A salt on treatment with dilute sulphuric acid produces a gas which turns lime-water milky but has no effect on acidified potassium dichromate solution. The anion present in the salt is:

Distinguish between the following pair of compounds using a reagent as a chemical test:

• Magnesium chloride and magnesium nitrate solution.

The _____ gas is released when hydrochloric acid is added to the test compound. It indicates the presence of carbonate ions.

$S_1:$ Like $C{O}_3^{2-},S{O}_3^{2-}$ ions also give test with baryta water, $\mathrm{\text{Ba}}{(OH)}_2.$

$S_2:A{g}_{2}S$ is insoluble in hot and concentrated $\mathrm{\text{HN}}{O}_3.$

$S_3:$ Triodide ions $(I_3^{-})$ produced by the reaction of $C{u}^{2+}$ and KI solution are not reduced by excess of sodium thiosulphate solution.

$S_4:$ Black precipitate of $\mathrm{\text{Cu}}{(SCN)}_2$ turns into white precipitate when it reacts with saturated solution of $S{O}_2$ in water.

${\mathrm{CaCO}}_3\stackrel{\mathrm{\Delta }}{⟶}\mathrm{A}+\mathrm{B} \left(\mathrm{g}\right)$

$\mathrm{A}+{\mathrm{H}}_2\mathrm{O}⟶\mathrm{C}$

$\mathrm{C}+\mathrm{B}⟶\underset{\left(\mathrm{white} \mathrm{precipitate}\right)}{\mathrm{D}}+{\mathrm{H}}_2\mathrm{O}$

$\mathrm{D}+\mathrm{B} \left(\mathrm{g}\right)\to \underset{\left(\mathrm{water} \mathrm{soluble}\right)}{\mathrm{E}}\underset{{\mathrm{BaCl}}_2}{\stackrel{\mathrm{Boil}}{\to }}\underset{(\mathrm{white} \mathrm{precipitate})}{\mathrm{F}}$

Note: Question details are changed from that of the book.

Select the correct option(s) for the white precipitate shown in the above reactions.

Then A may have: