Chemical Properties
of Water
Water
and Life

Life on Earth
began in
water and
evolved there
for 3 billion
years.
Modern life still remains tied
to water
 Cells are composed of 70%95% water

•Water is found
as a liquid over
71% of the
earth
•The abundance
of water is a
major reason
Earth is
habitable
The structure of water

Studied in isolation, the water molecule is
deceptively simple

Its two hydrogen atoms are joined to one
oxygen atom by single covalent bonds
H
H
O

But the electrons of the covalent bonds are not shared equally between
oxygen and hydrogen
Not in notes***
This unequal sharing makes water a polar molecule
Oxygen is more electronegative than hydrogen, so it has a
greater pull on the electrons
()
()
()
()

The polarity of
water results in
weak electrical
attractions
between
neighboring water
molecules
These
interactions
are called
hydrogen
bonds
()
Hydrogen bond
()
()
()
()
()
(b)
()
()
Polar Structure
Electronegativity of H20
Water’s Life Supporting
Properties

The polarity of water molecules and the
hydrogen bonding that results explain most
of water’s life-supporting properties
Water’s cohesive nature
 Water’s ability to moderate temperature
 Floating ice
 Versatility of water as a solvent

The Cohesion of Water

Water molecules stick
together as a result of
hydrogen bonding
Microscopic tubes
Cohesion is vital
for water transport
in plants
Surface tension is the measure of how difficult
it is to stretch or break the surface of a liquid
Can support animals like “water
striders” in ponds
Surface Tension

Water drops are round because all the
molecules on the edge are pulled to the
middle.
Water will also be
attracted to other
polar substances
 This is called
adhesion
 It is due to the polar
nature of the water
molecule

Caused by adhesion, the water
runs along the glass and does
not fall straight.
Capillary Action
Glass has polar
molecules.
 Glass can hydrogen
bond.
 Attracts the water
molecules.
 Some of the pull is up.

Water moves up from
ground using
cohesion and
adhesion.
Transpiration
pulls water out of
ground into tree,
water molecules
cohere to each other
and adhere to sides
of xylem tube in tree.
 Describe
an example of how
cohesion, adhesion, & capillary
action work in a living organism:
Meniscus
Water curves up
along the side.
 This makes the
meniscus.

How water moderates
temperature

Because of hydrogen bonding, water has
a strong resistance to temperature
change
Heat and temp
Heat is the measure of the amount of
kinetic energy in the environment and
molecules and object has
 Temperature measures avg. energy of the
molecules.

How water moderates
temperature

Whenever 2 objects meet, the cooler
object absorbs heat from the warmer
object until they are the same temperature
Water absorbs heat during warm periods, then
the shore absorbs that stored heat during
cooler periods.
 Despite the fact that water is losing/gaining
heat energy, it is not necessarily changing
temperature because it has a high specific
heat.

How water moderates
temperature

Water has a high specific heat

Specific heat = the amount of heat that must
be absorbed or lost to change the
temperature of 1g of the substance 1°C
How water moderates
temperature
Since water has a high specific heat, it will
not change temperature much when it
absorbs or loses heat
 This is because much of the absorbed
heat is used to break hydrogen bonds, not
increase the kinetic energy of the
molecules

How water moderates
temperature

So water can absorb
and store large
amounts of heat
while only changing
a few degrees in
temperature
Large bodies of water help to
moderate temperature
Earth’s giant water supply
causes temperatures to stay
within limits that permit life
 Evaporative cooling
removes heat from the
Earth and from organisms

 Compare
the temperature changes
(per season or per day) of coastal
habitats to inland habitats:
How water moderates
temperature
Water also has:
High heat of fusion
• The temp at which liquid turns solid
High heat of vaporization
• The temp at which liquid turns to gas
The Biological Significance of Ice
Floating

When water molecules get cold, they move
apart, forming ice
A chunk of ice has fewer molecules than an equal volume
of liquid water
Liquid
water
Ice

The density of ice is lower than liquid water

This is why ice floats
Hydrogen bond
Ice
Stable hydrogen bonds
Liquid water
Hydrogen bonds
constantly break and re-form
Figure 2.15
Change of State
Dipole Structure
Ice
floats in water because all ice
molecules are held in hexagons
The center of the
hexagon
is open space,
making ice 8% less
dense than
water.

Since ice floats, ponds, lakes, and even the
oceans do not freeze solid
Marine life could not survive if bodies of water froze solid

Floating ice insulates water below,
preventing freezing: critical for ocean
animals
Maximum density: 3.98oC


Below this temp, form hexagonal polymers and
decrease density
Above this, molecules are energetic, water behaves
like other liquids - expanding when warm and
contracting when cool
Water as the Solvent of Life

A solution is a liquid consisting of two or
more substances evenly mixed
The dissolving agent is called the solvent
The dissolved substance is called the solute
Ion in solution
Salt crystal

When water is the
solvent, the result is
called an aqueous
solution


Water is a good
solvent because it is
polar
Ionic (salts) and polar
(sugars) compounds
dissolve readily in
water
Solvent Properties


Water dissolves salts by surrounding the atoms in the
salt molecule and neutralizing the ionic bond holding the
molecule together
Free water is water not attached to solute. Any time
you add a solute you have less free water.
 What
is the “rule” for solubility?