$A$ sodium salt on treatment with $MgCl_2$ gives white precipitate only on heating. The anion of the sodium salt is

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

Which of the following is correctly matched?

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?

Sodium nitroprusside,when added to an alkaline solution of sulphide ions,produces a purple colour due to the formation of:

$X + H_2SO_4 \to Y$ (a colorless gas with a pungent smell).
$Y + K_2Cr_2O_7 + H_2SO_4 \to$ green colored solution.
What are $X$ and $Y$?