Hydration

HYDRATION, a chemical term denoting the process of adding water to any substance. This includes: I. The separation from an aqueous solution of any substance of crystals containing a different number of water molecules per molecule of the substance. The resulting compounds, for example, Na₂SO₄.10H₂O; CuSO₄.5H₂O; BaCl₂.2H₂O, etc., are called crystalline hydrates, and the water that is part of the composition of these crystals is called water of crystallization. One and the same substance can form crystalline hydrates with different amounts of water and, depending on this, possess different physical properties: different water vapor pressure, crystalline form, solubility, etc. Some crystalline hydrates easily lose water directly in the air (e.g., Na₂SO₄.10H₂O) (the process of efflorescence), while others give it up with difficulty, only upon heating (e.g., CuSO₄.5H₂O). When containing several particles of water, often a different amount of water is held with unequal force; for example, in CuSO₄.5H₂O, the first two molecules of water are released most easily, the next two with more difficulty, and the last only at 240–250°. II. The addition of water that enters into the chemical composition of a substance newly formed due to the addition of water: e.g., CaO + H₂O = Ca(OH)₂; or SO₃ + H₂O = H₂SO₄; reactions of the indicated type are accompanied by the release of heat and in these cases lead to the formation of stable bodies. Such compounds are called hydrates (see) or hydroxides, and the water entering into their composition is called constitutional water. Hydration can also take place in relation to organic compounds, e.g., the addition of elements of a water particle to chloral with the formation of chloral hydrate, to acetylene with the formation of acetaldehyde. III. Hydration of ions. In aqueous solutions, the majority of ions are surrounded by a water shell containing a certain number of water molecules, which varies for different ions. Consequently, it is not individual ions that move under the influence of an electric current, but ion-hydrates formed in aqueous solutions. In physical chemistry, there are known methods for determining the amount of water bound by one ion or another present in a solution; however, the method of the addition of water molecules by ions, colloidal particles, etc., remains unexplained. With an increase in the concentration of solutions, the hydration of ions decreases. IV. The phenomena of hydration also include the processes of swelling of colloids (e.g., gelatin). These processes are dependent on the electrolytes present and play an important role in the physiology and pathology of the living organism (phenomena of muscle contraction, changes in the aggregate state of protoplasm, phenomena of diuresis, formation of edema, and many others). V. Hydration accompanied by the splitting of the molecule of the initial substance is called hydrolysis (see). S. Severin.