The salt which responds to dilute and concentrated $H_2SO_4$ is

Which anion of a sodium salt gives a white precipitate upon heating with a magnesium salt?

$A$ reagent consisting of $1$-naphthylamine and sulphanilic acid in acetic acid is used for the detection of:

The solution of a chemical compound $X$ reacts with $AgNO_3$ solution to form a white precipitate of $Y$,which dissolves in $NH_4OH$ to give a complex $Z$. When $Z$ is treated with dilute $HNO_3$,$Y$ reappears. The chemical compound $X$ can be:

$A$ salt solution is treated with chloroform drops. Then it is shaken with chlorine water. The chloroform layer becomes violet. The solution contains:

$A$ mixture is heated with dilute $H_2SO_4$ and the lead acetate paper turns black due to the evolved gases. The mixture contains: