The Importance of Water to Life
and the Concept pH
Earth’s Environmental Conditions that
Sustain Life are Dependent on Water
• Over 75% of the
Earth’s surface is
covered by water in
one form or
another.
• Earth’s climate is
intrinsically tied to
water and its
properties.
Earth’s Living Organisms Contain
a High Volume of Water
• All life needs
water in one
way or another
for survival.
• All living
organisms
contain a high
volume of water
in their cells,
tissues, and
body.
Water Is a Polar Molecule
• In a water molecule,
Oxygen holds the
electrons that it
covalently shares with
the two Hydrogen
closer to its nucleus.
• Therefore, in water,
Oxygen picks up two
negative charges and
each Hydrogen has
one positive charge.
Water’s Polarity Leads to Hydrogen Bonding
• Hydrogen bonding
amongst water
molecules give water
its properties as a
substance.
• Water molecules are
held together by
hydrogen bonds to
make water a very
“solid-like” liquid.
Unique Properties of Water Sustain Life
Cohesion/
Adhesion
Surface
Tension
Specific
Heat
Density
Solvent
Attraction of
water molecules
for each other
and other
substances,
respectively.
Ex. H2O drops
Water has a
film-like surface
due to the
cohesion of the
molecules at
the surface.
Water
absorbs and
stores a large
amount of
heat, then
releases it
slowly.
Water is less
dense as a
solid and
more dense
as a liquid,
therefore, ice
floats.
Water is a
universal, most
versatile,
solvent.
Cohesion and Adhesion
• Cohesion – attraction
that water molecules
have for each other,
due to Hydrogen
bonds.
• Adhesion – attraction
that polar molecules
have for molecules of
other substances.
• Cohesion-tension
transports a H2O
column from roots to
leaves.
Water Moderates Temperature
• As H2O is heated Hydrogen bonds
break and reform, therefore, a
large amount of heat is absorbed
and stored with a minimal change
in temperature.
• As water cools, hydrogen bonds
form and absorb energy in the
process, therefore, slowing the
cooling time.
• Water slows “moderates”
temperature changes.
Water is Less Dense as a Solid
Than as a Liquid: Ice Floats!
• Frozen water is a crystal, where each H2O molecule is rigidly Hydrogen
bonded to four other water molecules. At cold temperatures, there is
insufficient energy to continuously break and form Hydrogen bonds.
This arrangement creates free space between the molecules, i.e. less
density.
• In liquid water there is plenty of energy to cause the constant breakage
and reforming of Hydrogen bonds. Therefore, the H2O molecules rotate
freely and are closer in proximity, than in cool conditions, i.e. more
dense.
Water a Most Versatile Solvent
• Solutions are made up
of solvents and solutes.
• Due to its polar nature,
in living things, water is
the most versatile
solvent.
• Water dissolves both
ionic and polar
substances by creating
spheres of hydration
around the solutes.
• Hydrophilic substances
easily dissolve in water.
Water Dissociates Into H+ and OH• In solutions, water
molecules split into OH(Hydroxide ions) and H+
(Hydrogen ions).
• OH- and H+ ions are very
reactive and must be
balanced. Neutral solutions
have similar amounts of
OH- and H+
• Buffers balance solutions.
• Acids donate H+ ions to
the solution.
• Bases accept H+ ions from
the solution.
• pH is the negative
log (base 10) of the
Hydrogen ion
concentration in a
solution.
• The pH scale
ranges from 0,
most acidic (most
H+); to 14, least
acidic (most OH-)
or most basic; with
7 being neutral,
(H+ = OH-).
• Buffers moderate
pH
Optimal Ranges to Maintain
Homeostatic Balance
• In order to maintain a stable and healthy internal
environment to allow life’s functions, organisms
must exist in optimum ranges or conditions.
• pH ranges that are too acid or basic may harm an
organism,
• Ex. Acidification of trees due to acid precipitation.