Water and the Fitness of the
Environment
AP Biology Chapter 3
Importance of Water
• It can be quite correctly argued that life
exists on Earth because of the abundant
liquid water. Earth is the only planet in
which water exists in all three states.
• Other planets have water, but they either
have it as a gas (Venus) or ice (Mars).
• The chemical nature of water is thus one
we must examine as it permeates living
systems: water is a universal solvent, and
can be too much of a good thing for some
cells to deal with.
Water
• Water is a tiny V-shaped molecule
with the molecular formula H2O.
Water has hydrogen
bonds, which confers
many characteristics.
Polarity of Water
• Water is polar
covalently bonded
within the molecule.
• This unequal sharing
of the electrons
results in a slightly
positive and a slightly
negative side of the
molecule.
Properties of Water
• Cohesion
• Surface tension
• Adhesion
• High Specific Heat
• Moderates Temperatures on Earth
• High Heat of Vaporization
• Evaporative Cooling
• Ice Floats
• Universal Solvent
Cohesion
Cohesion refers to the tendency of
water molecules to hydrogen bond to
each other
Cohesion contributes to a number of
water’s properties
These properties include the ability of
water to be siphoned as well the related
property of transport of water from the
roots to the leaves of plants
Surface
Tension
– Surface tension is an emergent property of
water that results from the tendency of
water molecules to stick to each other (by
hydrogen bonding) better than they adhere
to air molecules
– Surface tension “makes water behave as
though it were coated with an invisible
film.”
Adhesion
• Adhesion is the tendency of water
to stick to substances other than
water.
Specific Heat
– Water is able to absorb heat – without
increasing much in temperature – better
than many substances
– This is because for water to increase in
temperature, water molecules must be
made to move faster within the water;
this requires breaking hydrogen bonds,
and the breaking of hydrogen bonds
absorbs heat
– Water is also able to retain heat better
than many substances (i.e., resist
cooling).
– This is because for water to decrease in
temperature, water molecules must be
made to move more slowly within the
water; this requires the forming of
hydrogen bonds, and the forming of
hydrogen bonds gives off heat (hence
counteracting cooling tendencies as
heat is lost from liquid water).
Moderate Temperatures
– Water’s high specific heat serves to
buffer the internal temperature of
organisms, the temperatures of bodies
of water, and the temperatures of the
entire biosphere, all things that enhance
the ability of life to survive on this planet.
High Heat of Vaporization
– Water resists evaporating (i.e., vaporizing)
because hydrogen bonds must be broken in
order for water to transition from liquid to the
gas state
– This high heat of vaporization contributes to
the ability of water to serve as local heat sinks
(e.g., organisms, lakes, ponds) and as a
global heat sink (i.e., oceans) – these are
regions (volumes) that retain heat for longer
than surrounding substances (such as air or
rocks)
Evaporative Cooling
– The vaporization of water is a
consequence of individual water
molecules escaping the liquid state for
the gas (or vapor) state.
– Those water molecules that are most
energetic (i.e., moving fastest) are most
likely to escape liquid water.
– Faster moving water molecules carry
more heat than slower moving ones
(heat actually is simply a measure of
degree of molecular motion).
– “It is as if the 100 fastest runners at a college
transferred to another school; the average
speed of the remaining students would
decline.” (Campbell et al., 1999)
– This results in the average temperature of
liquid water declining with the loss of each
more-energetic water molecule to the vapor
phase.
– Evaporative cooling contributes to water’s
ability to serve as a temperature buffer.
– We use evaporative cooling when we sweat.
Ice Floats
– Unlike most substances, solid water (ice) has a lower
density than liquid water
– As a consequence, solid water floats upon liquid water,
rather than sinking beneath it
– The lower density of ice is a result of the water solid
phase containing on average more hydrogen bonds per
water molecule (i.e., approaching 4) than does liquid
water at any given moment
– More hydrogen bonds results in more structure which,
in water’s case, results in more unoccupied space, i.e.,
a lower density upon freezing
– Because ice floats, bodies of water freeze from the top
down rather than the bottom up
– Since ice serves as an insulator, this property of water
assures that the complete freezing of bodies of water
does not occur
Structures of Liquid Water and
Ice
Universal Solvent
– The most important property
of water to the existence of
life has to do with the ability
of water to dissolve some
substances and not others
– Water dissolves substances
to which it can readily
hydrogen bond (or is
otherwise attracted to
typically because the
substance contains a full or
partial charge)
pH
• To understand the chemistry of
water we need to know about pH.
• pH is the Log Scale Unit of
Measure used to express the
degree of acidity of a substance.
• A Water Molecule has one oxygen
atom and two hydrogen atoms.
• In pure water, most of the water
molecules remain intact. However,
a very small amount of them react
with each other in the following
manner.
• H2O + H2O ===> H3O+ + OH–
Acids
• When pure water reacts, it produces an equal
amount of H3O+ and OH–. Thus, it does not
have an excess of either ion. It is therefore
called a neutral solution.
• If a strong acid, such as hydrochloric acid ( HCl )
is added to water, it reacts with some of the
water molecules as follows:
• HCl + H2O <=====> H3O+ + Cl–
• Thus, the addition of HCl to water increases the
H3O+ or acid concentration of the resulting
solution.
Bases
• If a strong base, such as sodium
hydroxide, is added to water, it ionizes as
follows:
• NaOH <=====> Na+ + OH–
• Thus, the addition of NaOH to water
increases the OH– or alkali concentration
of the resulting solutions.
Acids II
• Acids are ionic compounds ( a compound
with a positive or negative charge) that
break apart in water to form a hydrogen
ion (H+).
*The strength of an acid is based on the
concentration of H+ ions in the solution.
The more H+ the stronger the acid.
Example: HCl (Hydrochloric acid) in water
Bases II
• Bases are ionic compounds that break
apart to form a negatively charged
hydroxide ion (OH-) in water.
*The strength of a base is determined by
the concentration of Hydroxide ions (OH-).
The greater the concentration of OHions the stronger the base.
pH Scale
• The strength of an acid or base in a
solution is measured on the pH scale.
**The pH scale is a measure of the
hydrogen ion concentration. It spans from
0 to 14 with the middle point (pH 7) being
neutral.
• Any pH number greater than 7 is
considered a base and any pH number
less than 7 is considered an acid.
pH
Scale