The brown ring test is used for the detection of which of the following?

$P_{(Coloured \ Solution)} + BaCl_2 \to Q \downarrow_{(White)} + R_{(Coloured \ Solution)}$
Then salt $P$ in above reaction is

Explain the brown ring test for nitrate ions.

To a solution of a colourless sodium salt, a solution of lead nitrate was added to form a white precipitate which dissolves in warm water and reprecipitates on cooling. Which of the following acid radicals is present in the salt?

To a solution of a colourless efflorescent sodium salt, when dilute acid is added, a colourless gas is evolved along with the formation of a white precipitate. Acidified dichromate solution turns green when the colourless gas is passed through it. The sodium salt is

$A$ chloride salt solution acidified with dil. $HNO_3$ gives a curdy white precipitate,$[A]$,on addition of $AgNO_3$. $[A]$ on treatment with $NH_4OH$ gives a clear solution,$B$. Identify $[A]$ and $B$.