Bhawna took a pale green substance A in a test tube and heated it over the flame of a burner. A brown coloured residue B was formed along with the evolution of two gases with burning smell of sulphur. Identify A and B. Write the chemical reaction involved

Substance A is ferrous sulphate (FeSO4).
Two gases, sulphur dioxide and sulphur trioxide are evolved.
Substance Sis ferric oxide (FeSO3).
The reaction involved is