1. Water & the Fitness of the Environment
Chapter 3

2. Some Extraordinary Properties of Water
Liquid water is cohesive!
Water has a high specific heat!
Water stabilizes temperature!
Water has a high heat of vaporization!
Water expands when it freezes!
Water is a versatile solvent!

3. Water
Water contributes to the fitness of the environment to support life.
Life on earth evolved in water
Living cells are 70%-95% water
Water covers about ¾ of the earth
In nature, water naturally exists in all 3 physical states of matter.
Water’s extraordinary properties are emergent properties resulting from its structure and molecular interactions.
Figure 3.1

4. Bonding
Polar covalent bonds in water molecules result in hydrogen bonding.
Structure of water
Made up of one atom of oxygen and two atoms of hydrogen (H2O)
Polar
Oxygen is so electronegative, that shared electrons spend more time around the O causing a weak positive charge near H’s.
Covalent is the sharing of two valence electrons by two atoms
In a polar covalent bond
The atoms have differing electronegativities
Share the electrons unequally
Water has a simple molecular structure. It is composed of one oxygen atom and two hydrogen atoms. Each hydrogen atom is covalently bonded to the oxygen via a shared pair of electrons. Oxygen also has two unshared pairs of electrons. Thus there are 4 pairs of electrons surrounding the oxygen atom, two pairs involved in covalent bonds with hydrogen, and two unshared pairs on the opposite side of the oxygen atom. Oxygen is an "electronegative" or electron "loving" atom compared with hydrogen.
Water is a "polar" molecule, meaning that there is an uneven distribution of electron density. Water has a partial negative charge () near the oxygen atom due to the unshared pairs of electrons, and partial positive charges (+) near the hydrogen atoms.

5. Hydrogen Bonding
The slightly negative oxygen atom from one water molecule is attracted to the slightly positive hydrogen end of another water molecule.
Each water molecule can form a maximum of four hydrogen bonds at a time.
The polar molecules of water are held together by hydrogen bonds.
Positively charged H of one molecule is attracted to the negatively charged O of another water molecule.

7. Four Emergent Properties
1) Cohesion
2) Moderation of temperature is possible because of water’s high specific heat
3) Insulation of bodies of water by floating ice
4) Water is an important solvent
Emergent: new properties or properties that emerge with each step upward in the hierarchy of life, owing to the arrangement and interactions of parts as complexity increases.
These properties contribute to the Earth’s suitability for life.
Hydrogen bonds are key to each property, and what makes water so unique.

8. 1) Cohesion The linking of water molecules.
Adhesion is the clinging of one substance to another.
Transpiration is the movement of water molecules up the very thin xylem tubes and their evaporation from the stomata in plants.
Water conducting cells
100 µm
Figure 3.3
Cohesion: Think “water molecule joined to water molecule” and visualize a water strider walking on top of a pond due to the surface tension that is the result of this property.
Surface Tension: is a measure of how hard it is to break the surface of a liquid
Adhesion: Think “water molecule attached to some other molecule” such as water droplets adhering to a glass windshield
Transpiration: The water molecules cling to each other by cohesion, and to the walls of the xylem tubes by adhesion.
Figure 3.4

9. 2) Moderation of temperature is possible because of water’s high specific heat
Specific heat is the amount of heat required to raise or lower the temperature of a substance by one degree Celsius.
Relative to most other materials, the temperature of water changes less when a given amount of heat is lost or absorbed. This high specific heat makes the temperature of Earth’s oceans relatively stable and able to support vast quantities of both plant and animal life.

