Properties of water


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CAPE Biology module 1

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  • 2 corners are orbitals with unshared electrons and a weak negative charge 2 are occupied by hydrogen atoms that have polar covalent bonds
  • Only a hydrogen ion is actually transferred The transferred proton binds to an unshared orbital of the second water molecule. The water molecule that loses the proton
  • There is only 1 molecule out of 554,000,000 water molecules that is dissociated.
  • Properties of water

    1. 1. Water – Chapter 3
    2. 2. Properties of Water <ul><li>Polar molecule </li></ul><ul><li>Cohesion and adhesion </li></ul><ul><li>High specific heat </li></ul><ul><li>Density – greatest at 4 o C </li></ul><ul><li>Universal solvent of life </li></ul>
    3. 3. Polarity of Water <ul><li>In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids </li></ul><ul><ul><li>Because oxygen is more electronegative, the region around oxygen has a partial negative charge. </li></ul></ul><ul><ul><li>The region near the two hydrogen atoms has a partial positive charge. </li></ul></ul><ul><li>A water molecule is a polar molecule with opposite ends of the molecule with opposite charges. </li></ul>
    4. 4. <ul><li>Water has a variety of unusual properties because of attractions between these polar molecules. </li></ul><ul><ul><li>The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. </li></ul></ul><ul><ul><li>Each water molecule can form hydrogen bonds with up to four neighbors. </li></ul></ul>Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.1
    5. 5. HYDROGEN BONDS <ul><li>Hold water molecules together </li></ul><ul><li>Each water molecule can form a maximum of 4 hydrogen bonds </li></ul><ul><li>The hydrogen bonds joining water molecules are weak, about 1/20 th as strong as covalent bonds. </li></ul><ul><li>They form, break, and reform with great frequency </li></ul><ul><li>Extraordinary Properties that are a result of hydrogen bonds. </li></ul><ul><ul><li>Cohesive behavior </li></ul></ul><ul><ul><li>Resists changes in temperature </li></ul></ul><ul><ul><li>High heat of vaporization </li></ul></ul><ul><ul><li>Expands when it freezes </li></ul></ul><ul><ul><li>Versatile solvent </li></ul></ul>
    6. 6. Organisms Depend on Cohesion <ul><li>Cohesion is responsible for the transport of the water column in plants </li></ul><ul><li>Cohesion among water molecules plays a key role in the transport of water against gravity in plants </li></ul><ul><li>Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels. </li></ul>Hydrogen bonds hold the substance together, a phenomenon called cohesion
    7. 7. <ul><li>Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. </li></ul><ul><ul><li>Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. </li></ul></ul><ul><ul><li>Water behaves as if covered by an invisible film. </li></ul></ul><ul><ul><li>Some animals can stand, walk, or run on water without breaking the surface. </li></ul></ul>Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.3
    8. 8. Moderates Temperatures on Earth <ul><li>What is kinetic energy? </li></ul><ul><li>Heat? </li></ul><ul><li>Temperature? </li></ul><ul><li>Calorie? </li></ul><ul><li>What is the difference in cal and Cal? </li></ul><ul><li>What is specific heat? </li></ul>  Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air. Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature. Celsius Scale at Sea Level 100 o C Water boils 37 o C Human body temperature 23 o C Room temperature 0 o C Water freezes
    9. 9. Specific Heat is the amount of heat that must be absorbed or lost for one gram of a substance to change its temperature by 1 o C . <ul><li>Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for: </li></ul><ul><li>Prevention of temperature fluctuations that are outside the range suitable for life. </li></ul><ul><li>Coastal areas having a mild climate </li></ul><ul><li>A stable marine environment </li></ul>
    10. 10. Evaporative Cooling <ul><li>The cooling of a surface occurs when the liquid evaporates </li></ul><ul><li>This is responsible for: </li></ul><ul><ul><li>Moderating earth’s climate </li></ul></ul><ul><ul><li>Stabilizes temperature in aquatic ecosystems </li></ul></ul><ul><ul><li>Preventing organisms from overheating </li></ul></ul>
    11. 11. Density of Water <ul><li>Most dense at 4 o C </li></ul><ul><li>Contracts until 4 o C </li></ul><ul><li>Expands from 4 o C to 0 o C </li></ul><ul><li>The density of water: </li></ul><ul><li>Prevents water from freezing from the bottom up. </li></ul><ul><li>Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. </li></ul><ul><li>Makes transition between season less abrupt. </li></ul>
