WATER Chapter 3

Polarity of Water The oxygen (O) atom is slightly negative (d-) and the hydrogen (H) atoms are each slightly positive (d+) POLAR COVALENT bonds hold the H to the O WITHIN the molecule HYDROGEN bonds form between water molecules. The d- O atom from one molecule is attracted to the d+ H atom of another molecule. Each water molecule can form up to 4 H bonds at a time.

4 properties of water contribute to earth being suitable for life Adhesion: water molecules stick to other molecules (think of the meniscus in a graduated cylinder) Transpiration: water travels against gravity from roots to leaves in plants due to adhesion and cohesion 1) Cohesion: the linking of like molecules; water molecules stick together, causing water to have high surface tension

4 properties of water contribute to earth being suitable for life 2) Moderation of temperature: water has a high specific heat; it must absorb or give off a lot of energy before its temperature will increase or decrease Water’s high specific heat makes temperature of oceans and large bodies of water very stable and able to support lots of plant/animal life. 3) Insulation of bodies of water by floating ice: ice floats at the top of large bodies of water, preventing them from freezing solid enabling a moderation of water temperature 4) “Universal Solvent”: Hydrophilic Substances (water loving) are water soluble and dissolve easily in water (ions and polar substances) Hydrophobic Substances (water fearing) do not easily dissolve in water (nonpolar substances like fats and oils)

pH scale The scale runs from 0-14, measuring the relative acidity and alkalinity (“basicity”) of aqueous solutions Acids: have excess H+ ions and a pH below 7.0 Bases: have excess OH- and a pH above 7.0 Pure Water: neutral with a pH of 7.0 (H+ = OH-) pH of 4 has 10x as much H+ as a substance with a pH of 5 pH of 5 has 100x as much H+ as a substance with a pH of 7

Buffers and pH H2O + CO2  H2CO3  HCO3- + H+ Buffers: substances that minimize the changes in pH within the body; a partnership between a weak acid and a weak base that forms as the acid dissolves in water Buffer systems bind or release H+ to keep pH steady Buffer systems in the blood moderate pH changes in the blood plasma H2O + CO2  H2CO3  HCO3- + H+