Question

what happens when dry rose petals are placed in the jar of sulphur dioxide

Answer 1

Sulphur dioxide , that is SO2 is a reductant.
With this property , it has the power to decolorise the pigment of the rose.
As a result, the rose turns white.
But this effect will not last for a very long time.
The oxygen in the air oxidizes it and it gets back its original pigment.
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Answer 2

as the atmosphere provided is dry no change in colour of rose petal will occur
Because bleaching action of SO2 is only in presence of moisture or water