Monohydrates

Monohydrates are crystalline compounds that incorporate exactly one water molecule per formula unit in their solid structure. They are a subset of hydrates, which are compounds containing water molecules chemically bound within the crystal lattice or acting as lattice or coordinated water. The water molecules in a monohydrate may be integral to the lattice, coordinating to metal centers or occupying structural sites, and cannot be removed without altering the crystal.

Hydration level depends on temperature, humidity, and pressure; some substances show different hydrates at different conditions, with the monohydrate representing a single-water species among others such as dihydrates and trihydrates. Dehydration by gentle heating can yield the anhydrous form, while exposure to water vapor can reform the monohydrate.

Characterization methods include X-ray diffraction to determine structure, thermogravimetric analysis to confirm water content, and infrared spectroscopy to detect O-H vibrations. Physicochemical properties such as solubility, melting point, and stability are affected by the hydration state.

In practice, monohydrates are encountered in minerals, pharmaceuticals, and materials science. The term is usually written as the compound followed by a water of crystallization designation (for example, compound·H2O, with the numeral 1 implied). The existence and stability of a monohydrate depend on the specific chemical system and external conditions. Monohydrates are one of several possible hydration states that a substance can adopt, and they are contrasted with higher hydrates and with anhydrous forms in discussions of hydration chemistry.