When sulfur dioxide (SO2) is bubbled through a solution of calcium carbonate (CaCO3) in water it is converted to the sulfite ion. The chemical equation is shown below.
SO2 + CaCO3 --> CaSO3 + CO2
The carbon dioxide bubbles away and helps drive the reaction forward. CaSO3, however, is not the most stable chemical that can be formed here. It can be oxidized with oxygen from the atmosphere dissolved in the water to form calcium sulfate, a highly stable solid.
2CaSO3 + O2 --> 2CaSO4
So the net equation is shown below:
2SO2 + 2CaCO3 + O2 --> 2CaSO4 + 2CO2
So we can see that the net result is that noxious SO2 gas is converted to harmless calcium sulfate (CaSO4).