Ferrous sulfate is an inorganic compound with the chemical formula FeSO₄, appearing as a white, odorless powder. Its crystalline hydrate is a heptahydrate at room temperature, commonly known as "green vitriol," a light green crystal that effloresces in dry air and oxidizes on the surface to form brown basic ferric sulfate in moist air. At 56.6°C, it becomes a tetrahydrate, and at 65°C, it becomes a monohydrate. Ferrous sulfate is soluble in water and almost insoluble in ethanol. Its aqueous solution oxidizes slowly in air when cold and more rapidly when hot. The addition of alkali or exposure to light accelerates its oxidation. It has a relative density (d15) of 1.897 and is irritating. Ferrous sulfate can be used as a reagent in chromatography, spot analysis for the determination of platinum, selenium, nitrites, and nitrates. It also serves as a reducing agent, in the manufacture of ferrites, water purification, polymerization catalysts, photomechanical printing, and more.