Water
and the Fitness of the
Environment
Polarity of Water
• Water is a polar
•
•
molecule
Hydrogen of one H2O
is attracted to the
Oxygen of another
H2O
Therefore, hydrogen
bonding occurs
Properties of Water
• Water has 4 properties that make Earth a
fitting environment for life:
– High surface tension/Cohesive behavior
– Ability to stabilize temperature
– Expansion upon freezing
– Versatility as a solvent for life
1. Cohesion
• Cohesion:
– Hydrogen bonds hold
water together in
liquid form
– Responsible for
transport of water
against gravity in
plants
Cohesion
• Adhesion = clinging
of one substance to
another
– Cohesion = adhesion
of water to itself
• Surface Tension =
how hard it is to
stretch or break the
surface of a liquid
2. Ability to Stabilize Temperatures
• Water…
– Absorbs heat from air that is warmer than
itself
– Releases stored heat to air that is cooler than
itself
Ability to Stabilize Temperatures
• Water’s high specific heat
– Specific heat
• amount of heat that must be absorbed or lost for
1g of that substance to raise its temperature by 1C
– Because of its high specific heat (due to
hydrogen bonding) water resists
temperature change
• LOTS of hydrogen bonds have to be broken before
water molecules can start moving faster
Ability to Stabilize Temperatures
• Application:
– Large body of water can absorb LOTS of heat
from the sun
– Then, the gradually cooling water warms the
air at night and in the winter
• Mild climate of coastal areas
– Ocean, lake, pond temperatures remain stable
– IMPORTANT!
3. Expansion Upon Freezing
• Water is less dense as a
solid than as a liquid:
– Because of hydrogen
bonding! 
• Therefore, ice floats
• Ice:
– Each water molecule
bonded to max of 4
“partners”
• Water:
– Bonds are constantly
breaking and reforming
Expansion Upon Freezing
• Application and
Importance:
– Allows marine life to
continue to live even
when the top layer of
water is frozen
– Ice acts as an
insulator
4. Solvent of Life
• Solution:
– A liquid that is a homogeneous mixture of 2 or more
substances
• Solvent:
– The dissolving agent
– Ie. water
• Solute:
– Substance that is dissolved
– Ie. Kool-aid powder
Solvent of Life
• Aqueous Solution:
– A solution in which water is the solvent
• Water is NOT a universal solvent
– Why not?
Solvent of Life
• Hydrophilic:
– “water-loving”
– Polar and ionic compounds
• Hydrophobic:
– “water-fearing”
– Nonpolar compounds
The pH Scale
• The dissociation of water
H2O  H+ + OH-
• In water:
[H+] = [OH-]
The pH Scale
• Acid:
– A substance that increases the H+
concentration of a solution
[H+] > [OH-]
• Base:
– A substance that decreases the H+
concentration of a solution
[H+] < [OH-]
The pH Scale
• Logarithmic scale of 0
•
•
– 14
Neutral pH = 7
Acidic pH = <7
– More H+ ions
• Basic pH = > 7
– Fewer H+ ions
• pH declines as H+
concentration
increases
Acid Precipitation
• pH of normal rain = 5.6
• pH of acid rain = as low as 1.5
(Sulfur oxides or nitrogen oxides) + water  acids
• Human blood pH = 7.4
– 7.0 or 7.8 = death!
• Importance:
– Aquatic animals, plants, etc…