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n Physical properties-the result of hydrogen bonding Studied in isolation, the water molecule is deceptively simple. Its tow hydrogen atoms are joined to the oxygen atom by single covalent bonds. Because oxygen is more electronegative than hydrogen, the electrons of the polar bonds spend more time closer to the oxygen atom. In other words, the bonds that hold together the atoms in a water molecule are polar covalent bonds, with the oxygen region of the molecule having a partial negative charge and the hydrogen having a partial positive charge. The water molecule, shaped something like a wide V, is a polar molecule, meaning that opposite ends of the molecule have opposite charges. The anomalous properties of water arise from attractions between these polar molecules. The attraction is electrical; slightly positive hydrogen of one molecule is attracted to the slightly negative oxygen of a nearby molecule. The two molecules are thus held together by a hydrogen bond (FIGURE 1). Each water molecule can form hydrogen bonds to a maximum f four neighbors, and at any given moment, many of the molecules in a sample of liquid water are linked in this way. The extraordinary qualities of water are emergent properties resulting from the hydrogen bonding that orders molecules into a higher level of structural organization. I will examine four of waterˇ¦s properties that contribute to the fitness of Earth as an environment for life: waterˇ¦s cohesive behavior, its ability to stabilize temperature, its expansion upon freezing, and its versatility as a solvent. 1. Cohesion Water molecules stick to each other as result of hydrogen bonding. When water is in its liquid form, its hydrogen bonds are very fragile, about 1/12 as strong as covalent bond. They form, break, and re-form with great frequency. Each hydrogen bond lasts only a few trillionths of a second, but the molecules are constantly forming new bonds with a succession of partners. Thus, at any instant, a substantial percentage of all the water molecules are bonded to their neighbors, making water more structured than most other liquids. Collectively, the hydrogen bonds holds the substance together, a phenomenon called cohesion. Cohesion due to hydrogen bonding contributes to the transport of water against gravity in plants. Water reaches the leaves through microscopic vessel that extend upward from the roots (FIGURE 2). Water that evaporates from a leaf is replaced by water from the vessels in the veins of the leaf. Hydrogen bonds cause water molecules leaving the veins to tug on molecules father down in the vessel, and the upward pull is transmitted along the vessel all the way down to the roots. Adhesion, the clinging of one substance to another, also plays a role. Adhesion of water to the walls of the vessel helps counter the downward pull of gravity. Related to cohesion is surface tension (FIGURE 3), a measure of how difficult it is to stretch or break the surface of a liquid. Water has a greater surface tension than most other liquids. At the interface between water and air is an ordered arrangement of water molecules, hydrogen-bonded to one another and to the water below. This makes the water heaves as though coated with an invisible film.
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