hexahydraat

Hexahydraat, or hexahydrate, is a chemical term for a compound that contains six water molecules per formula unit. This water can be either coordinated to a central metal ion as six aqua ligands, forming an octahedral aqua complex, or present as lattice water embedded in the crystal structure. Hexahydrates are common among metal salts crystallizing from aqueous solution and among coordination compounds of transition metals in which the metal center is surrounded by six water molecules.

Structure and formation: In many hexahydrates, six water molecules are directly bound to the metal, denoted as [M(H2O)6]n+, or as neutral aqua complexes. Other water molecules may reside in the crystal lattice as lattice water. Hexahydrates are typically formed by crystallizing the corresponding salt from water, by evaporating an aqueous solution, or by hydration of a dehydrated salt. The precise arrangement of water molecules and whether they are coordinated or lattice-bound can depend on temperature, pressure, and the specific ion.

Examples: A classic example is cobalt(II) chloride hexahydrate, CoCl2·6H2O, which forms characteristic colored crystals and undergoes color changes upon dehydration. While some salts crystallize as hexahydrates readily, others may form different hydration levels or remain anhydrous under similar conditions.

Properties and applications: Hexahydrates generally dissolve in water, and dehydration upon heating releases water and yields the anhydrous salt. They are used as laboratory reagents, as precursors to metal oxides, and as subjects of crystallography and coordination chemistry studies to understand hydration states and ligand environments.