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

$S^{2-}$ and $SO_3^{2-}$ can be distinguished by using

Sodium sulphite on heating with dilute $HCl$ liberates a gas which

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

$A$ substance on treatment with dilute $H_2SO_4$ liberates a colourless gas which produces:
$(i)$ turbidity with baryta water and
$(ii)$ turns acidified dichromate solution green.
The reaction indicates the presence of:

$M \text{ (Salt)} + \text{Dil. } HCl \xrightarrow{\text{Warm}} N \uparrow + P \downarrow$. Gas $N$ changes the colour of $FeSO_4$ solution into a yellow solution. Then,salt $M$ in the above reaction is: