Hydrationstate

Hydrationstate, or hydration state, refers to the degree to which water is associated with a substance. This can describe water incorporated into a crystal lattice as water of crystallization, or water molecules that surround solute particles in a solution, forming a hydration shell. In chemical formulas the hydration state is often indicated by a dot, as in CuSO4·5H2O, where the compound is a pentahydrate. The term also covers the general concept of an overall water content in a sample.

In solid-state chemistry and mineralogy, hydration states define how many water molecules are bound within a crystal structure. Dehydration, typically by heating or drying, removes these water molecules and may yield anhydrous form with different properties, such as altered color, solubility, or reactivity. In solution chemistry, the hydration number is the average count of water molecules directly coordinated to a solute ion or molecule; for many metal ions, the hydration shell profoundly affects ionic mobility, reactivity, and spectroscopy.

In biology and medicine, hydration state describes the body's water content and electrolyte balance; abnormal hydration can lead to dehydration or overhydration, impacting cellular function and physiology. Measurement approaches range from direct water content determination to osmolarity-based assessments and modern bioimpedance techniques.

Understanding hydration state is important for predicting solubility, phase behavior, and stability of materials, as well as for interpreting biological hydration and its clinical implications.