During the qualitative analysis of SO 3 2 - using dilute H 2 SO 4 , SO 2 gas is evolved which turns K 2 Cr 2 O 7 solution (acidified with dilute H 2 SO 4 ):

Step-by-step solution

On treating sulphite ( SO 3 2 - ) with warm dilute H 2 SO 4 , SO 2 gas is evolved which is suffocating with the smell of burning sulphur. Na 2 SO 3     +   H 2 SO 4   →   Na 2 SO 4   +   H 2 O   +   SO 2 The gas turns potassium dichromate paper acidified with dil. H 2 SO 4 , green. K 2 Cr 2 O 7     +   H 2 SO 4     +   3 SO 2   →   K 2 SO 4   +   Cr 2 ( SO 4 ) 3   +   H 2 O