1. 1

2. 2 Polarity of Water In a water molecule two hydrogen atoms form single polar covalent bonds with an oxygen atom. Gives water more structure than other liquids –Because oxygen is more electronegative, the region around oxygen has a partial negative charge. –The region near the two hydrogen atoms has a partial positive charge. A water molecule is a polar molecule with opposite ends of the molecule with opposite charges.

3. 3 Water has a variety of unusual properties because of attractions between these polar molecules. –The slightly negative regions of one molecule are attracted to the slightly positive regions of nearby molecules, forming a hydrogen bond. –Each water molecule can form hydrogen bonds with up to four neighbors. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.1

4. 4 Hydrogen bonding

5. 5 HYDROGEN BONDS Hold water molecules together Each water molecule can form a maximum of 4 hydrogen bonds The hydrogen bonds joining water molecules are weak, about 1/20 th as strong as covalent bonds. They form, break, and reform with great frequency Extraordinary Properties that are a result of hydrogen bonds. –Cohesive behavior –Resists changes in temperature –High heat of vaporization –Expands when it freezes –Versatile solvent

6. 6 Organisms Depend on Cohesion Cohesion among water molecules plays a key role in the transport of water against gravity in plants Adhesion, clinging of one substance to another, contributes too, as water adheres to the wall of the vessels. Hydrogen bonds hold the substance together, a phenomenon called cohesion

7. 7 Surface tension

8. 8 Surface tension, a measure of the force necessary to stretch or break the surface of a liquid, is related to cohesion. –Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface. –Water behaves as if covered by an invisible film. –Some animals can stand, walk, or run on water without breaking the surface. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.3

9. 9 Evaporative Cooling The cooling of a surface occurs when the liquid evaporates This is responsible for: –Moderating earth’s climate –Stabilizes temperature in aquatic ecosystems –Preventing organisms from overheating

10. 10 Density of Water Most dense at 4 o C Contracts until 4 o C Expands from 4 o C to 0 o C The density of water: 1.Prevents water from freezing from the bottom up. 2.Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation. 3.Makes transition between season less abrupt.

11. 11 –When water reaches 0 o C, water becomes locked into a crystalline lattice with each molecule bonded to its maximum of four partners. –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. –Ice is about 10% less dense than water at 4 o C. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Fig. 3.5

12. 12 Ice, water, vapor

13. 13 Hydrogen bonding (electric attraction) IceLiquid

14. 14 Are you afraid of water? What do you think the following two words mean? –Hydrophilic –Hydrophobic

15. 15 Solvent for Life Solution –Solute –solvent Aqueous solution Hydrophilic –Ionic compounds dissolve in water –Polar molecules (generally) are water soluble Hydrophobic –Nonpolar compounds

16. 16 “Like Dissolves Like” Polar Solutions (like water) are able to dissolve other polar molecules as well as ionic compounds. Why? Likewise, nonpolar solutions (like oil) will not dissolve polar and ionic compounds and will instead dissolve NONPOLAR molecules.

17. 17 So what about oil and water?

18. 18 Most biochemical reactions involve solutes dissolved in water. There are two important quantitative proprieties of aqueous solutions. –1. Concentration –2. pH

19. 19 Occasionally, a hydrogen atom shared by two water molecules shifts from one molecule to the other. –The hydrogen atom leaves its electron behind and is transferred as a single proton - a hydrogen ion (H + ). –The water molecule that lost a proton is now a hydroxide ion (OH - ). –The water molecule with the extra proton is a hydronium ion (H 3 O + ). Dissociation of Water Molecules Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings Unnumbered Fig. 3.47

20. 20 “Dissociation” of water

21. 21 A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion: –H 2 O H + + OH - This reaction is reversible. At equilibrium the concentration of water molecules greatly exceeds that of H + and OH -. In pure water only one water molecule in every 554 million is dissociated. –At equilibrium, the concentration of H + or OH - is 10 -7 M (25°C).

22. 22 pH pH = – log [H+] Log scale means 10X change per unit! [H+]= 10 -1 M [H+]= 10 -9 M

23. 23 pH Scale Measures the degree of acidity (0 – 14) Most biologic fluids are in the pH range from 6 – 8 Each pH unit represents a tenfold difference (scale is logarithmic) –A small change in pH actually indicates a substantial change in H + and OH - concentrations.

24. 24 Acids and Bases An acid is a substance that increases the hydrogen ion concentration in a solution. Any substance that reduces the hydrogen ion concentration in a solution is a base. –Some bases reduce H + directly by accepting hydrogen ions. Strong acids and bases complete dissociate in water. Weak acids and bases dissociate only partially and reversibly.

25. 25 Buffers A substance that eliminates large sudden changes in pH. Buffers help organisms maintain the pH of body fluids within the narrow range necessary for life. –Are combinations of H + acceptors and donors forms in a solution of weak acids or bases –Work by accepting H + from solutions when they are in excess and by donating H + when they have been depleted.

26. 26 Acid Precipitation Rain, snow or fog with more strongly acidic than pH of 5.6 West Virginia has recorded 1.5 East Tennessee reported 4.2 in 2000 Occurs when sulfur oxides and nitrogen oxides react with water in the atmosphere –Lowers pH of soil which affects mineral solubility – decline of forests –Lower pH of lakes and ponds – In the Western Adirondack Mountains, there are lakes with a pH <5 that have no fish.

27. 27 Overall lessons: Many properties of water are emergent properties due to hydrogen bonding. The cohesion of water molecules to each other is exploited by plants and animals. Water resists temperature changes by absorbing lots of heat. Lower density of ice causes it to float & insulate the water below. The polarity of water allows it to dissolve other polar molecules. Non-polar compounds are hydrophobic and not easily dissolved in water. Adding or removing hydrogen ions changes the pH of a solution. Buffers resist pH changes by accepting or donating H ions when [H+] changes.

28. 28 How would carbon bond with H? CH4 How would it look? CH4 contains nonpolar covalent bonds because although C and H are different elements, they pull on the electrons equally.