PHYSICAL CHEMISTRY OF WATER

The human body is made up principally of water. Consequently, the physical properties of water have enormous implication for maintenance of homeostasis. Water is a simple triatomic molecule. Its chemical formula is H₂ O, and its structural formula is H-O-H. Because the charge distribution of each covalent bond is unequal, the molecule has a polar conformation, and the H-O-H bond angle is 105 degrees. Water molecules attract and form hydrogen bonds with one another. These bonds are fundamental to the existence of life on this planet. Water has a high surface tension, low vapor pressure, high

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Water molecules are in continuous motion. Occasionally, when molecules collide, enough energy is produced to transfer a proton from one water molecule to another. Water slightly dissociates into a negatively charged hydroxyl ion (OH^- ) and positively charged hydronium ion (H₃ O⁺ ). Conventionally, this self-ionization of water is written as follows:

H₂ O ⇌ H⁺ + OH⁻

The symbol H⁺ is convenient because, although protons dissociating from water have many aliases (e.g., H₃ O⁺ , H₉ O₄ ⁺ ), most physicians and chemists refer to them as hydrogen ions.

The self-ionization of water is miniscule. In pure water at 25°C, the [H⁺ ] and [OH^- ] are 1.0 × 10⁻⁷ mmol/L. The tendency for water to dissociate into its component parts is represented by the following expression:

K_eq H₂ O = [H⁺ ][OH⁻ ]

The molarity of water is extremely high—55.5 M ("there is a lot of water in water"). Because the concentration of water and the K_eq are constants, the ion-product dissociation constant for water (Kw) can be expressed as follows:

K_eq H₂ O = K_eq (55.5) = Kw = [H⁺ ][OH⁻ ]

The implication is that the relative concentrations of hydroxyl and hydrogen are constant, and when there is an increase in the concentration of hydrogen ions, there is a concomitant decrease in the concentration of hydroxyl ions and vice versa.

Pure water is considered neutral because the relative concentrations of hydrogen and hydroxyl are equal at 1.0 × 10⁻⁷ mmol/L. A solution is considered acidic if the concentration of hydrogen ions exceeds that of hydroxyl ions ([H⁺ ] > 1.0 × 10⁻⁷ mmol/L, [OH⁻ ] < 1.0 × 10⁻⁷ mmol/L). A solution is considered alkaline if the hydroxyl ion concentration exceeds the hydrogen ion concentration.