Match List - I with List - II.

List-I (Anion)	List-II (gas evolved on reaction with dil. $$H_2SO_4$$)
(A) $$CO_3^{2-}$$	(I) Colourless gas which turns lead acetate paper black.
(B) $$S^{2-}$$	(II) Colourless gas which turns acidified potassium dichromate solution green.
(C) $$SO_3^{2-}$$	(III) Brown fumes which turns acidified KI solution containing starch blue.
(D) $$NO_2^-$$	(IV) Colourless gas evolved with brisk effervescence, which turns lime water milky.

Choose the correct answer from the options given below

We need to match each anion with the gas evolved when it reacts with dilute $$H_2SO_4$$.

(A) $$CO_3^{2-}$$ reacts with $$H_2SO_4$$ to give carbon dioxide: $$CO_3^{2-} + H_2SO_4 \rightarrow CO_2 + H_2O + SO_4^{2-}$$. Carbon dioxide is a colourless gas that is evolved with brisk effervescence and turns lime water milky: $$CO_2 + Ca(OH)_2 \rightarrow CaCO_3\downarrow + H_2O$$. Therefore it matches with (IV).

(B) $$S^{2-}$$ reacts with $$H_2SO_4$$ to give hydrogen sulfide: $$S^{2-} + H_2SO_4 \rightarrow H_2S + SO_4^{2-}$$. Hydrogen sulfide is a colourless gas with a rotten egg smell and turns lead acetate paper black: $$Pb(CH_3COO)_2 + H_2S \rightarrow PbS\downarrow(\text{black}) + 2CH_3COOH$$. Therefore it matches with (I).

(C) $$SO_3^{2-}$$ reacts with $$H_2SO_4$$ to give sulfur dioxide: $$SO_3^{2-} + H_2SO_4 \rightarrow SO_2 + H_2O + SO_4^{2-}$$. Sulfur dioxide is a colourless gas with a pungent smell; it acts as a reducing agent and turns acidified potassium dichromate solution green: $$K_2Cr_2O_7 + 3SO_2 + H_2SO_4 \rightarrow K_2SO_4 + Cr_2(SO_4)_3(\text{green}) + H_2O$$. Therefore it matches with (II).

(D) $$NO_2^-$$ reacts with $$H_2SO_4$$ to form nitrous acid which decomposes to nitrogen dioxide: $$NO_2^- + H_2SO_4 \rightarrow HNO_2 \rightarrow NO_2(\text{brown fumes})$$. The brown fumes oxidize iodide ions to iodine, turning starch solution blue: $$2NO_2 + 2KI \rightarrow 2KNO_2 + I_2$$ and $$I_2 + \text{starch} \rightarrow \text{blue colour}$$. Therefore it matches with (III).

The correct answer is Option D: A - IV, B - I, C - II, D - III.