Which test mixture gives a brown colouration at room temperature with sodium nitrite solution but no brown colouration or ring with sodium nitrate solution?

A $FeSO X_{4}$ solution with conc. $H X_{2} SO X_{4}$ run down the side of the tube
B $BaCl X_{2}$ solution
CFreshly prepared $FeSO X_{4}$ solution with dilute acetic acid
DLime water

Answer: C

Acetic acid is strong enough to liberate

HNO X_{2}

NO

from nitrite, which forms brown

[Fe (H X_{2} O) X_{5} NO] X^{2 +}

with

Fe X^{2 +}

. Nitrate needs concentrated

H X_{2} SO X_{4}

to be reduced to

NO

, so it gives nothing with dilute acetic acid. Conc.

H X_{2} SO X_{4}

with

FeSO X_{4}

gives the brown ring with both, and

BaCl X_{2}

and lime water give no characteristic result with either.