When ferrous sulphate crystals (FeSO4) are heated, they undergo thermal decomposition. The decomposition process involves the breakdown of the compound into its constituent elements or simpler compounds. Here's what happens when ferrous sulphate is heated:

1. Initial Dehydration: As ferrous sulphate crystals are heated, they first undergo dehydration. Water molecules present in the crystal lattice are driven off, resulting in the formation of anhydrous ferrous sulphate (FeSO4).

FeSO4·7H2O (ferrous sulphate heptahydrate) → FeSO4 (anhydrous ferrous sulphate) + 7H2O (water vapor)

2. Decomposition: Upon further heating, anhydrous ferrous sulphate begins to decompose. This decomposition occurs in several stages, each releasing different gases and producing various iron oxides:

a) Stage 1: In the first stage, ferrous sulphate decomposes into ferric oxide (Fe2O3), sulphur dioxide (SO2), and sulphur trioxide (SO3) gases.

2FeSO4 → Fe2O3 + SO2 + SO3

b) Stage 2: As heating continues, the remaining ferric oxide further reacts with sulphur dioxide to form ferrous oxide (FeO) and more sulphur trioxide.

Fe2O3 + 2SO2 → 2FeO + SO3

3. Final Products: The final products obtained from the complete decomposition of ferrous sulphate crystals are:

- Ferric oxide (Fe2O3): a reddish-brown solid

- Ferrous oxide (FeO): a black solid

- Sulphur dioxide (SO2): a colourless gas with a pungent odor

- Sulphur trioxide (SO3): a colourless gas that reacts with water to form sulphuric acid (H2SO4)

These decomposition products depend on the specific temperature and conditions of the heating process. In practical applications, the thermal decomposition of ferrous sulphate is often carried out as a method for producing iron oxides and sulphur compounds.