10. 3) Insulation of bodies of water by floating ice
Water is less dense as a solid than in its liquid state, whereas the opposite is true of most other substances.
Because ice is less dense than liquid water, ice floats. This keeps large bodies of water from freezing solid and therefore moderates temperature.
The density of ice is about 90 percent that of water, but that can vary because ice can contain air, too. That means that about 10 percent of an ice cube (or iceberg) will be above the water line.
This property of water is critical for all life on earth. Since water at about 39°F (4°C) is more dense than water at 32°F (0°C), in lakes and other water bodies the denser water sinks below less-dense water. If water was most dense at the freezing point, then in winter the very cold water at the surface of lakes would sink, the lake could freeze from the bottom up, and all life in them would be killed. And, with water being such a good insulator (due to its heat capacity), some frozen lakes might not totally thaw in summer.
The real-world explanation of water density is actually more complicated, as the density of water also varies with the amount of material that is dissolved in it. Water in nature contains minerals, gasses, salts, and even pesticides and bacteria, some of which are dissolved. As more material is dissolved in a gallon of water then that gallon will weigh more and be more dense—ocean water is denser than pure water.

11. Freezing Water!
Water expands as it freezes because it doesn’t have enough kinetic energy to overcome the formation of hydrogen bonds.
Expansion of water contributes to the fitness of the environment:
Prevents deep bodies of water from freezing bottom up
Density allows for surface freezing first
Makes the transitions between seasons less abrupt

12. 4) Water is an important solvent
Hydrophilic substances
Hydrophobic substances
The substance that something is dissolved in is called the solvent, whereas the substance being dissolved is the solute. Together they are called the solution.
Hydrophilic: substances are water- soluble. These include ionic compounds, polar molecules (example sugars), and some proteins.
Hydrophilic: substances such as oils are nonpolar and do not dissolve in water.

13. Acidic and basic conditions affect living organisms
pH scale
0 to 14
Measures the relative acidity and alkalinity of aqueous solutions
Acids, Bases, & Buffers
Carbonic Acid (H2CO3) important buffer
Increasingly Acidic
[H+] > [OH–]
Increasingly Basic
[H+] < [OH–]
Neutral
[H+] = [OH–]
Oven cleaner 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
Acids have an excess of hydrogen ions and a pH below 7.0
Bases have an excess of hydroxide ions and a pH above 7.0
Pure water is neutral, which means it has a pH of 7.0
Buffers are substances that minimize changes in pH. They accept hydrogen ions from solution when they are excess and donate hydrogen ions when they are depleted
Buffers help organisms maintain the pH of body fluids within the narrow range necessary for life (usually pH 6-8).
Important buffer in living systems. It moderates pH changes in blood plasma and the ocean.

14. Dissociation Continued
Only a hydrogen ion (proton with a +1 charge) is actually transferred.
Transferred proton binds to an unshared orbital of the second water molecule creating a hydronium ion (H3O+)
Water molecule that lost a proton has a net negative charge and is called a hydroxide ion (OH-)
Ionization of water is expressed as the dissociation into H+ and OH-
Reaction is reversible
At equilibrium, most of the water is not ionized

15. Stabilizing Temperature
It’s due to water’s high specific heat.
Because of hydrogen bonding:
It takes a relatively large heat loss or gain for each 1 degree change in temperature.
Hydrogen bonds must absorb heat to break, and they release heat when they form.
Much absorbed heat energy is used to disrupt hydrogen bonds before water molecules can move faster.

16. Heat Sink!
Large bodies of water absorb heat from sunlight (day and summer), and release heat (night and winter) resulting in:
Decreased temperature fluctuation
Milder climates in coastal areas
Marine environments with relatively stable temperatures.

17. Vaporization A.K.A.-Evaporation (transformation from liquid to gas)
Molecules with enough kinetic energy to overcome the mutual attraction of molecules in a liquid, can escape into the air.
Evaporative Cooling: cooling of a liquid’s surface when a liquid evaporates.

18. Water’s High Heat of Vaporization
Moderates the earth’s climate:
Solar heat absorbed by tropical seas dissipates when surface water evaporates
As moist tropical air moves poleward, water vapor releases heat as it condenses into rain.
Stabilizes temperature in aquatic ecosystems
Helps organisms from overheating by evaporative cooling.