The blue color obtained in the Lassaigne's test is due to the formation of which compound?

In the detection of halogens,what is formed by adding $AgNO_3$? What is its colour?

In the test for halogens,why is nitric acid added to the sodium fusion extract before adding silver nitrate?

The compound formed in the positive test for nitrogen with the Lassaigne solution of an organic compound is

Which of the following nitrogen-containing compounds does not give Lassaigne's test?

$A$ solution of $m-$chloroaniline,$m-$chlorophenol and $m-$chlorobenzoic acid in ethyl acetate was extracted initially with a saturated solution of $NaHCO_3$ to give fraction $A$. The left over organic phase was extracted with dilute $NaOH$ solution to give fraction $B$. The final organic layer was labelled as fraction $C$. Fractions $A$,$B$ and $C$ contain respectively