Nonahydrates

Nonahydrates are chemical hydrates in which nine water molecules are incorporated into the crystal structure of the compound per formula unit. They represent a high level of hydration relative to more common hydrates and are typically written as MX·9H2O for a salt M with an anion X, though the exact composition is determined by crystallography and can vary among compounds.

In nonahydrates, water can occupy lattice positions or exist as coordination water bound to metal centers. In most inorganic nonahydrates, the nine water molecules primarily act as lattice water within the crystal framework, but some structures may include coordinated water as well, depending on the geometry of the central ion.

Nonahydrates form by crystallizing from aqueous solutions under conditions that favor extensive hydration, such as low temperature or high water activity. They can be metastable and may dehydrate to lower hydrates or to the anhydrous salt upon heating, drying, or exposure to low humidity. Dehydration typically occurs in steps, with successive losses of water molecules at characteristic temperatures.

Physically, nonahydrates are usually crystalline solids with properties (solubility, stability, color) dependent on the specific salt. They are of interest in crystallography and thermogravimetric analysis for determining water content and hydration number, and they can influence the handling and storage of hydrated salts.

See also: hydrates, pentahydrate, heptahydrate, octahydrate, decahydrate.