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Chapter 18. PROPERTIES OF WATER OF IMPORTANCE IN PHYS IOLOGY Approximately 70% of the mass of higher animals is water. About two thirds of this water content is intracellular, and the remainder includes the solvent of interstitial fluids (17% of body weight), plasma (6% of body weight), and secretions in the ducts of the exocrine glandular system (approximately 2% of total). Water is so important to the function of cells and the body in general that a brief review of the physical chemistry of this molecule is in order. A reader lacking background in chemistry will benefit from reading Appendix A, or by consulting a high school or introductory college chemistry text for a description of the nature of atoms, ions, electron shells, and molecules. A. Water as a solvent Water molecules exhibit a high degree of charge dipole because the oxygen atom is more electronegative than are the hydrogens. Both hydrogen atoms exhibit weak positive charges, making them mutually repulsive around the oxygen, as represented in Figure 18-1. The charge asymmetry is referred to as dipole, and the charge distribution leads to the bonding between each hydrogen atom with the oxygen atom in water being about 60% covalent and 40% ionic. Because of this, the residual positivity on the hydrogen atom leads to its being attracted to the surplus negativity on the oxygen of an adjacent water molecule. This attractive Figure 18-1. The water dipole. Crude shading has been used to give a general impression of electron density around the three atomic nuclei. The figure shows the hydrogens to be relatively electron deficient because of the electrophilic behavior of the oxygen. The resulting asymmetry of charge is the basis for using the term dipole. force, which is termed hydrogen bonding, may be so great that the hydrogen covalently binds to the second oxygen atom. As illustrated in Figure 18-2, this colligative phenomenon results in formation of the hydronium (H 3 O + ) ion and hydroxyl (OH - ) ion. The dissociation of multiple molecules of water to hydronium and hydroxyl ions occurs reversibly with great lability. However, at a given time, water is predominantly in the associated form (H 2 O). Pure water at 25 C contains a mere 10 -7 moles each of H 3 O + and OH - per kilogram
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Figure 18-2. Hydrogen bonding in water. Water dipoles associate with one another through hydrogen bonding. The negativity of the oxygen attracts a hydrogen atom, with its relative positivity, from other water molecules. If the attracted hydrogen leaves its parent molecule to form a hydronium ion (H 3 O + ), the remainder becomes a hydroxyl ion (OH - ). associated water (0.0000000018 to 1)
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This note was uploaded on 03/23/2009 for the course ANSCI 1110 taught by Professor Brucecurrie during the Fall '08 term at Cornell University (Engineering School).

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