Nitroso amines $\left({\mathrm{R}}_2 \mathrm{N}-\mathrm{N}=\mathrm{O}\right)$ are soluble in water. on heating them with concentrated ${\mathrm{H}}_2{\mathrm{SO}}_4$ they give secondary amines. The reaction is called 

Liebermann's Nitroso reaction and This is a test for secondary amines. Secondary amines(aliphatic as well as aromatic) reacts with nitrous acid to form N-nitrosoamines.