1. Water: Chemistry and Behavior
SGN 3

2. Life’s biochemistry is shaped by the aqueous environment in which it works
Water is essential for life because it is the medium of biochemistry
Because it shapes biochemistry, a fundamental understanding of the chemistry and properties of water is critical to an understanding of biology

3. Water’s Chemistry H2O See angle of covalent bonds Small compound
Covalent bonds are polar Electrons shared unequally
Oxygen is very electronegative (measure of an atom’s attraction for electrons) so electrons spend more time near the oxygen atom
This gives a water molecule dipoles, or ends with opposite partial charge

4. This means that each of a water molecule’s three atoms can form hydrogen bonds or mixed bonds with other compounds
Cohesion – attraction of water molecules to other water molecules
Adhesion – attraction of water molecule for another type of compound

5. Water’s Behaviors
The ability to form many cohesive or adhesive hydrogen bonds, as well as the typically high concentration of water that exists in the medium of interest (solution, cell) allows water to behave in several different ways that are important for life
Water is a good solvent for many things but not all things
Water changes state on earth and translocates to land
Water resists temperature change
Solid water is less dense than liquid water
Surface tension and capillary action

6. Water is a good solvent for many things but not all things
Water is able to adhere to charged and polar moieties and dissolve hydrophilic substances; in their dissolved state solutes can interact and react
But water cannot adhere to neutral moieties so some substances will not dissolve in water and can form basically insoluble substances such as cell membranes or cell walls

7. 2. Water changes state on earth and translocates to land, allowing for plentiful terrestrial life

8. 3. Water resists temperature change
Water is able to absorb a lot of heat energy and release a lot of heat energy with relatively little change in temperature
As water goes from solid to liquid to gas a great deal of energy is absorbed, which is needed to break the many stabilizing hydrogen bonds between the molecules (resists change of state – requires energy input)
As water goes from gas to liquid to solid a great deal of energy is released, as stable hydrogen bonds form between the molecules

9. Water has a high specific heat and high heat of vaporization

10. Water helps absorb and distribute heat, stabilizing temperatures within organisms (for example, evaporative cooling), and in ecosystems (locally and globally)

11. Water stabilizes land temperatures

12. 4. Solid water is less dense than liquid water

13. 5. Capillary action and surface tension
Capillary action – ability of water to “climb” up small tubes composed of polar molecules, like glass or cellulose
Surface tension – stronger cohesion of water molecules at surface where water interfaces with air

14. Solute concentration in aqueous solutions
Students should understand the different ways to express the concentration of a solution
moles/molarity %
weight/volume

15. Acidic and Basic Aqueous Solutions
In liquid water a small number of H2O molecules disassociate to form hydronium (H3O+, often considered simply as a hydrogen ion/H+) and hydroxide/OH- ions
In a pure aqueous solution these two ions always equal each other in concentration
In any aqueous solution [H3O+] x [OH-] =10-14

16. The pH scale measures the degree to which a solution is acidic or basic
Body, blood and cellular cytsolic pH is approximately 7.4

17. In any aqueous solution [H3O+] x [OH-] =10-14 pH is the negative logarithm of the hydrogen ion concentration pH = -log10 [H3O+] pH + pOH = 14 Each pH unit (on a scale of 0 to 14) represents a 10-fold change in H3O+ concentration Find the pH of a M HCl solution. The HCl is a strong acid and is 100% ionized in water. The hydronium ion concentration is M. pH = - log (0.0025) = - ( ) = 2.60

18. What is the pH of a solution with a [H+] = 10-8 M
What is the pH of a solution with a [H+] = 10-8 M? What is the pH of a solution with a [OH-] of 10-2 M? Acetic acid has a part (collection of atoms) called a carboxyl group, which is symbolized with R–COOH. When acetic acid dissolves in water the hydrogen completely disassociates from the carboxyl group to form R–COO- and H+. You dilute acetic acid in water to a concentration of 10-4 M. What is the pH of this solution? What is the hydronium ion concentration in a solution that has a pH of 8.34? The pH of a solution is 6.5. What are the [H+] and [OH-] of the solution?

19. What is the pH of a solution with a [H+] = 10-8 M?
pH = -log10 [H3O+]
pH = -log [10-8]
pH = 8
What is the pH of a solution with a [OH-] of 10-2 M?
pOH = -log10 [OH-]
pH + pOH = 14
pOH = -log [10-2]
p0H = 2
14 – 2 = 10

20. What is the hydronium ion concentration in a solution that has a pH of 8.34? 8.34 = -log10 [H3O+] H3O+ = H3O+ = 4.57 x 10-9 M

21. Chemical reactions necessary for life can only occur properly within a narrow pH range.
Small pH changes within a cell, body or ecosystem can be destabilizing.
Biological systems (cells, multicellular organisms) must have buffering capacity to maintain homeostasis in regard to pH.

22. Example Mammalian blood buffering system
Buffers are typically weak acid-base pairs that combine reversibly to remove or donate H+, and thus minimize changes in the system’s pH
carbon dioxide + water <-> carbonic acid < - > bicarbonate ion + hydrogen ion
CO2 + H < -- > H2CO < -- > HCO3- + H+
Mammalian blood is typically around 7.4
If blood becomes acidic CO2 is breathed out, shifting the reaction to the left

23. Disruption of the body’s pH can lead to medical acidosis and alkalosis
If CO2 cannot be expelled (respiratory acidosis) or acid builds up due to other reasons (metabolic acidosis) blood will become too acidic
Caused by…
Lung disease blocking gas diffusion (emphysema, pneumonia, bronchitis, asthma)
Depression of respiratory system by drugs
Cardiopulmonary arrest
Stroke
Alkalosis is much less common (shock, anxiety attack leading to hyperventilating, expelling CO2 too rapidly)

24. Example –Ocean acidification 30% increase in ocean acidity since 1800 CO2 + H20  H2CO3  HCO3- + H+ CO3-2 + calcium = necessary for invertebrate shells, coral, etc. But in acidic conditions 2H++ CO3-2  H2CO3 Excess H+ reduces availability of carbonate ion and so impedes necessary biological processes in many organisms