When a green iron

(a) The salt of iron which is green in colour is Iron [II] Sulphate. It occurs in hydrated form and its chemical formula is FeSO₄.7H₂O [Ferrous Sulphate Heptahydrate]. It is also commonly called green vitriol.

(b) According to the above-mentioned description, the reaction is a thermal decomposition reaction as ferrous sulphate decomposes to iron [III] oxide and oxides of sulphur [sulphur dioxide and sulphur trioxide] on heating. So the black colour mass obtained at the end of the reaction is Iron [III] oxide and the odour of the burning of sulphur is due to oxides of sulphur formed in the reaction.

(c) The reaction is as follows:

2FeSO₄→ Fe₂O₃ + SO₂ + SO₃