What is the number of water molecules present in a Ferrous Sulphate crystal ?

Ferrous sulphate crystals, commonly known as green vitriol or copperas, contain water of crystallisation that gives them their characteristic light green colour. The most common crystalline form of ferrous sulphate is the heptahydrate, which has the chemical formula FeSO4·7H2O [2]. This indicates that there are 7 water molecules associated with each formula unit of iron(II) sulphate in its crystal lattice [2]. When these crystals are heated, they lose this water of crystallisation and change colour from green to white as they become anhydrous [1][3]. While other hydration states like the tetrahydrate (4H2O) and monohydrate (1H2O) exist, the standard 'ferrous sulphate crystal' referred to in general chemistry and mineralogy (melanterite) is the heptahydrate form [2].

Sources

1. [2] https://pubchem.ncbi.nlm.nih.gov/compound/Ferrous-sulfate-heptahydrate
2. [1] Science , class X (NCERT 2025 ed.) > Chapter 1: Chemical Reactions and Equations > Figure 1.4 > p. 8
3. [3] https://pubchem.ncbi.nlm.nih.gov/compound/Ferrous-Sulfate