Take a small amount of ferrous sulphate crystals in a dry test tube.
(a) Note the colour of the ferrous sulphate crystals.
Heat the test tube over the flame of a burner or spirit lamp.
(b) Observe the colour of the crystals after heating.
Objective – To study a thermal decomposition reaction.
(a) Before heating the colour of ferrous sulphate is green.
(b) On heating crystals of ferrous sulphate (FeSO4. 7H2O), first lose water and form anhydrous ferrous sulphate (FeSO4). The colour of the crystals changes from green to white. Ferrous sulphate decomposes further to brown ferric oxide (Fe2O3), sulphur dioxide (SO2) and sulphur trioxide (SO3). It gives a characteristic odour of burning sulphur.
FeSO4.7H2O (s) → FeSO4 (s) + 7H2O
Ferrous sulphate Anhyd. ferrous sulphate
FeSO4 (s) → Fe2O3 (s) + SO2 (g) + SO3 (g)
Conclusion – This reaction is an example of thermal decomposition, as it is carried out by heating.