Which of the following statements are correct for tests with dilute $H X_{2} SO X_{4}$ (warm if necessary)?

A $S X^{2 -}$ gives $H X_{2} S$ , which blackens lead acetate paper because PbS forms.
B $SO X_{3} X^{2 -}$ gives $SO X_{2}$ , which turns acidified $K X_{2} Cr X_{2} O X_{7}$ paper green.
C $NO X_{2} X^{-}$ gives NO, which forms brown $NO X_{2}$ on mixing with air.
D $HCO X_{3} X^{-}$ gives no gas with dilute acid in the cold.

Answer: A, B, C

Statements 1, 2 and 3 describe the standard gas tests: PbS is black, dichromate is reduced from orange to green by

SO X_{2}

, and

NO + \frac{1}{2} O X_{2} NO X_{2}

is brown. Statement 4 is false because hydrogencarbonates also evolve

CO X_{2}

with dilute acid. So 1, 2 and 3 are correct.