Observation:
When 2g of ferrous sulphate crystals were heated in a glass tube, the following observations were recorded:

1. Color change: The greenish-blue crystals of ferrous sulphate turned into a yellowish-white powder.
2. Formation of brown gas: A brown gas was observed evolving from the heated crystals.

Chemical reaction:
The chemical reaction for the observation can be represented as follows:

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

Explanation:
1. Decomposition of ferrous sulphate: Ferrous sulphate (FeSO4) undergoes thermal decomposition upon heating to form ferric oxide (Fe2O3), sulfur dioxide (SO2), and sulfur trioxide (SO3). The reaction can be described as follows:

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

2. Color change: The greenish-blue color of ferrous sulphate crystals is due to the presence of water molecules in the crystal lattice. As the crystals are heated, the water molecules are driven off, leading to the formation of anhydrous ferric oxide, which appears as a yellowish-white powder.

3. Formation of brown gas: The brown gas evolved during the heating process is a mixture of sulfur dioxide (SO2) and sulfur trioxide (SO3). These gases are formed due to the decomposition of ferrous sulphate. The brown color is attributed to the presence of sulfur dioxide.

Overall, the heating of ferrous sulphate crystals leads to the decomposition of the compound, resulting in the formation of ferric oxide and the release of sulfur dioxide and sulfur trioxide gases. The color changes observed are due to the formation of anhydrous ferric oxide and the evolution of the brown gas.