Hydration
Hydration is the incorporation or association of water molecules with another substance, a process occurring in chemical, biological, and environmental contexts. In chemistry, it involves reactions where water adds to ions, molecules, or compounds, often forming hydrates—stable compounds with definite water content, such as in inorganic salts or organic addition reactions.[1] Biologically, hydration refers to maintaining adequate water balance in living organisms, where water comprises about 60% of an adult human's body weight and supports essential functions like cellular homeostasis, temperature regulation, and nutrient delivery.[2] This balance is regulated through intake from fluids and food, countering losses via urination, sweating, and other means; humans can survive only a few days without water.[3] In health contexts, proper hydration prevents dehydration, which can impair physical and cognitive performance even at 2% body water loss, and reduces risks like kidney stones. As of 2005 guidelines from the U.S. National Academies (still current as of 2025), total daily water intake is recommended at 3.7 liters (15.5 cups) for adult men and 2.7 liters (11.5 cups) for adult women, including about 20% from food.[4][5] Further applications in materials science, geology, and agriculture are detailed in later sections.Chemical Hydration
Definition and Mechanisms
Hydration in chemistry refers to the process by which water molecules are incorporated into a substance, typically through bonding to ions or molecules, or by participating in the formation of new chemical compounds. This interaction often involves the addition of water across unsaturated bonds or the coordination of water molecules to charged species, resulting in structures such as hydrated ions or complexes. Unlike the broader concept of solvation, which describes the stabilization of solutes by any solvent through various intermolecular forces, hydration is specifically limited to water as the solvent and emphasizes ion-dipole interactions or hydrogen bonding.[6] The mechanisms of hydration vary depending on the nature of the substance involved. For ionic compounds, hydration proceeds via ion-dipole attractions, where the partial negative charge on oxygen in water molecules orients toward positively charged ions (e.g., Na⁺), forming a hydration shell that stabilizes the ion in solution; this process is driven by electrostatic forces and often occurs spontaneously upon dissolution. In contrast, for molecular compounds with unsaturated bonds, such as alkenes, hydration typically follows a nucleophilic addition mechanism under acidic conditions. Here, the reaction overcomes an activation energy barrier associated with the formation of a reactive intermediate, which can be lowered by catalysts like strong acids (e.g., H₂SO₄) that protonate the substrate, enhancing its electrophilicity. Enzymes can also catalyze hydration in biological contexts by providing a low-energy pathway, though this is less relevant to purely chemical processes.[6][7] Hydration reactions can be classified as exothermic or endothermic based on their enthalpy change (ΔH). Exothermic hydrations release heat, as the energy from new ion-dipole bonds exceeds the lattice energy required to break the solute's structure; for example, the dissolution of NaOH in water is exothermic with ΔH = -44.51 kJ/mol. Endothermic hydrations absorb heat, occurring when the endothermic step of separating solute particles dominates, such as in the hydration of KNO₃ (ΔH = +34.89 kJ/mol). Regarding reversibility, many hydration processes are reversible, particularly in salts where water can be removed by heating to yield anhydrous forms, as seen in the equilibrium between hydrated and anhydrous copper(II) sulfate: CuSO₄·5H₂O ⇌ CuSO₄ + 5H₂O. Irreversible hydrations, however, proceed to completion without favoring the reverse direction under standard conditions, often due to the formation of stable products like alcohols from alkenes.[6][8] A representative example of hydration via nucleophilic addition is the acid-catalyzed hydration of alkenes, which converts an alkene to an alcohol following Markovnikov's rule. The general reaction is: \text{R-CH=CH}_2 + \text{H}_2\text{O} \xrightarrow{\text{H}^+ } \text{R-CH(OH)-CH}_3 The mechanism involves three key steps:- Protonation of the double bond by the acid catalyst, forming a carbocation intermediate on the more substituted carbon (rate-determining step, involving breaking of the π-bond).
- Nucleophilic attack by water on the carbocation, yielding a protonated alcohol.
- Deprotonation by the conjugate base of the acid, regenerating the catalyst and forming the neutral alcohol.