Here's the chemical equation for the reaction:

K2Cr2O7 (orange) + 3 SO2 + H2O → Cr2(SO4)3 (green) + K2SO4 + H2SO4

Explanation:

* Potassium dichromate (K2Cr2O7) is an orange-colored compound.

* Sulfur dioxide (SO2) is a colorless gas with a pungent odor.

* Chromium(III) sulfate (Cr2(SO4)3) is a green-colored compound.

In this reaction, sulfur dioxide acts as a reducing agent, reducing the chromium in potassium dichromate from a +6 oxidation state to a +3 oxidation state. This change in oxidation state results in the formation of chromium(III) sulfate, which is green.

This reaction is a common test for the presence of sulfur dioxide. It's often used in chemical analysis and environmental monitoring.