Observation: When ferrous sulphate crystals are heated, they undergo a chemical change. The green colour crystals change to a white solid, and a brown gas is released.

Chemical equation: The chemical equation for the reaction is:

2 FeSO4(s) → Fe2O3(s) + SO2(g) + SO3(g)

Explanation: When ferrous sulphate crystals are heated, the water molecules present in the crystals are first removed. This leaves behind a solid compound of iron(II) oxide (Fe2O3) and sulphur trioxide (SO3). The sulphur trioxide then reacts with the oxygen in the air to produce sulphur dioxide (SO2). The sulphur dioxide gas is released into the atmosphere, while the iron(II) oxide remains as a solid.

This reaction is an example of a thermal decomposition reaction, in which a compound is broken down into simpler substances by the application of heat.