Sulphur dioxide

Sulphur dioxide is a reducing agent in the presence of water and is often used for bleaching by reduction.
SO2 (aq) + H2O (l) + dye => SO42− (aq) +2H+ (aq) + dye-O
But Iron (II) ion is also a reducing agent with stronger reducing power than SO2. There is NO reaction between SO2 and FeSO4.
[If you check the standard electrode potentials of the 2 half equations in the above answer, (e.g. http://en.wikipedia.org/wiki/Table_of_standard_electrode_potentials) then you will know that the said "reaction" will not take place. Or simply use COMMON SENSE. If SO2 + FeSO4 can produce H2SO4 and Iron, then air pollution can easily be improved and we don't need Contact process to produce sulphuric acid or get Iron from iron ores.]

2007-10-14 16:32:47 補充：
The answer of the above person is not correct. The reaction he said will not take place.

2007-10-14 16:33:03 補充：
There is no full equation for the bleaching action as it is only used to show that SO2 bleaches by reduction, i.e. removal of oxygen from the dye/colour pigment. There are many different dyes but the composition of them are not of concern in exams.

2007-10-14 16:33:11 補充：
I'm not a specialist in dye and cannot give you any full equation.

YES! Sulphur is a reducing agent , it carry out oxidation as the O.N. increase

SO2 -> SO4 (2-)
+ 4 +6

the ionic half equation of SO2
SO2 +2H2O -> SO4(2-) + 4H(+) + 2e(-)
the ionic half equation of Fe 2+
Fe2(+) + 2e(-) -> Fe

it reacts with FeSO4 as follow :
SO2 +Fe2(+) +2H2O ->SO4(2-) + 4H(+) +Fe

So, there would be a colour change from green to silver.