2 H2SO4 (aq) + 4 KClO3 (aq) → 2 H2O (l) + O2 (g) + 2 K2SO4 (aq) + 4 ClO2 (g)
This is an oxidation-reduction (redox) reaction:
4 ClV + 4 e- → 4 ClIV (reduction)
2 O-II - 4 e- → 2 O0 (oxidation)
This is a gas evolution reaction, O2 and ClO2 is the formed gas.