The brown ring test is used to detect the presence of which of the following ions?

When a solution of a mixture having two inorganic salts was treated with freshly prepared ferrous sulphate in an acidic medium,a dark brown ring was formed,whereas on treatment with neutral $FeCl_3$,it gave a deep red colour which disappeared on boiling and a brown-red precipitate was formed. The mixture contains:

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

Select the $CORRECT$ statement based on the following reactions:
$\text{Salt } 'X' (\text{soluble}) + \text{dil. } H_2SO_4 \to \text{gas } (B) + \text{neutral oxide } (C) + 'Y'$
$\text{Salt } 'X' (\text{soluble}) \xrightarrow{BaCl_2 \text{ solution}} \text{White ppt. } 'Z'$
$\text{aq. sol. of gas } (B) \xrightarrow{H_2S(g)} 'Y' + \text{neutral oxide } (C)$
Given these reactions,identify the correct statement regarding salt $'X'$.

During the detection of an acidic radical present in a salt,a student obtains a pale yellow precipitate that is soluble with difficulty in $NH_4OH$ solution when the sodium carbonate extract is first acidified with dil. $HNO_3$ and then $AgNO_3$ solution is added. This indicates the presence of:

On addition of conc. $H_2SO_4$ to a chloride salt,colourless fumes are evolved,but in the case of an iodide salt,violet fumes are observed. This is because: