Water’s Life support
Properties
Text pg. 25 - 26
Polar Covalent Bonds

Water has atoms with different electronegativities
–
–
–
Oxygen attracts the shared electrons more strongly
than hydrogen
So, the shared electrons spend more time near oxygen
The result is a polar covalent bond
 In
–
H2O the oxygen atom has a slight negative charge
and the hydrogens have a slight positive charge
Molecules with this unequal distribution of charges are
called polar molecules
Hydrogen Bonds

Hydrogen, within a polar covalent bond, is
attracted to electronegative atoms in other
molecules.
–
called a hydrogen bond
–
Is responsible for holding
together 2 separate water
molecules
Cohesion

Hydrogen bonding causes molecules to stick
together, a property called cohesion

Cohesion is related to surface tension - a
measure of how difficult it is to break the
surface of a liquid
–
Hydrogen bonds are responsible
for surface tension
Adhesion

Adhesion is the clinging of one substance to
another unlike itself.
Caused by adhesion the water runs
along the glass and does not fall
straight.
Capillary Action is a result of a
combination of cohesion and
adhesion



Capillary Action is the
ability of water to flow up a
thin tube against gravity
It is due to a combination
of cohesion and adhesion
Examples:


Water soaking up a paper
towel
Water turning up the side of
a test tube
Capillary action is the mechanism
behind transpiration
Transpiration is the
process by which
water is pulled from
the roots of a plant
all the way up and
out of the leaves.
Adhesion
Cohesion
Direction
of water
movement
Water-conducting
cells
Water - The Universal Solvent
Solvent – liquid that can dissolve other substances
 Solute – substance that dissolves in a solvent
HYDROPHILIC (WATER LOVING) SUBSTANCES
 Ionic substances are soluble (they dissolve) in water because the
poles of the polar water molecules interact with the ionic
substances and separate them into ions.
 Polar covalent substances are also soluble of the interaction with
their poles with water
HYDROPHOBIC (WATER HATING) SUBSTANCES
 Nonpolar covalent substances do not dissolve in water because
they lack charged poles
EX. WATER AND OIL BOTTLES

Water’s hydrogen bonds moderate
temperature

Has a greater ability to resist temperature change
because of hydrogen bonds.
–
Heat is the energy associated with movement of
atoms and molecules in matter
–
Temperature measures the intensity of heat
***** When it is cool outside, water will be warmer than the
air. When it is warm outside, the water will be colder than
the air.
Specific Heat Capacity


Specific heat capacity – the degree to which a
substance changes temperature in response to a
gain or loss of heat
Water has a high specific heat, with so many
hydrogen bonds between water molecules, it can
absorb a great deal of heat, with only a slight
temperature change.

Compare the water in a pool on a hot day, and a
metal chair next to the pool
Heat of fusion and heat of
vaporization

When water changes physical states, from solid
to liquid or liquid to gas, energy is absorbed but
the water temperature remains constant.
The absorbed energy is used only to change the
physical state of the water by breaking the hydrogen
bonds that tether the water molecules together.
 In the reverse reactions, from gas to liquid or liquid
to solid, the energy released reestablishes the
hydrogen bonds.

Heat of fusion and heat of
vaporization
Ice



Most types of matter are more dense as a solid
Water is different in that when water freezes, each molecule
forms a stable hydrogen bond with four neighbors
– A three-dimensional crystal results
– There is space between the water molecules (This is not
the case with most solids. This is the reason why you can
bite into and break ice.)
Ice is less dense than water, so it floats
Ice
Water molecules
Are spaced out
Liquid water
Water molecules
are closer together