Lassigne's test in not shown by diazonium salts and hydrazines `(NH_2NH_2)`. Why?

Explanation

Diazonium salts usually lose `N_2` on heating much before they react with fused NA metal. That is why they do not give the above test.
Hydrazine does ot have C atoms so it does not form `NaCN` and hence does not give a positive test for `N`. In ordeer to test the presence of N in such compounds, during fusion with `Na`, some chacoal or preferably starch (which contains C but not N, S, halohgens ets. ) is added under these conditions, C of starch or charcoal combines with N of the compound to form `NaCN` which will give a positive test for N.