1. ….. And the Reasons Water is So Special
Hydrogen Bonds
….. And the Reasons Water is So Special

2. Electronegativity  is a measure of the tendency of an atom to attract a bonding pair of electrons.

3. Polar bond (oxygen is more electronegative than hydrogen)

4. Each water molecule can bind with up to 4 other water molecules
Hydrogen bond (hydrogen atom of one polar molecule is attracted to the slightly negative charge of another polar molecule)
Each water molecule can bind with up to 4 other water molecules

5. Hydrogen Bonds
Results when hydrogen is bonded to 2 highly electronegative atoms at the same time
Application:
Water molecules bonding to each other and other polar molecules
DNA – 2 strands linked together (nitrogenous bases)
Secondary, Tertiary, Quaternary structures of proteins

6. Water is wonderful … and unique!
How do the unique chemical and physical properties of water make life on earth possible?
Polar molecule
Results in hydrogen bonding
Cohesion
Adhesion
Surface Tension
Moderates temperature
High specific heat
Evaporative cooling
Oceans don’t freeze solid
Solvent

7. Hydrogen bonding in water results in:
Cohesion
Transport of water against gravity in plants
Water exiting the leaves “tugs” on water molecules leading all the way down to the roots
Adhesion
Water clings to the walls (cellulose does not dissolve in water … nice quality) of plant vessels to help counter the downward pull of gravity
Surface tension
Measure of how difficult it is to stretch or break the surface of a liquid
Some animals can stand, walk, or run on water

8. Solid water (ice) is less dense than liquid water
When water freezes the hydrogen bonds are “locked” into place and creates an organized crystal structure with air pockets that make it less dense.

9. Water moderates temperature
Absorbs heat from air that is warmer
Hydrogen bonds break
Releases heat to air that is cooler
Hydrogen bonds form
High specific heat
This absorption and release of of heat from water only slightly changes its own temperature
Coastal areas generally have milder climates

10. Evaporative Cooling
Water molecules must have a relatively high kinetic energy to break hydrogen bonds and change states from liquid to gas
Prevents land organisms from overheating
Prevents tissues in leaves from becoming too warm in sunlight

11. Water is the solvent of life
The oxygen (-) regions of water cling to sodium (Na+) ions while the hydrogen (+) regions of water are attracted to chloride (Cl-) ions.
Eventually all ions are dissolved as a “hydration shell” surrounds them.

12. The Dissociation of Water Molecules

13. Acid
Increases the hydrogen ion concentration
Base
Reduces the hydrogen ion concentration
Increases the hydroxide ion concentration
Logarithmic scale
pH 1 is 10 X more acidic than pH 2
pH = - log [H+]
pH 7; [H+] = [OH-] = 10-7
[H+] x [OH-] = 10-14
If pH = 5 than [H+] = 5 and [OH-] = 9

14. Biological Application of pH
Enzymes have optimal pH’s in which they work best
Pepsin is found in the stomach
optimal pH is very acidic

15. HCO3 - (bicarbonate ion (base))
pH Buffers
Substances that minimize changes in concentrations of H+ and OH- in solution
Human blood pH stability
Uses Carbonic Acid – Bicarbonate buffering system
Response to rise in pH
H2CO3 (carbonic acid)
(H+ donor)
HCO3 - (bicarbonate ion (base))
(H+ acceptor)
H+ (hydrogen ion) +
Response to drop in pH