4 HClO4 + 10 HI + K2Cr2O7 → 2 Cr(ClO4) + 2 KClO4 + 5 I2 + 7 H2O
This is an oxidation-reduction (redox) reaction:
2 ClVII + 4 e- → 2 ClV (reduction)
10 I-I - 10 e- → 10 I0 (oxidation)
2 CrVI + 6 e- → 2 CrIII (reduction)
HClO4 is an oxidizing agent, HI is a reducing agent, K2Cr2O7 is an oxidizing agent.