1. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Chapter 3 – Water Three-quarters of the Earth’s surface is submerged in water The abundance of water is the main reason the Earth is habitable Figure 3.1

2. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Chapter 3 – Water Overview: The Molecule That Supports All of Life – Water is the biological medium here on Earth – All living organisms require water more than any other substance

3. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Polarity of Water Molecules Water is unique because of its structure and the interaction of its molecules Water is a polar molecule because oxygen is more electronegative than hydrogen

4. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The polarity of water molecules – Allows them to form hydrogen bonds with each other – Contributes to the various properties water exhibits Hydrogen bonds + + H H + +  – –  – –  – –  – – Figure 3.2

5. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Polarity of Water The hydrogen bonds between water molecules give water 4 unique properties: 1)Water’s cohesive behavior 2)Water’s ability to moderate temperature 3)Water’s expansion on freezing 4)Water’s versatility as a solvent

6. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Cohesion – Is the bonding of a high percentage of the molecules to neighboring molecules Water molecules pull each other along

7. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Cohesion – Helps pull water up through the microscopic vessels of plants Water conducting cells 100 µ m Figure 3.3

8. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Cohesion Adhesion is related to cohesion Adhesion – the clinging of one substance to another Water sticks to the walls of cells, which helps it resist gravity

9. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Cohesion Surface tension – Is a measure of how hard it is to break the surface of a liquid Figure 3.4

10. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Moderation of Temperature Water moderates air temperature – By absorbing heat from air that is warmer and releasing the stored heat to air that is cooler This is one reason why Cape Cod has milder winters than Worcester County

11. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Moderation of Temperature Water has high specific heat – That is, it takes a lot of energy to raise the temperature of water Why the ocean is still cold here in the summer

12. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Moderation of Heat Fast moving molecules can leave a liquid as gas Evaporative cooling – when the “hottest” or fastest moving molecules leave a liquid, thus cooling the remaining liquid

13. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Moderation of Heat Water’s evaporative cooling property helps stabilize the temperature of lakes and ponds and allows for the mechanism of temperature regulation in living things too

14. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Expansion of Water Water is less dense as a solid than as a liquid – This is very rare! Water expands as it solidifies (freezes) As a result, ice floats This plays an important role in Life

15. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Expansion of Water The hydrogen bonds in ice – Are more “ordered” than in liquid water, making ice less dense – The molecules are further apart in ice than liquid water Liquid water Hydrogen bonds constantly break and re-form Ice Hydrogen bonds are stable Hydrogen bond Figure 3.5

16. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Expansion of Water Since ice floats in water – Life can exist under the frozen surfaces of lakes and polar seas

17. Copyright © 2006 Cynthia Garrard publishing under Canyon Design At what temperature is water the densest?

18. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life Solution – a liquid that is completely homogenous mixture of two or more substances Solvent – the dissolving agent of a solution Solute – the substance dissolved Aqueous solution – solution in which water is the solvent

19. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life Water is a versatile solvent due to its polarity If an ionic compound (NaCl) is placed in water, the ions exposed at the surface are attracted to the polarity of the water molecules Result: Water molecules separate individual ions and surround them, effectively “breaking up” the salt

20. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life The different regions of the polar water molecule can interact with ionic compounds called solutes and dissolve them Negative oxygen regions of polar water molecules are attracted to sodium cations (Na + ). + + + + Cl – – – – – Na + Positive hydrogen regions of water molecules cling to chloride anions (Cl – ). + + + + – – – – – – Na + Cl – Figure 3.6

21. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life Compounds don’t have to be ionic to be affected Water can also interact with polar molecules such as proteins or sugars This oxygen is attracted to a slight positive charge on the lysozyme molecule. This oxygen is attracted to a slight negative charge on the lysozyme molecule. (a) Lysozyme molecule in a nonaqueous environment (b) Lysozyme molecule (purple) in an aqueous environment such as tears or saliva (c) Ionic and polar regions on the protein’s Surface attract water molecules. ++ –– Figure 3.7 These compounds are still water soluble

22. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life A hydrophilic substance – Has an affinity for water Anything ionic Polar molecule Substance doesn’t have to dissolve in water

23. Copyright © 2006 Cynthia Garrard publishing under Canyon Design The Solvent of Life A hydrophobic substance – Does not have an affinity for water – Non-ionic and non-polar substances – Seem to repel water

24. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH Polar water molecules don’t just break up ionic and polar compounds Water can also disassociate itself H Hydronium ion (H 3 O + ) H Hydroxide ion (OH – ) H H H H H H + – + Figure on p. 53 of water dissociating

25. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH Water can dissociate into – Hydrogen ions – single protons with a charge of 1+ – Hydroxide ions – charged with a 1- This reaction is reversible and will reach a state of dynamic equilibrium – When the water dissociates at the same rate that it is being re-formed H Hydronium ion (H 3 O + ) H Hydroxide ion (OH – ) H H H H H H + – + Figure on p. 53 of water dissociating

26. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH In pure water there is 1 molecule disassociated for every 554 million molecules of water. Despite its rarity, disassociated water molecules are extremely important to life

27. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH Certain solutes can alter the concentration of these ions An acid – Is any substance that increases the hydrogen ion concentration of a solution A base – Is any substance that reduces the hydrogen ion concentration of a solution

28. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH Acids and bases that dissolve completely in water are considered strong Weak acids and bases bind reversibly to H + or OH - and will eventually reach a state of equilibrium

29. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH The pH of a solution – Is determined by the relative concentration of hydrogen ions – Is low in an acid – Is high in a base

30. Copyright © 2006 Cynthia Garrard publishing under Canyon Design pH scale ranges from 0 – 14 – Less than 7 is acidic The lower the number the stronger the acid – 7 is neutral – Greater than 7 is basic (alkaline) The higher the number the stronger the base

31. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH The pH scale and pH values of various aqueous solutions Increasingly Acidic [H + ] > [OH – ] Increasingly Basic [H + ] < [OH – ] Neutral [H + ] = [OH – ] Oven cleaner 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 pH Scale Battery acid Digestive (stomach) juice, lemon juice Vinegar, beer, wine, cola Tomato juice Black coffee Rainwater Urine Pure water Human blood Seawater Milk of magnesia Household ammonia Household bleach Figure 3.8

32. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH The pH scale is a logarithmic scale Therefore, a solution with a pH of 3 isn’t twice as acidic as one with a pH of 6, it is 1000 times more acidic! If a small change in the number of H + has such a big change on the pH level, how can concentrations be maintained?

33. Copyright © 2006 Cynthia Garrard publishing under Canyon Design Acids, Bases and pH Buffers – Are substances that minimize changes in the concentrations of hydrogen and hydroxide ions in a solution – Consist of an acid-base pair that reversibly combines with hydrogen ions – Works by accepting H + from the solution when they are in excess and donating H + to the solution when they have been depleted