`Na_(2)SO_(3)` reacts with dilute `H_(2)SO_(4)` to give `SO_(2)` gas which reacts with `K_(2)Cr_(2)O_(7) +` dil`H_(2)SO_(4)` to give a green solution due to the formation of `Cr_(2)(SO_(4))_(3)`. As whole of `SO_(2)` gas reacts away , on passing the resulting gas through lime water, it remains clear.