On treating sulphite (SO32−) with warm dilute H2SO4, SO2 gas is evolved which is suffocating with the smell of burning sulphur.
Na2SO3+H2SO4→Na2SO4+H2O+SO2
The gas turns potassium dichromate paper acidified with dil. H2SO4, green.
K2Cr2O7+H2SO4+3SO2→K2SO4+Cr2(SO4)3+H2O