1. Water – Chapter 3

2. Water is a POLAR colvalent molecule
Oxygen exerts a greater pull over the shared electrons and they move a little closer to the oxygen.
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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.

4. HYDROGEN BONDS
The attraction between water molecules are called hydrogen bonds.
The hydrogen bonds joining water molecules are weak, about 1/20th as strong as covalent bonds.
They are pretty weak alone, but their great numbers give them strength (like the tiny teeth in a zipper).
They form, break, and reform with great frequency.

5. Organisms Depend on Cohesion
Hydrogen bonds hold the substance together, a phenomenon called cohesion

6. Water is also Adhesive
Water will stick to other surfaces.

7. Cohesion and Adhesion Contribute to:
Capillary Action-water will move up xylem against gravity
Capillary Action

8. Cohesion and Adhesion also contribute to:
Surface Tension
Water has a greater surface tension than most other liquids because hydrogen bonds among surface water molecules resist stretching or breaking the surface.

9. Cohesion forms drops, surface tension keeps them sperical and adhesion sticks them to the leaf!

10. The Argyroneta aquatica

11. Thermal Properties of Water
Water stabilizes air temperatures by absorbing heat from warmer air and releasing heat to cooler air.
Water can absorb or release relatively large amounts of heat with only a slight change in its own temperature due to its HIGH SPECIFIC HEAT CAPACITY.
Celsius Scale at Sea Level
100oC
Water boils
37oC
Human body temperature
23oC
Room temperature
0oC
Water freezes

12. 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

13. Water is a Coolant
Because it takes a lot of energy for water to evaporate it is a GREAT coolant! It removes a lot of heat from a system!
It moderates Earth’s climate
It stabilizes temperatures in aquatic ecosystems
It prevents organisms from overheating

14. It is also useful for animals who use water or mud to cool off during a hot day!

15. Density of Water Most dense at 4oC Contracts until 4oC
Expands from 4oC to 0oC
The density of water:
Prevents water from freezing from the bottom up.
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.
Supports the mass of large and small organisms.

16. 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.

17. Water is a transport medium
Water dissolves nutrients, gasses and waste products that can be carried in the circulatory system of animals, the xylem and phloem of plants.
The human body is 70% water!

18. Water is a Solvent for Life
Water is a good solvent because it is a POLAR molecule and it will dissolve polar solutes easily.
Polar attractions cause water molecules to surround and isolate the solute molecules.
Dissolved particles are able to move around and diffuse making more likely they will collide leading to a reaction.
Metabolic reactions take place in water.

19. Ex) Sodium Chloride in water

20. All metabolic reactions occur in solutions
There are two important quantitative proprieties of solutions:
Concentration
2. pH

21. Concentration of a Solution
Molecular weight – sum of the weights of all atoms in a molecule
Mole – amount of a substance that has a mass in grams numerically equivalent to its molecular weight.
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
The concentration of a material in solution is called its molarity or M.
A one molar (1M) solution has one mole of a substance dissolved in one liter (L) of solvent, typically water.
Therefore the units of molarity of M = mols/L.

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+).

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. 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.

28. 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
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.