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Overview: The Molecule That Supports All of Life All living organisms require water more than any other substance Most cells are surrounded by water, and cells themselves are about 70–95% water Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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The polarity of water molecules results in hydrogen bonding The water molecule is a polar molecule: The opposite ends have opposite charges Polarity allows water molecules to form hydrogen bonds with each other Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings – – H + + H O — — + + – –
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Hydrogen bond – – H + + H O — — + + + + + + – – – – – –
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Hydrogen bond – – H + + H O — — + + + + + + – – – – – – Why Does Water Have a High Specific Heat?
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Evaporative Cooling Evaporation is transformation of a substance from liquid to gas Heat of vaporization is the heat a liquid must absorb for 1 g to be converted to gas Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Evaporative Cooling As a liquid evaporates, its remaining surface cools, a process called evaporative cooling Evaporative cooling of water helps stabilize temperatures in organisms and bodies of water
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Fig. 3-6 Hydrogen bond Liquid water Hydrogen bonds break and re-form Ice Hydrogen bonds are stable Ice floats in liquid water because hydrogen bonds in ice are more “ordered,” making ice less dense Q. Why Does ice Float?
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Hydrophilic and Hydrophobic Substances A hydrophilic substance is one that has an affinity for water A hydrophobic substance is one that does not have an affinity for water Oil molecules are hydrophobic because they have relatively nonpolar bonds Copyright © 2008 Pearson Education, Inc., publishing as Pearson Benjamin Cummings
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Which of these substances is (are) hydophilic? Which of these substances is (are) hydophobic?
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