Sulfates are a broad class of chemical compounds that contain the sulfate anion with the formula

SO₄²⁻ at their core. This anion consists of a central sulfur atom covalently bonded to four oxygen atoms in a tetrahedral geometry. The divalent negative charge of the sulfate anion is typically balanced by metallic cations (such as Na⁺, Ca²⁺, Fe²⁺) or ammonium groups (NH₄⁺), resulting in the formation of sulfate salts.

From a historical perspective, sulfates have long been known in the form of alum and were widely used in dyeing, leather tanning, and water treatment industries. Today, sulfates occur naturally in minerals such as gypsum (CaSO₄·2H₂O), anhydrite (CaSO₄), and barite (BaSO₄). They are also present in natural waters, soils, and even the atmosphere as suspended particles (aerosols).

Key properties of sulfates include:

• They are generally white or colorless crystalline solids.
• Their solubility in water varies; most sulfates of alkali and alkaline earth metals (such as sodium sulfate and magnesium sulfate) are highly soluble, whereas sulfates of heavy metals like lead, barium, and calcium (in anhydrite form) are sparingly soluble or insoluble.
• Many sulfates are capable of forming hydrates (waters of crystallization). A well-known example is copper(II) sulfate pentahydrate (CuSO₄·5H₂O), commonly known as blue vitriol.