Chemistry of Water Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Outline Chemistry of Water Properties of Water pH Scale Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

All organisms are approximately 2/3 water Chemistry of Water All organisms are approximately 2/3 water ¾ of the Earth is covered in water Life existed for about 3 billion years in water before moving to land Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Structure of Water The water molecule is simple and stable Covalent bonds Octet rule satisfied One oxygen atom bound to two hydrogen atoms by two single covalent bonds. Polar Molecule - Has distinct ends, each with a partial charge. Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Hydrogen Bonds In a water molecule, both oxygen and hydrogen atoms attract shared electrons in the covalent bonds (electronegativity). Shared electrons more likely to be found near the O nucleus than near the H nuclei. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Properties of Water Cohesion - Attraction to other water molecules. Responsible for surface tension. Reason why water is a liquid at Earth’s temperatures Ex: As transpiration occurs from leaf surface, water is pulled up the xylem from the roots. Adhesion - Attraction to other substance. Capillary action also helps water move up the xylem of a plant Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Properties of Water Polarity of water causes it to be attracted to other polar molecules. More narrow tube, the greater the electrostatic forces between water and the wall of the tube Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Water Moderates Temperature Heat Storage - Because of the large number of hydrogen bonds, a large input of thermal energy is necessary to break apart water molecules. High Specific Heat -amount of energy needed to allow molecules to break free (break H-bonds) & increase in temperature Amount of energy required to raise 1g of a substance by 1oC Water = 1calorie/g/oC Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Water Moderates Temperature (continued) High Heat of Vaporization (energy required to change liquid water to gas) Water = 586 cal/g Evaporation of water has a cooling effect on the surface from which it evaporates Ex: sweating cools your body Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

As water freezes, crystals form Properties of Water As water freezes, crystals form Ice is less dense than liquid water due to the crystal lattice Thus, ice floats. Otherwise, inland bodies of water far from equator would stay solid year-round Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Ex: NaCl forms ions as it dissolves (quickly) Properties of Water Solvent - Water is an effective solvent because of its ability to form hydrogen bonds. Water molecules gather around any substance bearing an electrical charge. Ex: NaCl forms ions as it dissolves (quickly) Ex: sugar has polar –OH groups (doesn’t dissolve as fast as salt) Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

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Hydrophobic - Nonpolar molecules shrink from contact with water. Properties of Water Organization of Nonpolar Molecules - Water molecules act to exclude nonpolar molecules. Hydrophobic - Nonpolar molecules shrink from contact with water. (Water Hating) Hydrophilic - Polar molecules readily form hydrogen bonds with water. (Water Loving) Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Ionization - Spontaneous ion formation. Water Ionizes Ionization - Spontaneous ion formation. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Water Ionizes pH Scale - Measures negative logarithm of hydrogen ion concentration in the solution. Acids - Substances that dissociate in water to increase concentration of H+ ions. pH values below 7. Bases - Substances that combine with H+ ions (and lower the concentration) when dissolved in water. pH values above 7. Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Raven - Johnson - Biology: 6th Ed Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Water Ionizes Ex: Carbonic acid and bicarbonate (base) Buffer - A substance that acts as a reservoir for hydrogen atoms. Donates hydrogen atoms to solution when concentration falls, and removes them from the solution when concentration rises. Minimize change in pH. Ex: Carbonic acid and bicarbonate (base) Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies

Copyright © McGraw-Hill Companies Permission required for reproduction or display Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies