During the qualitative analysis of \( \mathrm{SO_3^{2-}} \) using dilute \( \mathrm{H_2SO_4} \), \( \mathrm{SO_2} \) gas is evolved which turns \( \mathrm{K_2Cr_2O_7} \) solution (acidified with dilute \( \mathrm{H_2SO_4} \)).
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