A solution gives a white precipitate with $BaCl X_{2}$ . On adding dilute HCl the precipitate dissolves only partly, a colourless gas with a suffocating smell is evolved that turns acidified $K X_{2} Cr X_{2} O X_{7}$ paper green, and a white residue remains. The solution contained

Answer: C

BaSO X_{3}

dissolves in dilute HCl and evolves

SO X_{2}

, which reduces dichromate to green

Cr X^{3 +}

. The residue that does not dissolve is

BaSO X_{4}

. A carbonate would evolve

CO X_{2}

, which does not turn dichromate green. Hence both sulphite and sulphate are present.