NOTES: 2.2 –
Properties of Water
Water Overview…
• Water is perhaps the most important compound
in living organisms
• Because so many substances can dissolve in
water:
molecules and ions are free to
move and collide with one
another in the reactions of life;
water serves as a means of
material transportation
(i.e. blood, plant sap)
Water Overview…
• Living cells are 70%-95%
water
• Water covers approx. ¾ of
earth.
• In nature, water naturally
exists in all 3 states of
matter: solid, liquid and gas
Water is a POLAR molecule
• the bonds that hold the hydrogen and oxygen
atoms together are POLAR COVALENT
BONDS…
 this means that the electrons that make up
the covalent bonds are shared UNEQUALLY
…they spend more time near the oxygen atom
than they do near the
hydrogen atom
so, a water molecule has
a positive end and a
negative end
Water is POLAR…
• POLAR WATER MOLECULES attract
one another, as well as ions and other
polar molecules
Water Molecules…
• The polarity of water molecules
results in HYDROGEN BONDING
• a HYDROGEN BOND is the
attraction of opposite charges
between hydrogen and oxygen
• Each water molecule can form a maximum of
4 hydrogen bonds with neighboring water
molecules
Properties of Water (as a result of
polarity and H-bonding):
► COHESION: molecules are held
together by H bonds
• contributes to upward
movement of water in
plants from roots to leaves
Tension–
Cohesion Model
Properties of Water
► ADHESION: water
sticks to other
surfaces (by polarity
or H bonds); can
counteract gravity in
plant vessels
► SURFACE TENSION: measure of how
difficult it is to stretch the surface of a
liquid
-water has greater surface tension than
most liquids because at the air/water
interface the surface water molecules
are H-bonded to each other and to the
water molecules below
-causes water to “bead”
-creates a “skin” on the surface
► HIGH SPECIFIC HEAT (resists changes
in temp)
• Specific Heat: amt. of heat that must be
absorbed or lost for 1 gram of a
substance to change its temp by 1°C
• Water’s high specific heat means that it
resists temp. changes when it absorbs
or releases heat
*Heat is absorbed to break H-bonds; and
given off when they form
► HIGH HEAT OF VAPORIZATION
• Heat of vaporization = amt. of heat a
liquid must absorb for 1 g to be
converted to gas state
• for water molecules to evaporate, H
bonds must be broken, which requires
heat energy
► EVAPORATIVE COOLING: after
high temp. molecules have
evaporated, the remaining liquid is
cooler (ex: sweating)
-stabilizes temp. in aquatic ecosystems
-protects organisms from overheating
*(as 1 g of water evaporates
from our skin, 539 g
of body cools by 1°C)
The air in the shower
stall is at the same
temperature as the air
outside, but there's
less water vapor
outside to condense
on the skin.
EVAPORATIVE COOLING
HEAT OF VAPORIZATION
(WHILE CHANGING STATE)
SPECIFIC HEAT
(WHILE WATER IS HEATING)
► WATER EXPANDS WHEN IT FREEZES
• because of H-bonding, water is less dense as
a solid than it is as a liquid...ice floats!
• since ice is less dense, it forms on the surface
at first
• as water freezes, it releases heat to the water
below and insulates it!
• Oceans and lakes don’t freeze solid!!
cell
► WATER IS THE SOLVENT OF
LIFE
• due to its polarity, water is a
versatile solvent
Water forms Solutions
– All the components of a solution are
evenly distributed throughout the solution.
– In a saltwater solution, table salt is the
solute—the substance that is dissolved.
– Water is the solvent—the substance in
which the solute dissolves.
Water forms Suspensions
– Some materials do not dissolve in water, but
separate into small pieces. Such mixtures of water
and nondissolved material are known as
suspensions.
– Some of the most important biological fluids are
both solutions and suspensions.
– Blood is mostly water. It contains many
dissolved compounds, but also cells and other
undissolved particles that remain in suspension as
the blood moves through the body.
Acids, Bases, and pH
•
Water molecules sometimes split apart to form
hydrogen ions and hydroxide ions.
•
This reaction can be summarized by a chemical
equation in which double arrows are used to show that
the reaction can occur in either direction.
pH Scale
• Chemists devised a measurement system called the
pH scale to indicate the concentration of H+ ions in
solution.
• The pH scale ranges from 0 to 14.
• At a pH of 7, the concentration of H+ ions and OH– ions
is equal. Pure water has a pH of 7.
– Acids have a pH less than 7.
– Bases have a pH greater than 7.
Buffers
– The pH of the fluids within most cells in the human body
must generally be kept between 6.5 and 7.5 in order to
maintain homeostasis. If the pH is lower or higher, it will
affect the chemical reactions that take place within the
cells.
– One of the ways that organisms control pH is through
dissolved compounds called buffers, which are weak
acids or bases that can react with strong acids or bases
to prevent sharp, sudden changes in pH.