Chromyl chloride (B) is produced when a chloride ion reacts with conc.H2SO4 and K2Cr2O7.
Passing the chromyl chloride gas through NaOH turns the solution yellow. This is due to the formation of Na2CrO4 (C).
The reaction is as follows,
NaCl+conc.H2SO4+K2Cr2O7→(Reddishbrown)(B)CrO2Cl2+KHSO4+NaHSO4+H2O
CrO2Cl2+NaOH→YellowColour(C)Na2CrO4+NaCl+H2O