1. Properties of Water Polar molecule Cohesion and adhesion
High specific heat
Density – greatest at 4oC
Universal solvent of life

2. Polarity of Water Water is a Polar Molecule! What is a polar molecule?
Water molecule is called a “polar molecule” because opposite ends of the molecule have opposite charges.
Polar covalent bonds:
A type of bond formed when electrons are not shared equally
Water is a polar molecule:
Oxygen is more electronegative so it pulls the shared electrons closer creating a partial negative charge around the oxygen atom.
The region near the two hydrogen atoms has a partial positive charge.
Gives water more structure than other liquids
2 corners are orbitals with unshared electrons and a weak negative charge
2 are occupied by hydrogen atoms that have polar covalent bonds

3. Water has a variety of unusual properties because of attractions between these polar molecules.
Because each water molecule is polar (slightly charged): 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.
Fig. 3.1
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

4. HYDROGEN BONDS give water the following properties:
Hold water molecules together
Each water molecule can form a maximum of 4 hydrogen bonds
The hydrogen bonds joining water molecules are weak, about 1/20th 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

5. Organisms Depend on Cohesion
Hydrogen bonds hold the substance together, a phenomenon called cohesion
Cohesion: the act of sticking together
example: water clinging to other water molecules
Cohesion among water molecules plays a key role in the transport of water against gravity in plants
Adhesion: clinging of one substance to another
Ex. water clings to the sides of a vessel

6. Let’s talk! Cohesion vs. adhesion
Cohesion is different from adhesion because…… When you have an answer, put your pencil down and nod.

7. Water has a high surface tension (a measure of the force required to break the surface of a liquid) because the molecules are cohesive.
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.
A surfactant (wetting agent) will reduce the surface tension of water. Soap is an example of a surfactant.
Fig. 3.3
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

8. Video!
Surface Tension and Capillary Action Video

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 1oC.
Three-fourths of the earth is covered by water. The water serves as a large heat sink responsible for:
Prevention of temperature fluctuations that are outside the range suitable for life.
Coastal areas having a mild climate
A stable marine environment

10. Moderates Temperatures on Earth
Water has a high specific heat so it can absorb or release relatively large amounts of heat with only a slight change in its own temperature.
Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.
Celsius Scale at Sea Level
100oC
Water boils
37oC
Human body temperature
23oC
Room temperature
0oC
Water freezes

11. 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

12. Density of Water Most dense at 4oC Contracts until 4oC
Expands from 4oC to 0oC
The density of water:
1. Ice is less dense than water so ice floats.
2. Prevents lakes from freezing from the bottom up.
3. Ice forms on the surface first—the freezing of the water releases heat to the water below creating insulation.
Makes transition between season less abrupt.

13. Ice is about 10% less dense than water at 4oC.
When water reaches 0oC, water becomes locked into a crystalline lattice with each molecule bonded to to the 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 4oC.
Fig. 3.5
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

14. Video!
Water Freezing at Molecular Level

15. Its just the tip of the iceberg…

16. Water is the Universal Solvent
Water will dissolve:
Ionic compounds (salt)
Polar molecules
“like dissolves like”
Water will not dissolve:
Nonpolar compounds
Ex. oil

17. Word Pictures Game Get into lab groups.
Form 2 teams within each group. One team on each side of the table.
Select one of the words from a card and illustrate it on the board.
The opposing team gets a point for a correct guess.
The other team illustrates another word.

18. Polar Compounds
Polar compounds will dissolve in water because water is polar (like dissolves like)
Polarity can be predicted by shape
Polar substances are not symmetrical (having no center of symmetry)
Sucrose (sugar)
Hydrogen fluoride (HF)
H2O

19. Non-polar compounds
Non-polar compounds will not (usually) dissolve in water
Non-polar compounds are symmetrical
Boron tri-fluoride
Carbon dioxide
Methane (CH4)

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

21. Concentration of a Solution
Molecular weight – sum of the weights of all atoms in a molecule (daltons)
Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular weight in daltons.
Avogadro’s number – 6.02 X 1023
A mole of one substance has the same number of molecules as a mole of any other substance.

22. Molarity Calculate a one molar solution of sucrose, C12H22O16.
The concentration of a material in solution is called its molarity.
A one molar solution has one mole of a substance dissolved in one liter of solvent, typically water.
Calculate a one molar solution of sucrose, C12H22O16.
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?

23. Dissociation of Water Molecules
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 (H3O+).
Unnumbered Fig. 3.47
Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

24. This reaction is reversible.
A simpler way to view this process is that a water molecule dissociates into a hydrogen ion and a hydroxide ion:
H2O <=> 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 10-7M (25°C) .
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

25. 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.
There is only 1 molecule out of 554,000,000 water molecules that is dissociated.

26. pH Scale The pH scale in any aqueous solution :
[ H+ ] [OH-] = 10-14
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.

27. 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

28. 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.

29. 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.