be distinguished using the azo-dye test. A dye is obtained when aromatic amines react with HNO₂ (NaNO₂ + dil.HCl) at 0-5°C, followed by a reaction with the alkaline solution of 2-naphthol. The dye is usually yellow, red, or orange in colour. Aliphatic amines give a brisk effervescence due (to the evolution of N₂ gas) under similar conditions.