CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT

CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT -...

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CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT Water's extraordinary properties are emergent properties resulting from water's structure and molecular interactions. I. Water's Polarity and Its Effects A. The polarity of water molecules results in hydrogen bonding Water is a polar molecule. Its polar bonds and asymmetrical shape give water molecules opposite charges on opposite sides. - Four valence orbitals of O point to corners of a tetrahedron. -Two corners are orbitals with unshared pairs of electrons and weak negative charge. -Two corners are occupied by H atoms which are in polar covalent bonds with O. Oxygen is so electronegative, that a H shared electrons spend more time around the O causing a weak positive charge near H's. Hydrogen bonding orders water into a higher level of structural organization. -The polar molecules of water are held together by hydrogen bonds. -Positively charged H of one molecule is attracted to the negatively charged O of another water molecule. Water has extraordinary properties that emerge as a consequence of its polarity and hydrogen-bonding. Some of these properties are that water: - has cohesive behavior - resists changes in temperature - has a high heat of vaporization and cools surfaces as it evaporates - expands when it freezes - is a versatile solvent
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B. Organisms depend on the cohesion of water molecules. Cohesion = Phenomenon of a substance being held together by hydrogen bonds. -Though hydrogen bonds are transient, enough water molecules are hydrogen bonded at any given time to give water more structure than other liquids. -Contributes to upward water transport in plants by holding the water column together. Adhesion of water to vessel walls counteracts the downward pull of gravity. Surface tension = Measure of how difficult it is to stretch or break the surface of a liquid. -Water has a greater surface tension than most liquids; function of the fact that at the air/H 2 Ointerface, surface water molecules are hydrogen bonded to each other and to the water molecules below. -Causes H 2 O to bead (shape with smallest area to volume ratio and allows maximum hydrogen bonding). C. Water moderates temperatures on Earth 1. Heat and temperature Kinetic energy = The energy of motion. Heat = Total kinetic energy due to molecular motion in a body of matter. Temperature = Measure of heat intensity due to the average kinetic energy of molecules in a body of matter. Calorie (cal) = Amount of heat it takes to raise the temperature of one gram of water by one degree Celsius. Conversely, one calorie is the amount of heat that one gram of water releases when it cools down by one degree Celsius. NOTE: The "calories" on food packages are actually kilocalories (kcal).
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This note was uploaded on 03/18/2008 for the course BS 110 taught by Professor S.lawrence during the Spring '07 term at Michigan State University.

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CHAPTER 3 WATER AND THE FITNESS OF THE ENVIRONMENT -...

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