While performing an experiment, a student observes that when he heats some green crystals in a boiling tube, the colour of the crystals changes to brown and a gas evolves which smells like burning sulphur. Interpret the observations and results.
The green colour crystals that the student heated were of ferrous sulphate crystals (FeSO4). These crystals when heated undergoes decomposition reaction and breaks into ferric oxide, sulphur dioxide and sulphur trioxide (SO3). The green colour of FeSO4 crystals thus changes to reddish brown ferric oxide (Fe2O3) and then the smell of burning sulphur (SO2) is evolved.
2FeSO4→ Fe2O3 + SO2 + SO3 (In presence of heat, the reaction proceeds.)