    12. 12. <ul><ul><li>When water reaches 0 o C, water becomes locked into a crystalline lattice with each molecule bonded to to the maximum of four partners. </li></ul></ul><ul><ul><li>As ice starts to melt, some of the hydrogen bonds break and some water molecules can slip closer together than they can while in the ice state. </li></ul></ul><ul><ul><li>Ice is about 10% less dense than water at 4 o C. </li></ul></ul>Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.5
    13. 13. Solvent for Life <ul><li>Solution </li></ul><ul><ul><li>Solute </li></ul></ul><ul><ul><li>solvent </li></ul></ul><ul><li>Aqueous solution </li></ul><ul><li>Hydrophilic </li></ul><ul><ul><li>Ionic compounds dissolve in water </li></ul></ul><ul><ul><li>Polar molecules (generally) are water soluble </li></ul></ul><ul><li>Hydrophobic </li></ul><ul><ul><li>Nonpolar compounds </li></ul></ul>
    14. 14. Most biochemical reactions involve solutes dissolved in water. <ul><li>There are two important quantitative proprieties of aqueous solutions. </li></ul><ul><ul><li>1. Concentration </li></ul></ul><ul><ul><li>2. pH </li></ul></ul>
    15. 15. Concentration of a Solution <ul><li>Molecular weight – sum of the weights of all atoms in a molecule (daltons) </li></ul><ul><li>Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular weight in daltons. </li></ul><ul><li>Avogadro’s number – 6.02 X 10 23 </li></ul><ul><ul><li>A mole of one substance has the same number of molecules as a mole of any other substance. </li></ul></ul>
    16. 16. Molarity <ul><li>The concentration of a material in solution is called its molarity. </li></ul><ul><ul><li>A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water. </li></ul></ul>Calculate a one molar solution of sucrose, C 12 H 22 O 16 . C = 12 daltons H = 1 dalton O = 16 daltons 12 x12 = 144 1 x 22 = 22 16 x 11 = 176 342 For a 2M solution? For a .05 M solution? For a .2 M solution?
    17. 17. <ul><li>Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. </li></ul><ul><ul><li>The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H + ). </li></ul></ul><ul><ul><li>The water molecule that lost a proton is now a hydroxide ion (OH - ). </li></ul></ul><ul><ul><li>The water molecule with the extra proton is a hydronium ion (H 3 O + ). </li></ul></ul>Dissociation of Water Molecules Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Unnumbered Fig. 3.47
    18. 18. <ul><li>A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion: </li></ul><ul><ul><li>H 2 O <=> H + + OH - </li></ul></ul><ul><li>This reaction is reversible. </li></ul><ul><li>At equilibrium the concentration of water molecules greatly exceeds that of H + and OH - . </li></ul><ul><li>In pure water only one water molecule in every 554 million is dissociated. </li></ul><ul><ul><li>At equilibrium, the concentration of H + or OH - is 10 -7 M (25°C) . </li></ul></ul>
    19. 19. Acids and Bases <ul><li>An acid is a substance that increases the hydrogen ion concentration in a solution. </li></ul><ul><li>Any substance that reduces the hydrogen ion concentration in a solution is a base . </li></ul><ul><ul><li>Some bases reduce H + directly by accepting hydrogen ions. </li></ul></ul><ul><li>Strong acids and bases complete dissociate in water. </li></ul><ul><li>Weak acids and bases dissociate only partially and reversibly. </li></ul>
    20. 20. pH Scale <ul><li>The pH scale in any aqueous solution : </li></ul><ul><ul><li>[ H + ] [OH - ] = 10 -14 </li></ul></ul><ul><li>Measures the degree of acidity (0 – 14) </li></ul><ul><li>Most biologic fluids are in the pH range from 6 – 8 </li></ul><ul><li>Each pH unit represents a tenfold difference (scale is logarithmic) </li></ul><ul><ul><li>A small change in pH actually indicates a substantial change in H + and OH - concentrations. </li></ul></ul>
    21. 21. Problem How much greater is the [ H + ] in a solution with pH 2 than in a solution with pH 6? Answer: pH of 2 = [ H + ] of 1.0 x 10 -2 = 1/100 M pH of 6 = [ H + ] of 1.0 x 10 -6 = 1/1,000,000 M 10,000 times greater
    22. 22. Buffers <ul><li>A substance that eliminates large sudden changes in pH. </li></ul><ul><li>Buffers help organisms maintain the pH of body fluids within the narrow range necessary for life. </li></ul><ul><ul><li>Are combinations of H + acceptors and donors forms in a solution of weak acids or bases </li></ul></ul><ul><ul><li>Work by accepting H + from solutions when they are in excess and by donating H + when they have been depleted. </li></ul></ul>
    23. 23. Acid Precipitation <ul><li>Rain, snow or fog with more strongly acidic than pH of 5.6 </li></ul><ul><li>West Virginia has recorded 1.5 </li></ul><ul><li>East Tennessee reported 4.2 in 2000 </li></ul><ul><li>Occurs when sulfur oxides and nitrogen oxides react with water in the atmosphere </li></ul><ul><ul><li>Lowers pH of soil which affects mineral solubility – decline of forests </li></ul></ul><ul><ul><li>Lower pH of lakes and ponds – In the Western Adirondack Mountains, there are lakes with a pH <5 that have no fish. </li></ul></ul>