Green vitriol (iron(II) sulfate, FeSO₄·7H₂O) has been known since the 13th century and is traditionally obtained by crystallization from iron or iron-based materials reacted with dilute sulfuric acid.

High Solubility & Multiple Hydration Forms：The heptahydrate forms green monoclinic crystals with high water solubility (296 g/L at 25 °C) and can be converted into mono-, tetra-, or anhydrous forms through controlled dehydration processes.

Thermal Transformation Capability：Upon heating, iron(II) sulfate undergoes systematic dehydration and eventually decomposes at high temperatures to form iron(III) oxide (Fe₂O₃), releasing sulfur dioxide and sulfur trioxide.

Industrial Relevance of Iron(III) Oxide：The resulting iron(III) oxide, also known as hematite, is a stable, high-temperature-resistant material widely used in industry. It dissolves in dilute acids to form iron(III) salts and can be reduced to metallic iron under controlled conditions.